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Article

Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses

by
Jiaqi Xia
and
Neil Mattson
*
School of Integrative Plant Science, Cornell University, Ithaca, NY 14850, USA
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(10), 1106; https://doi.org/10.3390/horticulturae10101106
Submission received: 4 September 2024 / Revised: 12 October 2024 / Accepted: 14 October 2024 / Published: 18 October 2024
(This article belongs to the Special Issue Indoor Farming and Artificial Cultivation)
Browse Figures
Review Reports Versions Notes

Abstract

:
Petunia stands as the top-selling bedding plant in the U.S., and improved lighting control in greenhouses holds the potential to reduce crop production time and optimize crop quality. This study investigated the impact of four distinct daily light integral (DLI) conditions with and without supplemental far-red (FR) radiation on the growth of petunia liners and subsequent development of finish plants. Two experiments were conducted in spring (9 April to 18 June 2021) and winter (28 October 2021 to 6 January 2022). Petunia cuttings were rooted in a common environment and then transferred to four greenhouse sections with different DLI treatments: 6, 9, 12, and 15 mol·m−2·d−1 for four weeks. Within each DLI condition, half of the plants were exposed to 28 μmol·m−2·s−1 supplemental FR radiation for 16 h daily (equivalent to 1.61 mol·m−2·d−1 light integral). The number of flower buds and open flowers were tracked daily. Representative liners were destructively harvested and evaluated after four weeks of lighting treatments. The remaining plants were transplanted and moved to a common DLI condition of 15 mol·m−2·d−1 for an additional three weeks before being destructively harvested and evaluated as finish plants. The primary finding reveals the promoting effect of DLI on flowering, branching, morphology, and biomass accumulation of petunia liners, with many effects persisting into the finish stage. A threshold DLI of 9 mol·m−2·d−1 was identified, as lower DLI (6 mol·m−2·d−1) resulted in extensive stem elongation, rendering the plants unmarketable. Higher DLI levels were found to be optimal in terms of flowering and morphology. Supplemental FR accelerated flowering by up to three days in the summer experiment and up to 12 days in the winter experiment. However, FR had limited impact on the number of flower buds and open flowers, branching, and shoot and root weight of the finish plants. Interactions between DLI and FR were observed on some parameters, whereby FR effects were more pronounced under lower DLI. Overall, both higher DLI and supplemental FR exhibited beneficial effects, but DLI had a more pronounced effect. Thus, DLI during petunia liner production appears more important than adding FR. This study well simulated the commercial propagation and production of petunia plants, providing practical insights for decision-making regarding lighting strategies.

