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Article

Exogenous Sucrose Enhances Growth and Physiological Performance of Tomato Seedlings Under Suboptimal Light Conditions in Passive Greenhouses

by
Miguel Gómez-Cabezas
* and
Ángelo España
Grupo de Investigación en Agrobiodiversidad, Seguridad y Soberanía Alimentaria—GIASSA, Facultad de Ingeniería en Ciencias Agropecuarias y Ambientales, Universidad Técnica del Norte (UTN), Av. 17 de Julio 5-21 y Gral. José María Córdova, Ibarra 100150, Ecuador
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(12), 1337; https://doi.org/10.3390/horticulturae10121337
Submission received: 5 November 2024 / Revised: 20 November 2024 / Accepted: 20 November 2024 / Published: 13 December 2024
(This article belongs to the Special Issue Effects of Biostimulants on the Growth and Development of Horticultural Crops)
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Abstract

:
Tomato is an important crop worldwide. Commonly, the production process is initiated in nurseries that provide seedlings to greenhouse growers. Many factors influence crop production, one of which is the seedlings’ quality. Light has an enormous effect on seedlings; however, in passive greenhouses, its control is quite difficult. In this situation, plants are usually affected by low or high light intensities which induces poor growth on plants. On the other hand, there is some evidence that sucrose applications could compensate for the adverse effects caused by low light intensities and other abiotic factors like salinity, drought, and temperature. In this way, this research aimed to assess the impact of exogenous sucrose on the morphology, quality, and growth of tomato seedlings cultivated under low-tech greenhouse conditions commonly observed in tropical and subtropical commercial nurseries. Four sucrose treatments were proposed (0, 1, 10, and 100 mM). On days 28, 32, 36, 40, and 44 after sowing, several morphological, physiological and growth measurements were evaluated. Sucrose-treated plants displayed higher leaf areas and chlorophyll contents, facilitating light absorption. Therefore, the relative growth rate (RGR) was enhanced and better explained by a higher net assimilation rate (NAR). Consequently, a higher dry matter accumulation and Dixon quality index (DQI) were achieved. Plants under treatment at 100 mM exhibited the best performance.