1. Introduction

Petunia (Petunia × hybrida) stands as one of the top-selling bedding plants in the U.S., with an annual sales value of $238 million in 2019 [1]. Horticultural techniques that reduce crop production time could have significant economic potential. Petunias are propagated by seed or vegetatively. While seed varieties are popular, it takes longer for breeders to develop a stable cultivar that is true to type. Vegetative propagation via stem cuttings has allowed new cultivars to enter the marketplace quicker [2]. Commercial propagators root stem cuttings and sell these rooted liners to greenhouse operations to finish. High-quality liners with well-developed roots, vigorous shoots, and pre-initiated flowering are essential for a quick and quality finish. Typically, petunias are considered marketable when they have one or more open flowers and a well-branched plant that fills out the container [3].
Various environmental factors such as temperature [4,5,6,7,8,9], daily light integral (DLI) [5,6], photoperiod [10,11,12,13], fertigation [7], and carbon dioxide [14] drive the establishment of unrooted cuttings and the subsequent growth of petunia plants. Among these factors, DLI is often the least consistently controlled in greenhouses [15]. Increasing DLI has been shown to promote both flowering and branching [5,6]. Petunias are long-day plants, with most commercial varieties exhibiting a facultative response (that is, long daylength, i.e., short night length, is not required but speeds up flowering developmentally) [10,11,12,13].
While petunias can be sold year-round in the Southern U.S., they are primarily produced for spring sales in the Northern U.S. due to the cooler winter climate. As a result, propagation typically takes place during the winter months when light levels can be limiting. Light, as a core driving force of plant growth, can affect plants in many different ways. The total amount of photons received by the plant, quantified as DLI, can largely affect crop yield/biomass and quality. The composition of light wavelengths, often presented as a light spectrum, can regulate plant morphology and quality. For flowering bedding plants like petunia, key performance metrics include plant size and shape, time to flower, number of flowers, and flower size, shape, and color [3]. While flower characteristics are primarily governed by breeding programs, other traits, particularly those related to plant architecture, can be regulated by environmental factors, such as light. With the development of light-emitting diode (LED) technology and its increasing use in horticulture, customized lighting strategies including minimum DLI targets and light spectra can be applied to modulate the morphology and quality of petunia plants.
Traditionally, photosynthetically active radiation (PAR) has been defined as light wavelengths ranging from 400 to 700 nm. DLI, in turn, is the total amount of PAR a plant receives over a 24-h period. Research has shown the benefits of DLI on liners and finishing of bedding plants. Erwin and Warner [16] and Mattson and Erwin [17] conducted extensive screening of herbaceous ornamentals to assess their flowering and growth responses to varied photoperiods and light intensity, both of which contribute to DLI, and they found that some species that exhibited delayed flowering could be prompted to flower earlier, while some species accumulated more biomass but did not flower earlier, through an extended photoperiod and/or increased light intensity. Other studies have also explored the impact of DLI on cuttings and the subsequent development of petunia. Lopez and Runkle [18] observed that an increase in DLI from 1.2 to 10.7 mol·m−2·d−1 significantly improved the quality of cuttings (increased root number, increased root and shoot dry weight [DW], and higher relative biomass allocation to roots) and subsequently accelerated flowering time by up to 22 days. Additional research on petunia has shown that increased DLI leads to higher biomass accumulation, reduced plant height, accelerated flowering, and increased flower number [5,19,20].
Wavelengths above 700 nm were not traditionally considered photosynthetically active, because these wavelengths alone could drive little photosynthesis [21,22,23,24]. However, far-red (FR) wavelengths, especially those of 700–730 nm, synergize with PAR and play an important role in photosynthesis [22,25,26]. As early as 1957, it was found that simultaneous FR and red (R, 600–700 nm) light resulted in a greater photosynthesis rate than FR or R light alone, which is known as the Emerson enhancement effect [27]. This is because Photosystem I (PSI) antennas contain pigments that absorb wavelengths above 700 nm, named “long wavelength chlorophylls” by Gobets et al. [28]. Despite constituting only 3–10% of total chlorophylls, they play a large role in energy transfer and trapping in the whole PSI system [28]. However, these pigments are not observed in Photosystem II (PSII) antennas. Therefore, in the absence of FR light, PSI is under-excited relative to PSII and the overall photosynthetic efficiency is limited by this imbalance between the excitations of PSI and PSII [29]. Recent studies showed the significant role of FR in photosynthesis. Zhen and Bugbee [25] quantified the gross photosynthesis rate resulting from FR addition across 14 different species, indicating FR photons had equal efficiency as white (W, 400–700 nm) photons when up to 40% of FR was added to the background of W light. Other research has demonstrated that FR enhanced plant biomass but had a mixed effect on nutritional content [30,31,32].
Beyond photosynthesis and biomass, FR can impact plant morphology. FR promoted plant growth by increasing leaf expansion and light capturing. For example, Jin et al. [33] observed that supplementing 52 µmol·m−2·s−1 FR to a background of 218 µmol·m−2·s−1 R and blue (B) light resulted in a 46–77% increase in lettuce (Lactuca sativa) DW and 58–75% increase in leaf surface area (LSA) depending on planting density. In their study, the net leaf photosynthesis rate was not significantly affected by FR, but they observed faster leaf expansion and increased light interception, which explained the gain in DW. Other studies also establish lettuce as a shade tolerator, which typically exhibit increased LSA or canopy area under shade conditions [30,32,34,35,36,37,38,39,40,41]. In contrast, shade avoiders, such as petunia, tend to show stem elongation in response to a lower R:FR ratio [42,43,44,45].
Since FR radiation is drawing more attention in recent lighting research, it is imperative to investigate the effect of FR on flowering bedding plants. FR-induced shade avoidance responses can include stem elongation and earlier flowering for some species [46]. Park and Runkle [47] reported the effect of substitutional and additional FR with varying phytochrome photoequilibria (PPE) (i.e., the ratio of the FR light-absorbing form of phytochrome [PFR] to the total of PFR and the R light-absorbing form of phytochrome [PR]) on the seedlings of four flowering bedding plants: geranium (Pelargonium × hortorum), petunia, snapdragon (Antirrhinum majus), and impatiens (Impatiens walleriana). Lower PPE (i.e., a lower R:FR ratio or elevated FR) resulted in taller plants, a greater LSA (geranium and snapdragon) and lower chlorophyll content (geranium, petunia, and snapdragon). Additional or substitutional FR at the seedling stage decreased the days to flower for snapdragon, but not for the other species, including petunia. Mah et al. [48] observed that a lower R:FR ratio resulted in an earlier appearance of the first flower bud of petunias, but had no effect on the days to first flower. Comparatively, Kohler and Lopez [49] and Zhang et al. [38] found FR treatment on petunia seedlings had no effect on the subsequent time to visible flower bud and time to flower. However, Kim et al. [50] filtered out FR from sunlight (increasing the R:FR ratio from 1.21 to 1.57 using the narrow band definition of R and FR [R = 655–665 nm, FR = 725–735 nm], or from 1.09 to 1.69 using the broad band definition of R and FR [R = 600–700 nm, FR = 700–800 nm]) and observed that petunia flowering time was delayed by up to 13 days, and plant height was decreased by up to 73%, under low FR. Other research also demonstrated that FR promoted earlier flowering (3–15 days) and stem elongation (15–58%) of petunia [35,51,52]. For flowering bedding plants, it is important to control the flowering time (i.e., promote earlier flowering in many cases to reduce crop overhead costs) as well as to maintain the plant shape (i.e., keep plant compact rather than stretching too much). LED manufacturers and growers should consider this tradeoff between FR’s positive effect on flowering and negative effect on stem elongation when adding FR to their R and B spectrum.
Light quantity (DLI) can interact with FR in regulating the growth of flowering bedding plants. Park and Runkle [53] investigated the effect of FR and photosynthetic photon flux density (PPFD) on petunia, geranium, and coleus (Solenostemon scutellarioides) seedlings. They found decreasing the R:FR ratio, independent of PPFD or DLI, led to increased stem elongation, LSA, and DW; however, increasing PPFD, independent of the R:FR ratio, led to increased whole-plant assimilation, DW, and specific leaf area, but decreased chlorophyll concentration. Notably, the R:FR ratio interacted with PPFD in regulating the subsequent flowering of petunia. A lower R:FR ratio at the seedling stage promoted earlier flowering, and this promoting effect was more pronounced at lower PPFD levels (7 days earlier at higher PPFD and 11 days earlier at lower PPFD). It is noteworthy that B (400–500 nm) photon flux density remained constant in this study, yet the B:R ratio was also previously reported as an important factor influencing photomorphogenesis [54,55]. Owen et al. [56] observed a similar interaction effect of FR and DLI in a greenhouse environment. Petunia exhibited earlier flowering under high DLI (16–19 mol·m−2·d−1) compared to low DLI (6–9 mol·m−2·d−1), and substitutional FR resulted in significantly earlier flowering under low DLI but showed no such effect under high DLI.
The early intervention for plant growth, such as the application of plant growth regulators (PGRs) and the implementation of environmental control strategies, may significantly affect the eventual performance of the flowering plants at their maturation. Given the increased interest in FR radiation, commercial propagators may consider installing FR LED lights alongside existing high-pressure sodium (HPS) lights. However, most previous studies investigated the effect of DLI or FR separately. The limited studies examining the interaction effects of DLI and FR include some conducted in growth chambers, which do not accurately simulate real-world growing conditions, including a matching solar spectrum. Compared to the greenhouse environment where natural sunlight contains approximately 18–19% FR [57], growth chambers with fluorescent lights or W LEDs or R and B LEDs are relatively low in FR (e.g., 2–3% from our own prior studies). Others were conducted in greenhouses where shade clothes were utilized to create DLI treatments [5,18,20,56], leading to a lack of precise DLI control. In contrast, the current study employed an algorithm for fine control of DLI by controlling supplemental lighting and shading decisions and designed a broader range of DLI treatments. By installing commercially available FR light bars into the existing greenhouse lighting system and conducting experiments in both summer and winter, this study aimed to simulate real-life production scenarios and provide applied information on the early-stage response of petunia to both DLI and supplemental FR, along with any potential carry-over effects on the subsequent finish plants within the greenhouse environment. Conducting the experiments in two seasons was crucial to capture the potential effects of seasonal variations in light and temperature on petunia growth, ensuring more robust recommendations for lighting strategies that can be implemented year-round to optimize production timing and plant quality.

2. Materials and Methods

2.1. Plant Material and Propagation Stage

Three petunia cultivars—‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ (Pleasant View Gardens, Loudon, NH, USA)—were used for the experiment. Unrooted petunia cuttings were placed in 72 Enhanced™ Trays (Pleasant View Gardens) filled with LM-1 Germination Mix (Lambert Peat Moss, Riviere-Ouelle, QC, Canada) and placed in a propagation greenhouse. Cuttings were misted with clear water for 6 s per 10 min. At day 7, when cuttings started to show callus or root initials, the misting frequency was reduced to 6 s per 20 min. At day 14, misting was stopped, and cuttings were hand watered once a day with 21 N–2.2 P–16.5 K Jack’s All Purpose Fertilizer (JR Peters Inc., Allentown, PA, USA) at a concentration of 150 mg·L−1 N. The experiment was conducted twice. In Experiment 1 (from 9 April 2021 to 18 June 2021), the propagation greenhouse had a 22.9 ± 0.9 °C (mean ± std dev.) day and night temperature. In Experiment 2 (from 28 October 2021 to 6 January 2022), the propagation greenhouse had a 22.9 ± 0.9 °C day and night temperature. As instructed by the plant material supplier, ethephon (Collate, Fine Agrochemicals, Whittington, UK) was applied as a spray at 500 ppm concentration when roots were about 1/3 down the cell (about 1–2 cm long, at day 14 for Experiments 1 and 2). At day 21, rooted cuttings were transferred to the research greenhouse for DLI/FR treatments. At day 21, plants were also pinched by removing 1 cm of the apical meristem to facilitate lateral growth.

2.2. Radiation Treatments and Liner Stage

The experiment design included four DLI treatments (6, 9, 12, and 15 mol·m−2·d−1) and two FR treatments (with or without an additional 28 μmol·m−2·s−1 FR radiation [700–750 nm] for 16 h a day [equivalent to 1.61 mol·m−2·d−1 light integral]) resulting in eight interactive treatments. The DLI treatments were randomly assigned to four greenhouse sections, and the DLI targets were achieved using the Argus Titan II System (Argus, Surrey, BC, Canada) and the Lighting and Shading System Implementation (LASSI) algorithm [15], which predicts ambient DLI based on light insolation on the first few hours of the day and achieves target DLI using supplemental greenhouse lighting and shading systems. In the summer crop cycle (Experiment 1), the liners under 6 mol·m−2·d−1 were grown under 50% shade cloth to achieve the DLI target. The actual DLIs were 5.7 ± 0.5, 9.3 ± 0.4, 11.9 ± 0.7, and 14.9 ± 0.3 mol·m−2·d−1 (mean ± std dev.), respectively, for the four DLI treatments in Experiment 1, and 6.3 ± 0.1, 8.8 ± 0.5, 12.1 ± 0.5, and 15.0 ± 0.1 mol·m−2·d−1 in Experiment 2. Two benches were set up in each greenhouse section, one of which was used for the FR treatment and the other as the control. A 0.9 m × 0.9 m × 0.9 m (length × width × height) frame built by PVC pipes was placed on the bench to hang two FR light bars (FGI 730 nm Far Red LED Grow Light, Forever Green Indoors, Seattle, WA, USA). The same PVC frame was placed on the control bench. The FR light bars distributed FR within the 0.9 m × 0.9 m area with a mean of 28.14 μmol·m−2·s−1 and a mean relative deviation (MRD) of 0.074. According to the Ciolkosz et al. [58] proposed measurement of light uniformity, the light distribution in this experiment provided good uniformity within the specified area. FR percentage (the light intensity of 700–750 nm radiation divided by that of extended photosynthetically active radiation (ePAR, 400–750 nm radiation)) was measured in six different weather scenarios, as shown in Table 1. The R:FR ratio was calculated using three different methods, Park and Runkle [47], Kusuma and Bugbee [59], and Smith [60], and is shown in Table A1. In the absence of sunlight, HPS had 5.1–6.1% of the total ePAR photons of FR, and the FR light bars had 87.8–92.4% FR. Adding FR light bars to HPS increased the FR percentage of ePAR to 18.2–25.8%. Under representative sunny conditions, ambient light had 12.8–16.6% FR in summer and 10.7 to 12.1% FR in winter. The FR treatments increased these percentages to 14.0–17.4% in summer and 22.6–32.7% in winter, depending on DLI treatments. Closing the shade cloth in summer reduced the total light intensity, amplifying the incremental effect of FR treatments and increasing the FR percentage to 20.7–30.9%. Turning on HPS in winter lowered the FR percentage to 6.5–10.9%. In this condition, the FR percentage under FR treatments was also lowered to 15.8–19.1%. Under representative cloudy conditions, ambient light had 12.6–13.7% FR in summer and 8.2–11.7% FR in winter. The FR treatments increased these percentages to 33.5–45.3% in summer and 49.4–67.8% in winter. Turning on HPS in cloudy days increased the total light intensity but lowered the FR percentage to 7.2–8.4% in summer and 5.5–6.4% in winter. In this condition, the FR treatments increased the FR percentage to 15.7–31.7% in summer and 18.0–25.3% in winter.
Plants were randomly placed within the 0.9 m × 0.9 m area. Plants were fertigated with 21 N–2.2 P–16.5 K Jack’s All Purpose Fertilizer at a concentration of 150 mg·L−1 N. According to the plant supplier’s guidance, the average daily temperature target should be 15.6–17.8 °C, and morning DROP (lowering the temperature below night temperature for a few hours at daybreak) could be used to offset warmer days. In Experiment 1 (the summer crop cycle), morning DROP was applied from 0700 to 1100 HR, and the actual temperature during this period was 18.8 ± 1.9 °C (mean ± std dev.). The day temperature (from 1200 to 2100 HR) was 21.5 ± 1.1 °C, and the night temperature (from 2200 to 0600 HR) was 19.0 ± 1.2 °C. The average temperature throughout the 24-h period was 20.3 ± 2.3 °C. In Experiment 2 (the winter crop cycle), the day temperature (from 0800 to 1900 HR) was 17.4 ± 1.1 °C and the night temperature (from 2000 to 0700 HR) was 14.6 ± 0.7 °C. The average temperature was 16.1 ± 1.6 °C. According to the plant supplier’s guidance, paclobutrazol should be applied 3–4 days after pinching to avoid excessive stem elongation. In Experiment 1, paclobutrazol was not applied because the liner rooting was somewhat delayed and the liners were not fully rooted by this stage. In Experiment 2, paclobutrazol drench (Piccolo, Fine Americas, Walnut Creek, CA, USA) was applied at day 25 (4 days after pinching/light treatment) at, respectively, 5, 10, and 2.5 ppm on ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’. At day 28 (a week after treatments), liners were spaced to 42 cm2/plant. The number of flower buds and open flowers was recorded daily. In Experiment 1, since the cuttings we received were pre-initiated, most of them had flowered by the time of transplanting. In Experiment 2, none of the liners had flowered at the time of transplanting. At day 49 (4 weeks after treatments), ten representative liners from each treatment and each cultivar (a total of 240 liners) were harvested and measurements were taken on the following: plant height (from soil surface to the tip of the main stem), number of main branches (originating from nodes along the main stem; secondary and tertiary branches not counted), number of open flowers (petals separated, exposing the reproductive organs), number of flower buds (including open flowers), and the fresh weight (FW) and DW (following 72 h in an oven at 70 °C) of shoot and root (washed to remove substrate particles and gently dried with paper towels).

2.3. Adult Plant/Finish Stage

The rest of the liners were transplanted to 10 cm diameter × 9 cm tall containers (524 cm3 volume) filled with LM-111 All Purpose Mix (Lambert Peat Moss). Plants were spaced pot to pot in 15-cell trays, and all moved to the same greenhouse section with 15 mol·m−2·d−1 target DLI in order to simulate a commercial finishing growth condition. No additional FR was added at the finish stage. The plants were grown for an additional three weeks. In Experiment 1, plants received 14.9 ± 0.3 mol·m−2·d−1 (mean ± std dev.) actual DLI. In Experiment 2, plants received 15.1 ± 0.0 mol·m−2·d−1 actual DLI. In Experiment 1, morning DROP was applied from 0300 to 0900 HR, and the actual temperature during this period was 19.1 ± 2.5 °C. The day temperature (from 1000 to 2000 HR) was 24.2 ± 2.6 °C, and the night temperature (from 2100 to 0200 HR) was 19.3 ± 1.5 °C. The average temperature throughout the 24-h period was 21.2 ± 3.1 °C. In Experiment 2, the greenhouse had a 17.3 ± 1.3 °C day temperature, and a 14.5 ± 0.4 °C night temperature. The average temperature was 16.8 ± 1.8 °C. According to the plant supplier’s guidance, paclobutrazol should be applied at the aforementioned concentrations if the plant canopy covers the container surface area two weeks after transplanting. In Experiment 1, Piccolo drench was applied at day 63 (two weeks after transplanting). In Experiment 2, Piccolo drench was not applied because the plant canopy had not filled out to cover the entire container surface area. At day 70 (21 days after transplanting), ten representative adult plants from each treatment and each cultivar (a total of 240 adult plants) were harvested, and the same measurements were taken with the addition of plant width (two perpendicular width measurements) and the exclusion of root measurements.

2.4. Experimental Design and Statistical Analysis

Since cuttings were received at two different times in development stages (flowering versus vegetative), the results from Experiment 1 and 2 were not treated as blocks but analyzed separately and compared side by side. Each experiment had a completely randomized design (CRD). Plants were randomly placed in the propagation greenhouse and research greenhouse, and DLI treatments were randomly assigned to the four greenhouse sections in Experiment 1 and 2. The data were analyzed using JMP Pro 17.0.0 (SAS Institute, Cary, NC, USA). In Experiment 1, the data were fit into two models: (1) a multiple linear regression model incorporating DLI, FR, and the interaction term DLI × FR as factors; and (2) a quadratic regression model incorporating DLI, DLI2, FR, and DLI × FR as factors. For each parameter under analysis, the model with better fit was selected, and then analysis of variance (ANOVA) and effect testing were conducted to distinguish the effects of DLI and FR, as well as revealing any interaction effect between DLI and FR. In Experiment 2, technical issues arose in the greenhouse section designed for 15 mol·m−2·d−1, and therefore data from this treatment was not collectable. The remaining data were fit into the multiple linear regression model, followed by ANOVA and effect testing.