1. Introduction

Tomato is an important crop worldwide. In 2022, the world production was 186 million tons in approximately 5 million hectares [1]. Commonly, the production process is initiated in nurseries that provide seedlings to greenhouse growers. Although many biotic or abiotic factors influence crop production directly, starting a production cycle with seedlings of good quality increases the possibilities of having successful yields [2].
Light intensity plays an important role in the morphological and physiological characteristics that determine the quality of seedlings [3,4,5,6]. Low light intensities alter plant growth and productivity negatively due to lower photosynthetic rates, while excessive light intensities could have similar effects by impairing the photosynthetic apparatus.
Zheng et al. [3] found that tomato seedlings grown at different light intensities (60, 150, 240, and 330 μmol m−2 s−1), provided by light-emitting diodes (LED) with a fixed red–blue ratio of 7:3, and an extra treatment provided by a full spectrum of sunlight, differed in many morphological and physiological characteristics, which altered Dixon’s quality index. Light intensities below or higher than 240 μmol m−2 s−1 harmed chlorophyll content, dry matter accumulation, stem diameter, and the quality index. Net photosynthetic rate had the same trend; however, plants under sunlight showed similar values as observed at 240 μmol m−2 s−1.
Similar results were obtained by [4], who applied different light intensities (50, 150, 300, 450, and 550 μmol m−2 s−1), given by LED lights, with a fixed red–blue ratio of 1:1, to cherry tomato seedlings. Their results revealed that light intensities below or higher than 300 μmol m−2 s−1 reduce the photochemical efficiency of PSII (Fv/Fm), fresh and dry matter, stem diameter, and quality index. Furthermore, the chlorophyll content showed a similar trend, with an exception observed at 50 μmol m−2 s−1, having similar values to the registered ones at 300 μmol m−2 s−1.
Fan et al. [5] exposed tomato seedlings to different light treatments (50, 150, 200, 300, 450, and 550 μmol m−2 s−1) given by LED lights, with a fixed red–blue ratio of 1:1. One of the outcomes of this research was that light treatments, in the range from 50 to 200 μmol m−2 s−1, reduced the accumulation of dry matter and increased plant height, causing a reduced health index. Alternatively, higher light intensities (300 to 550 μmol m−2 s−1) promoted more accumulation of dry matter, with the treatment of 550 μmol m−2 s−1 being the one with the greatest average for this variable. In addition, regarding the quality index, even though seedlings grown under the latter range of light intensities had similar performance, it was found that the net photosynthetic rate was the best at 300 μmol m−2 s−1, indicating possible damage in the photosynthetic apparatus of seedlings grown under 450 and 550 μmol m−2 s−1.
As consequence of the previous literature, it can be understood that the most suitable light intensity to produce high-quality tomato seedlings is approximately 300 μmol m−2 s−1. In low-tech greenhouses, shading screens are used to accomplish this requirement when sunlight intensities are extremely high, as commonly observed in tropical and subtropical regions or during the summer in temperate regions. Nevertheless, when sunlight intensities are lower, shading screens could reduce the amount of light to levels below the optimal or lead to a reduced photoperiod, as mentioned by Fan et al. [5], affecting the quality of the seedlings. Moreover, to save space, tomato seeds are usually sown at high densities in different kinds of trays, which induces spindly growth as a consequence of undesirable stem elongation [2,7], negatively affecting the quality of the seedlings.
Alternatively, sucrose is an important product of photosynthesis that provides plants with energy and structural material [8,9,10,11]. Furthermore, this molecule is involved in metabolic processes that regulate growth, development, and plant responses to abiotic factors [8,10,12,13,14,15]. According to Huang et al. [10], it is widely known that exogenous applications of sucrose can compensate for the negative effects caused by non-optimum environmental conditions; however, there is not much research concerning its effects on plants under low-light conditions. For instance, sucrose feeding reversed shade-induced kernel losses in maize plants, meaning that sucrose counteracted the inhibition of photosynthesis caused by shading [16]. Gao et al. [17] found that sucrose sprays increased the yields of soapberry feedstock forests, which are usually grown at high canopy densities and under low-light conditions. Similarly, Berrie [18] observed that tomato seedlings grown under different light intensities and photoperiods had a higher dry weight when treated with sucrose sprays.
As a result of this literature, it is possible to state that exogenous sucrose could help plants overcome physiological impairments caused by low light intensities. However, as far as we know, there is still missing information related to the influence of exogenous sucrose over certain morphological and physiological parameters that affect the growth and the quality of tomato seedlings. Therefore, the current study aims to assess the effect of exogenous sucrose on the morphology, quality, and growth of tomato seedlings cultivated under low-tech greenhouse conditions commonly observed in tropical and subtropical commercial nurseries.

2. Materials and Methods

2.1. Growth Conditions and Plant Material

This experiment was established in the greenhouse facilities of the Universidad Técnica del Norte, located in the province of Imbabura, Ecuador, at an altitude of 2340 m.a.s.l. (0°21′56″ N; 78°12′22″ O). The greenhouse had a plastic cover with a transmittance of approximately 60%. Under it, a 20% transmittance shading screen was installed. The photosynthetic active radiation and the temperatures registered during the experiment are detailed in Figure 1. No supplemental light or technology to control temperature was used.
Tomato seeds (Solanum lycopersicum) cv. “Zodiac” were sown in a mixture of peat and sterilized river sand (weight ratio: 1 to 0.75). Plug trays of 200 cells (length: 54 cm, width: 28 cm) were used. The volume of each cell was 14 cm3 and contained one seed per cell. Tap water was used to irrigate the trays during the first 14 days. Over the next 10 days, the seedlings were fertigated with a halved concentration of the following nutrient solution: 65 ppm N-NO3−, 5 ppm N-NH4+, 50 ppm P, 120 ppm K, 150 ppm Ca, 40 ppm Mg, 50 ppm S, 0.7 ppm B, 0.2 ppm Cu, 2.8 ppm Fe, 0.8 ppm Mn, and 0.05 ppm Mo, with a pH of 6 ± 0.5 and an electrical conductivity (EC) of 0.7 dS m−1 [19]. After this period, the full concentration solution was used to fertigate the seedlings. A leaching fraction of ~25% was used to avoid root-zone salinization.

2.2. Treatments and Experimental Design

The treatments consisted of four sucrose solutions: 0 mM (control), 1 mM, 10 mM, and 100 mM. They were applied on days 22, 29, 35, 39, and 43 after sowing. The solutions were sprayed on the seedlings until the surface of the leaves was completely moistened. Sucrose solutions were prepared on each one of the mentioned days. The sucrose used in this experiment was 99% pure. Tap water was used as the solvent to elaborate the sucrose solutions.
The treatments were arranged in a randomized complete block design with three blocks, having 24 plug trays in total (2 per treatment in each block). Each plug tray was divided into four equal parts to form 4 experimental units with 50 plants in each one. Excluding the border effect, each experimental unit was composed of 24 seedlings, of which 16 were randomly selected for the evaluations. Each treatment had 8 experimental units per block; nevertheless, only 5 of them were considered in the experiment to analyze growth parameters over time and have a similar seedling production cycle to the commercial nurseries.

2.3. Morphological, Physiological, and Growth Measurements

Measurements of dry matter (DM) of leaves, roots, and stems (mg) were realized on days 28, 32, 36, 40, and 44 after sowing. Measurements of leaf area (cm2) were performed in the same days, with an exception for day 28. On day 44, after sowing, the following variables were measured: stem diameter (mm), lengths of hypocotyls and roots (cm), plant height (cm), and chlorophyll concentration (μmol m−2). As suggested by [11,20], the Dixon quality index (DQI) was calculated as (plant DM; g)/[(plant height; cm/stem diameter; mm) + (shoot-to-root ratio)]. Dry matter partitioning was also evaluated.
Stem diameter was measured with an electronic Vernier caliper at the surface of the substrate. The plant height and the lengths of the roots and hypocotyls were measured with a ruler. Chlorophyll concentration was measured with an optical chlorophyll meter (model MC-100, Apogee Instruments, Logan, UT, USA). To obtain dry matter measurements, each seedling was dissected into leaves plus petioles, stems, and roots. Then, these organs were oven-dried separately at 105 °C to a constant weight. Leaf area was determined using a leaf area meter (model LI-3000; LI-COR, Lincoln, NE, USA). Specific leaf area (SLA; m2 g−1), a measure of leaf thickness, was calculated as (A/WL), where A is the plant leaf area (m2) and WL is the dry weight of the leaves (g).
Relative growth rate (RGR; g g−1 d−1) describes the efficiency of growth with respect to an initial biomass and was calculated as (lnWt2 − lnWt1)/(t2 − t1), where W is the plant total dry weight and t is the harvest time. Additionally, the components of RGR were evaluated; for instance, the net assimilation rate (NAR; g m−2 d−1) and leaf area ratio (LAR; m2 g−1). The first factor indicates the efficiency of leaves in generating biomass and was calculated as (Wt2 − Wt1)/(t2 − t1)] × [(lnAt2 − lnAt1)/(At2 − At1)]. The second factor represents the amount of photosynthetic area relative to respiratory biomass and was calculated as [(At1/Wt1) + (At2/Wt2)] × (1/2) [21,22]. To analyze fluctuations in RGR over successive time periods, day 28 after sowing was initially selected as the starting point. However, due to technical issues, the leaf area on day 28 was not measured. Consequently, day 32 was used as the starting point for calculating NAR and LAR. Day 32 also served as the baseline for assessing the effects of increasing sucrose concentrations on RGR and its components throughout the entire growth period.

2.4. Statistical Analysis

Once the data were checked for normality assumption, an analysis of variance (ANOVA) using the general linear model module of the InfoStat Software (Version 2020) (InfoStat Group, Agricultural Sciences College, National University of Cordoba, Cordoba, Argentina) was performed. When differences were observed, a comparison of means with Fisher’s least significant difference test (α = 0.05) was conducted.