3. Results

3.1. Flowering

In both crop cycles, plants exhibited earlier and accelerated flowering under higher DLI, with a more pronounced effect observed in the presence of supplemental FR within each DLI condition (Figure 1). Increasing DLI from 6 to 15 mol·m−2·d−1 and supplementing FR for 4 weeks during the liner production stage accelerated flowering time by 3–5 days and 13–16 days, respectively, in the summer crop cycle (Experiment 1) and winter crop cycle (Experiment 2), However, due to the fact that not all plants had flowered by the time of harvest, the average days to flower were not calculated. Instead, flowering was assessed by evaluating the percentage of plants with open flowers at the time of harvest, as well as the number of flower buds and open flowers on individual plants. In the summer crop cycle (Experiment 1), given the pre-initiated nature of the received cuttings, open flowers were observed across all treatments during the liner stage, but the proportion of liners with open flowers at the time of liner harvest showed a positive trend with higher DLI levels and the addition of FR (Figure 1a–c). For example, in the case of ‘Bermuda Beach’, 88% of liners under 15 mol·m−2·d−1 DLI with the addition of FR had open flowers at the time of liner harvest, while only 5% of the liners under 6 mol·m−2·d−1 DLI without the addition of FR exhibited open flowers. Higher DLI also resulted in an increased number of flower buds and open flowers on individual plants at liner harvest, while additional FR had some promoting effects depending on cultivars (Figure 2 and Figure 3a–f, Table 2). Specifically, ‘Bordeaux’ exhibited a significant increase in the number of open flowers with the addition of FR, while ‘Bermuda Beach’ and ‘Royal Velvet’ showed an increase in the number of flower buds, but not open flowers, with the addition of FR. Nevertheless, when moved to the 15 mol·m−2·d−1 DLI condition during the finish stage, all plants flowered within one or two days after transplanting. The number of flower buds and open flowers on individual finish plants showed a quadratic trend, with the 6 mol·m−2·d−1 and 15 mol·m−2·d−1 treatments exhibiting the highest number of flower buds and open flowers (Figure 4 and Figure 5a–f, Table 3). However, the effect of FR on flowering observed at liner harvest did not carry over to the finish plants under any DLI conditions.
In the winter crop cycle (Experiment 2), open flowers were observed during the finish stage. Consistent with observations from the summer crop cycle, with higher DLI and the addition of FR, a higher proportion of plants exhibited open flowers at the time of finish plant harvest (Figure 1d–f). Higher DLI also had a promoting effect on both the number of open flowers and flower buds on individual finish plants, while FR had little to no effect (Figure 6 and Figure 7a–f, Table 3).

3.2. Branching

In the summer crop cycle (Experiment 1), subtle differences in branching were observed in liners under different DLI conditions (Figure 2 and Figure 3g–I, Table 2). The number of branches of ‘Bermuda Beach’ increased from three to five when DLI was increased from 6 to 15 mol·m−2·d−1, while the number of branches of other cultivars showed no significant difference between DLI treatments. However, the observed difference became more pronounced when harvested after the finish stage (Figure 4 and Figure 5g–I, Table 3). Specifically, plants treated with higher DLIs, especially 15 mol·m−2·d−1, exhibited enhanced branching during the finish stage. FR had no significant effect on branching throughout both the liner and finish stage (Figure 2, Figure 3g–I, Figure 4 and Figure 5g–I, Table 2 and Table 3). In contrast, in the winter crop cycle (Experiment 2), at liner harvest the number of branches increased with DLI, and this trend persisted in the finish plants (Figure 6, Figure 7g–I and Figure 8a–c, Table 2 and Table 3). Similar to the summer crop cycle, FR had no effect on branching in the winter crop cycle.

3.3. Morphology

In the summer crop cycle (Experiment 1), liners under 6 mol·m−2·d−1 exhibited considerable elongation compared to their counterparts exposed to higher DLIs (Figure 2 and Figure 3j–l, Table 2). This elongation effect carried over to the finish plants, rendering them excessively tall and unsuitable for sale (Figure 4 and Figure 5j–l, Table 3). Conversely, plants treated with DLIs ranging from 9 to 15 mol·m−2·d−1 had similar height at the liner stage (Figure 2 and Figure 3j–l, Table 2). However, as the plants treated with 9 to 15 mol·m−2·d−1 progressed to the finish stage, their height increased with higher DLI, yet all remained within acceptable/sellable range (i.e., considerably shorter than those under 6 mol·m−2·d−1) (Figure 4 and Figure 5j–l, Table 3). Additionally, a cultivar-dependent interaction between FR and DLI was observed concerning liner height, since FR increased the liner height of ‘Bermuda Beach’ and ‘Bordeaux’ significantly under 6 mol·m−2·d−1 but exhibited no impact under higher DLI conditions (Figure 2 and Figure 3j–l, Table 2). However, neither the FR effect nor the interaction of FR and DLI was significant in the finish plants (Figure 4 and Figure 5j–l, Table 3). Furthermore, the canopy area of the finish plants showed a quadratic trend (Figure 4 and Figure 5m–o, Table 3). The plant canopy area decreased from 6 to 9 mol·m−2·d−1, followed by an increase with higher DLIs. Notably, plants treated with 15 mol·m−2·d−1 had a canopy area similar to those treated with 6 mol·m−2·d−1. Combining plant height and canopy area data, it is evident that plants achieved a larger size when the DLI increased from 9 to 15 mol·m−2·d−1. Those treated with 6 mol·m−2·d−1 exhibited excessive stretching, making them unsellable.
In the winter crop cycle (Experiment 2), the elongation effect observed in plants exposed to 6 mol·m−2·d−1 during the summer crop cycle was notably absent (Figure 6, Figure 7j–o and Figure 8d–f, Table 2 and Table 3). Instead, most plants, excluding ‘Bordeaux’, exhibited a positive trend in size, including both height and canopy area, as the DLI increased from 6 to 12 mol·m−2·d−1. FR demonstrated an interaction with DLI concerning plant size, leading to a more significant increase in both plant height and canopy area under 6 mol·m−2·d−1 compared to higher DLIs (Figure 6, Figure 7j–o and Figure 8d–f, Table 2 and Table 3). However, the effect of FR and the interaction were also cultivar-dependent.

3.4. Biomass

In the summer crop cycle (Experiment 1), both the shoot FW and DW for both liners and finish plants showed a quadratic trend with DLI, as previously described (Figure 2, Figure 3m–r, Figure 4 and Figure 5p–u, Table 2 and Table 3). FR only had a promoting effect on the shoot DW of certain cultivars (i.e., ‘Bordeaux’ and ‘Royal Velvet’) during the liner stage, but this effect was not present by the end of the finish stage. In the winter crop cycle (Experiment 2), both the shoot FW and DW for both liners and finish plants increased with higher DLI (Figure 6, Figure 7p–u and Figure 8g–l, Table 2 and Table 3). FR increased the shoot FW and DW for the ‘Bordeaux’ cultivar by the end of the finish stage.
In the summer crop cycle (Experiment 1), the impact of DLI on the root FW and DW of liners varied depending on the cultivar (Figure 3s–x, Table 2). Specifically, ‘Bermuda Beach’ and ‘Bordeaux’ generally exhibited increased root FW and DW with increasing DLI, while ‘Royal Velvet’ displayed a quadratic trend, with the greatest root FW and DW observed at 12 mol·m−2·d−1. In contrast, in the winter crop cycle (Experiment 2), the root FW and DW of liners increased linearly with higher DLI (Figure 8m–r, Table 2). FR demonstrated a promoting effect on the root FW and DW of ‘Bordeaux’ in both crop cycles (Figure 3s–x and Figure 8m–r, Table 2).

4. Discussion

High-quality liners exhibit traits such as accelerated flowering, robust branching, and well-developed root systems. The primary finding of this study underscores the positive impact of DLI on the flowering, branching, morphology, and root health of petunia liners, with many impacts extending into the finish stage. In this study, increasing DLI from 6 to 15 mol·m−2·d−1 for 4 weeks during the liner production stage accelerated the flowering time by up to 16 days, comparable to the 10–12-day acceleration in petunia flowering observed by Blanchard et al. [5] when DLI was increased from 4 to 14 mol·m−2·d−1 and the 21-day acceleration reported by Lopez and Runkle [18] when DLI was increased from 1.2 to 10.7 mol·m−2·d−1. It is worth noting that the promoting effect of DLI on flowering may be influenced by other factors, such as the generative status of stock plants, as the acceleration effect observed in the summer crop cycle (3–5 days acceleration) was not as significant as that in winter. In comparison, Faust et al. [20] observed only a 5-day acceleration in petunia flowering even when DLI was increased from 5 to 43 mol·m−2·d−1. DLI is the product of light intensity and photoperiod. Haliapas et al. [61] observed a two-week acceleration in petunia bud formation by increasing light intensity from 40 to 360 μmol·m−2·s−1 while maintaining a constant photoperiod at 16 h per day (equivalent to increasing DLI from 2.3 to 20.7 mol·m−2·d−1). Similar findings in other research indicate that higher light intensity, with a constant photoperiod, promoted flowering in geranium, chrysanthemum (Chrysanthemum × morifolium), marquis wheat (Triticum aestivum), and meadowfoam (Limnanthes alba) [62,63,64,65]. In long day plants, a longer photoperiod has been correlated with earlier flower initiation in some species [11]. In this study, light intensity fluctuated based on natural sunlight, while our lighting control algorithm adjusted supplemental lighting and shading to achieve a DLI target. The daily photoperiod was a minimum of 16 h. While both instantaneous light intensity and photoperiod can contribute to accelerating flowering, DLI, as an integrative measure of the two, holds more practical significance in overall plant growth and development. The results of this study provided quantified information on how DLI could accelerate flowering time. Additionally, with earlier flower initiation, plants under higher DLI developed more flower buds and open flowers at the time of harvest, as observed in both summer and winter crop cycles of this study.
In general, plants treated with higher DLI exhibited improved quality, evidenced by increased branching, size (height and canopy area), and shoot and root biomass, except for the response of plants under 6 mol·m−2·d−1 in the summer crop cycle. Although some research showed that higher DLI resulted in more compact plants, especially in terms of reduced height [5,53], it remains consistent that higher DLI is advantageous for rooting, gross photosynthesis, and biomass accumulation [18,19,20,56]. In this study, the liners under 6 mol·m−2·d−1 in the summer crop cycle exhibited considerable stem elongation, which is a typical shade avoidance response. Light quantity (intensity) and light quality (typically R:FR ratio regulated by the photoreceptor phytochromes) are the central signals to regulate shade avoidance response, where multiple genetic and hormonal pathways are involved [46,66,67,68,69]. Upon transitioning to the common finishing condition of 15 mol·m−2·d−1, the larger-sized liners were capable of capturing more light, expanding the growth gap between 6 mol·m−2·d−1 and other treatments. Practically, a threshold effect was observed in this study, as plants at 6 mol·m−2·d−1 were excessively tall and unmarketable. A minimum of 9 mol·m−2·d−1 was required and higher DLIs were optimal for achieving both earlier flowering and enhanced plant quality. It is worth noting that no such shade avoidance response was observed in the winter crop cycle. A potential explanation for this discrepancy is the differential application of PGR. In the summer crop cycle, paclobutrazol was not applied as directed by the plant supplier’s protocol, as the liners were not fully rooted by this stage. In contrast, during the winter crop cycle, paclobutrazol was applied after pinching as the liners exhibited satisfactory rooting. The interaction between PGR and DLI might contribute to the observed variation, particularly for liners grown under the low light condition (6 mol·m−2·d−1), potentially making them more susceptible to the inhibitory effect of PGR. Soster and Lopez [70] observed that paclobutrazol application on succulents under both high and low DLI suppressed stem elongation, resulting in plants of similar height, despite the excessive elongation caused by low DLI. Similarly, Collado et al. [71] found that petunias were more responsive to the combination of supplemental lighting (i.e., higher DLI) and paclobutrazol compared to other species such as dianthus (Dianthus chinensis) and geranium. While dianthus and geranium remained more compact with paclobutrazol application but exhibited no additional response to supplemental lighting, petunia plants became even more compact when both supplemental lighting and paclobutrazol were applied. In the current study, paclobutrazol might have mitigated the elongation effects associated with low DLI. However, the winter plants reached a reasonable size at the time of harvest, while the summer plants were somewhat overgrown, even with paclobutrazol application at a later time (a week before finish plant harvest). The higher average daily temperature during the summer crop cycle might also contribute to the different growth responses observed. Elevated temperature, combined with lower DLI, are typically associated with increased stem elongation [72,73,74]. Therefore, the warmer conditions during the summer crop cycle likely exacerbated elongation, particularly under low light conditions (6 mol·m−2·d−1), resulting in the observed overgrowth. Future research could delve into the interactions between PGR, temperature, and light conditions, but it is noteworthy that the current study well simulates real-life production scenarios in terms of time schedule and PGR application practices with a background of sunlight (rather than some prior studies in controlled environments with an absence of sunlight). While supplemental lighting is more needed in the winter time when ambient light is lower, this study provides important information regarding plant responses to varying light conditions across different production seasons.
In this study, additional FR resulted in earlier and faster flowering, accompanied by stem elongation, particularly notable in the winter crop cycle. The additional FR decreased the R:FR ratio, which induces the transition of PFR to PR, leading to downstream alterations in gene regulation and hormone levels. This ultimately leads to shade avoidance responses, including early flowering and stem elongation [69,75]. Bachman and McMahon [35] found that the increased cell length of epidermal, cortex, and pith cells contributed to the stem elongation of petunia. Eylands [76] found a similar effect of FR on epidermal, protoxylem, phloem, and parenchyma cells of lettuce plants. While stem elongation due to FR is consistent in various studies, the effect on flowering is mixed, with some observing earlier flowering [50,51,53,56] and others finding no significant impact [38,47,48,49]. Differences in these outcomes could be attributed to the period of FR treatment application. Studies showing no impact on flowering often applied FR at specific stages of the plant life cycle (e.g., only during the seedling stage or only after transplanting), whereas studies reporting earlier flowering typically applied treatments for longer durations. Additionally, variations in DLI backgrounds across these studies could influence the impact of FR on flowering. Acceleration of flowering by FR was more commonly observed under lower DLI conditions. Furthermore, differences in cultivar response to FR radiation may also play a role in these varying results. The tradeoff between earlier flowering and stem elongation is crucial in real-life production. From the results of this study, the addition of FR to the existing lighting system may be advantageous, considering its potential to accelerate flowering without significantly detrimental effects on stem elongation. In fact, as depicted in Figure 4 and Figure 7, FR generally contributed to the production of better-looking finish plants across all DLI conditions. For ornamental plants, registered PGRs could be used to mitigate the negative effects of stem elongation. However, it is important to weigh the capital expenditure associated with installing FR bars on any possible benefits. If the cost of lighting fixtures falls within budget, growers facing challenges in meeting market deadlines may consider a combination of high DLI and supplemental FR as a strategy to accelerate flowering. Besides, although FR led to earlier and faster flowering, it did not necessarily translate into an increase in flower buds and open flowers on the finish plants. There was neither enhanced branching or biomass accumulation for some cultivars. While other research, particularly those conducted in growth chambers, found significant effects of FR on various growth aspects, the dynamic greenhouse lighting environment, and the constant fluctuation of the R:FR ratio, may influence these effects. Moreover, natural sunlight already contains approximately 18–19% FR [57], potentially diminishing the impact of FR supplementation, especially during sunny periods. In general, DLI acted as a more effective tool than FR in improving flowering time, flower number, and overall plant quality.
An interaction effect of FR and DLI on the plant height and canopy area was observed for some cultivars. The effect of FR on plant height and canopy area was more pronounced at lower DLI levels compared to higher DLI conditions. This could logistically be explained by the lower absolute amount of both R and FR under low DLI conditions, resulting in a more substantial decrease in the R:FR ratio with the addition of FR. Interestingly, Park and Runkle [53] found an interactive effect of FR and PPFD on the flowering time of petunia, but FR independently led to increased stem elongation. Additionally, the lighting treatments exhibited interaction with cultivars. For example, while FR had little to no effect on biomass accumulation for ‘Bermuda Beach’ and ‘Royal Velvet’, FR, independent from DLI, increased the shoot and root weight of ‘Bordeaux’. Therefore, the decisions regarding lighting strategies need to be tailored and specific to each cultivar, considering their unique responses to FR, DLI, and their interactions.

5. Conclusions

This study holds significant practical implications for floriculture producers by elucidating the impact of DLI and supplemental FR radiation on petunia liners and the subsequent development of finish plants within a controlled greenhouse environment. A critical threshold DLI during the liner stage of 9 mol·m−2·d−1 was identified for ensuring marketable petunia finish plants. Higher DLIs were also optimal for enhancing various aspects of plant development including flowering, branching, root health, and plant architecture. Supplemental FR can be a valuable tool for accelerating flowering, especially in the winter time. However, FR had limited impact on other parameters including the number of flower buds and open flowers, branching, and shoot and root biomass. This may be attributed to the presence of abundant FR in natural sunlight and the fluctuating light conditions within the greenhouse environment. The tradeoff between accelerated flowering time and increased stem elongation observed in this study fell within an acceptable range. Furthermore, interactions between DLI and FR were observed in terms of plant height and canopy area for some cultivars. Future research could be conducted with higher levels of FR targeting specific R:FR ratios as well as the interaction of FR and PGR application strategies. Overall, both higher DLI and supplemental FR were found to play beneficial roles in the development of petunia liners and subsequent finish plants, but DLI had a more pronounced effect than FR. Lastly, it is important to consider other factors such as PGR and specific plant cultivars when making decisions on lighting strategies.

Author Contributions

Conceptualization, J.X. and N.M.; methodology, J.X. and N.M.; software, J.X.; formal analysis, J.X.; investigation, J.X.; resources, N.M.; data curation, J.X.; writing—original draft preparation, J.X.; writing—review and editing, J.X. and N.M.; visualization, J.X.; supervision, N.M.; project administration, N.M.; funding acquisition, N.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Lighting Approaches to Maximize Profit (LAMP) Project received from the Specialty Crop Research Initiative (SCRI), National Institutes of Food and Agriculture (NIFA), U.S. Department of Agriculture (USDA) (award 2018-51181-28365), and the Floriculture and Nursery Research Initiative Project received from the Agricultural Research Service (ARS), USDA (project number 0500-00059-001-000-D). Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the USDA.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We wish to express our sincere gratitude to Nicholas Kaczmar for his valuable technical support, as well as members of Mattson lab and greenhouse staff at Guterman Bioclimatic Research Lab at Cornell University for contributions to plant care and maintenance. We would also like to thank Pleasant View Gardens for the supply of unrooted plant material and technical guidance on growth protocols.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Red to far-red ratios (R:FR ratios) calculated from snapshot spectroradiometer measurements in six different scenarios. The R:FR ratio is calculated by dividing the light intensity of red (R) radiation by that of far-red (FR) radiation. The three numbers indicate different wavelength ranges for R and FR: R (600–700 nm) and FR (700–800 nm) following the method of Park and Runkle [47]; R (600–700 nm) and FR (720–740 nm) following the method of Kusuma and Bugbee [59]; and R (655–665 nm) and FR (725–735 nm) following the method of Smith [60]. “−Shade” denotes the shade curtain uncovered/open. “+Shade” denotes the shade curtain covered/closed. “−HPS” denotes the HPS lights off. “+HPS” denotes the HPS lights on. For the treatments, the minus sign indicates treatments without additional FR radiation, and the plus sign indicates treatments with additional FR radiation. The numbers indicate the four daily light integral (DLI, mol·m−2·d−1) treatments in adjacent greenhouses where the measurements were taken in Experiments 1 and 2.
Table A1. Red to far-red ratios (R:FR ratios) calculated from snapshot spectroradiometer measurements in six different scenarios. The R:FR ratio is calculated by dividing the light intensity of red (R) radiation by that of far-red (FR) radiation. The three numbers indicate different wavelength ranges for R and FR: R (600–700 nm) and FR (700–800 nm) following the method of Park and Runkle [47]; R (600–700 nm) and FR (720–740 nm) following the method of Kusuma and Bugbee [59]; and R (655–665 nm) and FR (725–735 nm) following the method of Smith [60]. “−Shade” denotes the shade curtain uncovered/open. “+Shade” denotes the shade curtain covered/closed. “−HPS” denotes the HPS lights off. “+HPS” denotes the HPS lights on. For the treatments, the minus sign indicates treatments without additional FR radiation, and the plus sign indicates treatments with additional FR radiation. The numbers indicate the four daily light integral (DLI, mol·m−2·d−1) treatments in adjacent greenhouses where the measurements were taken in Experiments 1 and 2.
Weather ScenarioTreatments
6−6+9−9+12−12+15−15+
Experiment 1
Sunny/−Shade/−HPS1.0/0.9/1.01.1/0.8/0.81.3/1.3/1.31.2/1.1/1.11.3/1.3/1.21.2/1.0/1.01.3/1.3/1.31.2/1.1/1.0
Sunny/+Shade/−HPS1.2/1.2/1.20.7/0.4/0.31.2/1.2/1.20.9/0.6/0.61.2/1.2/1.20.9/0.6/0.51.2/1.2/1.20.9/0.6/0.6
Cloudy/−HPS1.1/1.3/1.30.4/0.1/0.11.1/1.2/1.20.5/0.2/0.21.1/1.2/1.20.5/0.3/0.21.1/1.1/1.10.5/0.3/0.3
Cloudy/+HPS3.2/2.9/2.41.0/0.3/0.32.9/2.6/2.21.5/0.7/0.52.4/2.2/1.91.6/0.9/0.72.6/2.3/2.01.5/0.9/0.7
Night/−HPSN/A0.1/0.0/0.0N/A0.1/0.0/0.0N/A0.1/0.0/0.0N/A0.1/0.0/0.0
Night/+HPS4.6/3.7/2.90.9/0.3/0.25.1/4.1/3.21.6/0.6/0.45.0/3.9/3.11.9/0.7/0.54.5/3.3/2.61.8/0.8/0.6
Experiment 2
Sunny/−Shade/−HPS1.5/1.3/1.30.6/0.2/0.21.6/1.4/1.50.8/0.4/0.41.4/1.2/1.20.9/0.5/0.51.5/1.4/1.40.9/0.5/0.5
Sunny/+Shade/−HPS2.4/2.0/1.71.3/0.7/0.62.5/2.1/1.91.5/0.7/0.61.8/1.6/1.51.3/0.8/0.83.5/2.9/2.41.5/0.7/0.6
Cloudy/−HPS1.8/1.3/1.10.2/0.0/0.02.6/2.1/2.70.3/0.1/0.11.7/1.4/1.30.4/0.1/0.12.6/2.3/2.00.4/0.1/0.1
Cloudy/+HPS4.2/3.4/2.71.3/0.5/0.44.5/3.6/3.11.7/0.7/0.53.9/3.3/2.61.8/0.8/0.64.4/3.6/2.81.5/0.6/0.4
Night/−HPSN/A0.1/0.0/0.0N/A0.1/0.0/0.0N/A0.1/0.0/0.0N/A0.1/0.0/0.0
Night/+HPS4.6/3.7/2.90.9/0.3/0.25.1/4.1/3.21.6/0.6/0.45.0/3.9/3.11.9/0.7/0.54.5/3.3/2.61.8/0.8/0.6
N/A indicates the measure is not applicable.

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Horticulturae 10 01106 g001
Figure 1. Effect of daily light integral (DLI) and far-red (FR) on the percentage of petunia plants with open flowers over time. (a–c) are the results of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, from the liner stage in the summer crop cycle (Experiment 1). (d–f) are the results of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, from the finish stage in the winter crop cycle (Experiment 2). Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues. Numbers 6, 9, 12, and 15 in the legends denote the DLI levels. The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day. The minus signs denote treatments without additional FR radiation.
Figure 1. Effect of daily light integral (DLI) and far-red (FR) on the percentage of petunia plants with open flowers over time. (a–c) are the results of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, from the liner stage in the summer crop cycle (Experiment 1). (d–f) are the results of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, from the finish stage in the winter crop cycle (Experiment 2). Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues. Numbers 6, 9, 12, and 15 in the legends denote the DLI levels. The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day. The minus signs denote treatments without additional FR radiation.
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Horticulturae 10 01106 g002
Figure 2. Liners of the petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the summer crop cycle (Experiment 1). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day. The minus signs denote treatments without additional FR radiation. Photos were taken on day 49 after sticking cuttings.
Figure 2. Liners of the petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the summer crop cycle (Experiment 1). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day. The minus signs denote treatments without additional FR radiation. Photos were taken on day 49 after sticking cuttings.
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Horticulturae 10 01106 g003aHorticulturae 10 01106 g003b
Figure 3. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), shoot fresh weight (m–o), shoot dry weight (p–r), root fresh weight (s–u), and root dry weight (v–x) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in the summer crop cycle (Experiment 1). (a,d,g,j,m,p,s,v) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t,w) are the results of ‘Bordeaux’. (c,f,i,l,o,r,u,x) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Each graph shows either a linear or quadratic model, based on the higher degree of fit.
Figure 3. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), shoot fresh weight (m–o), shoot dry weight (p–r), root fresh weight (s–u), and root dry weight (v–x) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in the summer crop cycle (Experiment 1). (a,d,g,j,m,p,s,v) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t,w) are the results of ‘Bordeaux’. (c,f,i,l,o,r,u,x) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Each graph shows either a linear or quadratic model, based on the higher degree of fit.
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Figure 4. Finish plants of petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the summer crop cycle (Experiment 1). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day during the liner stage. The minus signs denote treatments without additional FR radiation. Photos were taken on day 70 after sticking cuttings.
Figure 4. Finish plants of petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the summer crop cycle (Experiment 1). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day during the liner stage. The minus signs denote treatments without additional FR radiation. Photos were taken on day 70 after sticking cuttings.
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Horticulturae 10 01106 g005aHorticulturae 10 01106 g005b
Figure 5. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), canopy area (m–o), shoot fresh weight (p–r), and shoot dry weight (s–u) of ‘Bermuda Beach’, Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in the summer crop cycle (Experiment 1). (a,d,g,j,m,p,s) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t) are the results of ‘Bordeaux’. (c,f,I,l,o,r,u) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Each graph shows either a linear or quadratic model, based on the higher degree of fit.
Figure 5. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), canopy area (m–o), shoot fresh weight (p–r), and shoot dry weight (s–u) of ‘Bermuda Beach’, Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in the summer crop cycle (Experiment 1). (a,d,g,j,m,p,s) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t) are the results of ‘Bordeaux’. (c,f,I,l,o,r,u) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, 12, and 15 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Each graph shows either a linear or quadratic model, based on the higher degree of fit.
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Horticulturae 10 01106 g006
Figure 6. Finish plants of petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the winter crop cycle (Experiment 2). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day during the liner stage. The minus signs denote treatments without additional FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues. Photos were taken on day 70 after sticking cuttings.
Figure 6. Finish plants of petunia cultivars ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’ grown under 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) without (−) and with (+) supplemental far-red (FR) radiation in the winter crop cycle (Experiment 2). The plus signs denote treatments with an additional 28 μmol·m−2·s−1 FR radiation for 16 h a day during the liner stage. The minus signs denote treatments without additional FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues. Photos were taken on day 70 after sticking cuttings.
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Horticulturae 10 01106 g007aHorticulturae 10 01106 g007b
Figure 7. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), canopy area (m–o), shoot fresh weight (p–r), and shoot dry weight (s–u) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in the winter crop cycle (Experiment 2). (a,d,g,j,m,p,s) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t) are the results of ‘Bordeaux’. (c,f,I,l,o,r,u) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues.
Figure 7. Number of open flowers (a–c), number of flower buds (d–f), number of branches (g–i), plant height (j–l), canopy area (m–o), shoot fresh weight (p–r), and shoot dry weight (s–u) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in the winter crop cycle (Experiment 2). (a,d,g,j,m,p,s) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q,t) are the results of ‘Bordeaux’. (c,f,I,l,o,r,u) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues.
Horticulturae 10 01106 g007aHorticulturae 10 01106 g007b
Horticulturae 10 01106 g008aHorticulturae 10 01106 g008b
Figure 8. Number of branches (a–c), plant height (d–f), shoot fresh weight (g–i), shoot dry weight (j–l), root fresh weight (m–o), and root dry weight (p–r) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in the winter crop cycle (Experiment 2). (a,d,g,j,m,p) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q) are the results of ‘Bordeaux’. (c,f,i,l,o,r) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues.
Figure 8. Number of branches (a–c), plant height (d–f), shoot fresh weight (g–i), shoot dry weight (j–l), root fresh weight (m–o), and root dry weight (p–r) of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in the winter crop cycle (Experiment 2). (a,d,g,j,m,p) are the results of ‘Bermuda Beach’. (b,e,h,k,n,q) are the results of ‘Bordeaux’. (c,f,i,l,o,r) are the results of ‘Royal Velvet’. The x-axis represents 6, 9, and 12 mol·m−2·d−1 daily light integral (DLI) treatments. The blue lines represent treatments without supplemental far-red (FR) radiation. The red lines represent treatments with supplemental FR radiation. Data from the 15 mol·m−2·d−1 DLI condition in the winter crop cycle were not collectable due to greenhouse technical issues.
Horticulturae 10 01106 g008aHorticulturae 10 01106 g008b
Table 1. Far-red (FR) percentages calculated from snapshot spectroradiometer measurements in six different scenarios. The FR percentage is calculated by dividing the light intensity of FR (700–750 nm) radiation by that of extended photosynthetically active radiation (ePAR, 400–750 nm radiation). “−Shade” denotes the shade curtain uncovered/open. “+Shade” denotes the shade curtain covered/closed. “−HPS” denotes the HPS lights off. “+HPS” denotes the HPS lights on. Measurements taken during cloudy or night conditions did not use shade (i.e., −Shade). For the treatments, the minus sign indicates treatments without additional FR radiation, and the plus sign indicates treatments with additional FR radiation. The numbers indicate the four daily light integral (DLI, mol·m−2·d−1) treatments in adjacent greenhouses where the measurements were taken in Experiment 1 and Experiment 2.
Table 1. Far-red (FR) percentages calculated from snapshot spectroradiometer measurements in six different scenarios. The FR percentage is calculated by dividing the light intensity of FR (700–750 nm) radiation by that of extended photosynthetically active radiation (ePAR, 400–750 nm radiation). “−Shade” denotes the shade curtain uncovered/open. “+Shade” denotes the shade curtain covered/closed. “−HPS” denotes the HPS lights off. “+HPS” denotes the HPS lights on. Measurements taken during cloudy or night conditions did not use shade (i.e., −Shade). For the treatments, the minus sign indicates treatments without additional FR radiation, and the plus sign indicates treatments with additional FR radiation. The numbers indicate the four daily light integral (DLI, mol·m−2·d−1) treatments in adjacent greenhouses where the measurements were taken in Experiment 1 and Experiment 2.
Weather ScenarioTreatments
6−6+9−9+12−12+15−15+
Experiment 1
Sunny/−Shade/−HPS16.6%17.4%12.8%14.0%13.4%15.1%13.2%15.0%
Sunny/+Shade/−HPS14.2%30.9%13.8%20.7%13.8%21.9%14.0%20.7%
Cloudy/−HPS13.0%45.3%13.6%38.5%12.6%33.5%13.7%34.0%
Cloudy/+HPS7.2%31.7%7.6%19.5%8.3%15.7%8.4%16.1%
Night/−HPS0.0%87.8%0.0%89.7%0.0%88.0%0.0%92.4%
Night/+HPS5.8%25.8%5.1%21.2%5.1%18.6%6.1%18.2%
Experiment 2
Sunny/−HPS10.7%32.7%12.0%28.3%12.1%23.0%12.1%22.6%
Sunny/+HPS9.0%19.1%9.0%18.5%10.9%15.8%6.5%17.9%
Cloudy/−HPS11.7%67.8%10.3%58.4%10.9%49.4%8.2%55.0%
Cloudy/+HPS6.0%25.3%5.7%19.7%6.4%18.0%5.5%21.8%
Night/−HPS0.0%87.8%0.0%89.7%0.0%88.0%0.0%92.4%
Night/+HPS5.8%25.8%5.1%21.2%5.1%18.6%6.1%18.2%
Table 2. Analysis of variance (ANOVA) and effect tests of the daily light integral (DLI), DLI2, far-red (FR), and the interaction between DLI and FR (DLI × FR) on the number of open flowers, number of flower buds, number of branches, plant height, shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in both summer (Experiment 1) and winter crop (Experiment 2) cycles.
Table 2. Analysis of variance (ANOVA) and effect tests of the daily light integral (DLI), DLI2, far-red (FR), and the interaction between DLI and FR (DLI × FR) on the number of open flowers, number of flower buds, number of branches, plant height, shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at liner harvest in both summer (Experiment 1) and winter crop (Experiment 2) cycles.
Summer Crop Cycle (Experiment 1)Winter Crop Cycle (Experiment 2)
Bermuda BeachBordeauxRoyal VelvetBermuda BeachBordeauxRoyal Velvet
Number of Open Flowers
ModelLinearLinearLinearLinearLinearLinear
DLI*********N/AN/AN/A
DLI2N/AN/AN/AN/AN/AN/A
FRNS*NSN/AN/AN/A
DLI × FRNSNS*N/AN/AN/A
Number of Flower Buds
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI********N/AN/AN/A
DLI2*******N/AN/AN/A
FR**NS***N/AN/AN/A
DLI × FRNSNS*N/AN/AN/A
Number of Branches
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI***NSNS*********
DLI2********N/AN/AN/A
FRNSNS*NSNSNS
DLI × FRNS*NS******NS
Plant Height
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI************NS***
DLI2*********N/AN/AN/A
FR***NSNS***NSNS
DLI × FR*****NS*****NS
Shoot Fresh Weight
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI******************
DLI2*********N/AN/AN/A
FRNSNSNSNSNSNS
DLI × FRNSNSNSNS***NS
Shoot Dry Weight
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI******************
DLI2*********N/AN/AN/A
FRNS******NSNSNS
DLI × FRNSNSNSNS***NS
Root Fresh Weight
ModelLinearLinearQuadraticLinearLinearLinear
DLI**************
DLI2N/AN/A***N/AN/AN/A
FR*NSNSNS**NS
DLI × FRNSNSNSNSNSNS
Root Dry Weight
ModelLinearLinearQuadraticLinearLinearLinear
DLI****************
DLI2N/AN/A***N/AN/AN/A
FRNS***NSNS***NS
DLI × FRNSNSNSNSNSNS
NS, *, **, or *** denote non-significance or significance at p ≤ 0.05, 0.01, or 0.001, respectively. N/A indicates the measure is not applicable.
Table 3. Analysis of variance (ANOVA) and effect tests of the daily light integral (DLI), DLI2, far-red (FR), and the interaction between DLI and FR (DLI × FR) on the number of open flowers, number of flower buds, number of branches, plant height, canopy area, shoot fresh weight, and shoot dry weight of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in both summer (Experiment 1) and winter (Experiment 2) crop cycles.
Table 3. Analysis of variance (ANOVA) and effect tests of the daily light integral (DLI), DLI2, far-red (FR), and the interaction between DLI and FR (DLI × FR) on the number of open flowers, number of flower buds, number of branches, plant height, canopy area, shoot fresh weight, and shoot dry weight of ‘Bermuda Beach’, ‘Bordeaux’, and ‘Royal Velvet’, respectively, at finish plant harvest in both summer (Experiment 1) and winter (Experiment 2) crop cycles.
Summer Crop Cycle (Experiment 1)Winter Crop Cycle (Experiment 2)
Bermuda BeachBordeauxRoyal VelvetBermuda BeachBordeauxRoyal Velvet
Number of Open Flowers
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLINS*************
DLI2*********N/AN/AN/A
FRNSNSNSNS*NS
DLI × FRNSNSNSNSNS*
Number of Flower Buds
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLINS**************
DLI2*********N/AN/AN/A
FRNSNSNS*NSNS
DLI × FRNSNSNSNSNS**
Number of Branches
ModelLinearLinearQuadraticLinearLinearLinear
DLI****************
DLI2N/AN/A***N/AN/AN/A
FRNSNSNSNSNS*
DLI × FRNSNSNSNSNSNS
Plant Height
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLI************NS***
DLI2*********N/AN/AN/A
FRNSNSNS*********
DLI × FRNSNSNSNS****
Canopy Area
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLINSNSNS***NS***
DLI2*********N/AN/AN/A
FR*NSNSNS***NS
DLI × FRNSNSNSNS*****
Shoot Fresh Weight
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLINSNS*********
DLI2*********N/AN/AN/A
FRNSNS*NS***NS
DLI × FRNSNSNSNSNSNS
Shoot Dry Weight
ModelQuadraticQuadraticQuadraticLinearLinearLinear
DLINSNS*********
DLI2*********N/AN/AN/A
FRNSNSNSNS**NS
DLI × FRNSNSNSNS**
NS, *, **, or *** denote non-significance or significance at p ≤ 0.05, 0.01, or 0.001, respectively. N/A indicates the measure is not applicable.
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Xia, J.; Mattson, N. Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses. Horticulturae 2024, 10, 1106. https://doi.org/10.3390/horticulturae10101106

AMA Style

Xia J, Mattson N. Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses. Horticulturae. 2024; 10(10):1106. https://doi.org/10.3390/horticulturae10101106

Chicago/Turabian Style

Xia, Jiaqi, and Neil Mattson. 2024. "Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses" Horticulturae 10, no. 10: 1106. https://doi.org/10.3390/horticulturae10101106

APA Style

Xia, J., & Mattson, N. (2024). Daily Light Integral and Far-Red Radiation Influence Morphology and Quality of Liners and Subsequent Flowering and Development of Petunia in Controlled Greenhouses. Horticulturae, 10(10), 1106. https://doi.org/10.3390/horticulturae10101106

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