3. Results

3.1. Morphological and Physiological Parameters

Exogenous sucrose affected almost all morphological parameters of tomato seedlings (Table 1). Plants under sucrose treatments had higher leaf area, plant, leaf, stem, and root DM than those under treatment at 0 mM (control), indicating that sucrose can potentially increase leaf area and, consequently, dry matter accumulation. However, it can be observed that the behavior of these parameters does not follow a clear pattern with an increasing dose of sucrose. Among sucrose treatments, the highest plant DM was registered for the 100 mM treatment, followed by the 10 mM treatment. The 1 mM treatment behaved similarly to the latter treatments. Changes for leaf DM were not significant for all sucrose treatments. The highest values for stem DM and leaf area were induced by 100 mM treatment. Between the 1 and 10 mM treatments, there were not significant differences. The highest root DM was observed for the 10 and 100 mM treatments. Moreover, the highest plants and the thickest stems were induced by the 100 mM treatment, while the shortest plants and thinnest stems were observed in plants of treatments at 0 and 10 mM. In addition, hypocotyl length was the highest for plants under the 100 mM treatment and the lowest for plants under treatments at 0 and 10 mM. Plants from the 1 mM treatment achieved similar hypocotyl lengths to the other treatments. On the other hand, the treatments that promoted the longest roots were the 1 and 100 mM treatments. The shortest roots were observed in the control treatment plants. The root length from treatment at 10 mM was like the other treatments. The specific leaf area was not affected by sucrose treatments at all. The shoot-to-root biomass ratio was highest for plants under treatments at 0 and 1 mM, while the lowest values were for the remaining treatments.
Concerning the Dixon quality index (Figure 2), it can be observed that plants under sucrose treatments exhibited higher values in relation to the control. However, plants under treatments at 10 and 100 mM presented the highest values. In terms of dry matter partitioning, plants treated with 100 mM sucrose showed reduced dry matter allocation to leaves, while allocation to stems was higher (Figure 3). There was no variation in dry matter allocation to leaves or stems among plants from treatments at 0, 1, and 10 mM. In the same order, the treatments at 10, 0, and 1 mM induced a relatively higher dry matter partitioning towards the roots. Plants from the 100 mM treatment had similar behavior to those observed for treatments at 0 and 10 mM. Moreover, chlorophyll content was higher in sucrose treatments than in the control treatment, as observed in Figure 4. Nevertheless, there were no significant differences among sucrose treatments.

3.2. Plant Growth Analysis

Exogenous sucrose had a positive impact on the RGR of tomato seedlings (Figure 5). Even though a decrease in RGR was registered for all the treatments over time, it is noticeable that this descent was lower for plants under sucrose applications, considering that during period 1 (days 28–32), all treatments had a similar RGR. Indeed, the decrease from period 1 to period 4 (days 28–44) was 49, 47, and 39% for treatments at 1, 10, and 100 mM, respectively; meanwhile, for the control treatment, it was 70%. Additionally, from period 2 (days 28–36) to 4, all sucrose treatments had better RGRs than the control treatment, with an exception observed in period 2, where the 10 mM treatment had a similar behavior to the control treatment.
NAR analysis reveals that, throughout the experiment, sucrose-treated plants generally exhibited greater leaf area efficiency for biomass production compared to the control group (Figure 6a). NAR values for plants treated with 100 mM and 1 mM sucrose solutions exceeded those of the control group by factors that ranged from 4 to 7.9 and 3.7 to 6.1, respectively. A similar tendency was observed for plants treated with 10 mM sucrose solutions during periods 2 (days 32–40) and 3 (days 32–44) with factors that range from 4.2 to 3. Nevertheless, during period 1 (days 32–36), the same plants showed NAR values comparable to those of the control group, which consistently recorded the lowest values. Plants subjected to 100 mM treatment demonstrated the highest NAR values through this research, with an exception found in the third period in which these values resemble those observed in plants from treatment at 1 mM.
Additionally, two distinct trends emerged among the different sucrose treatments. Plants exposed to 1 mM and 100 mM treatments demonstrated reductions in NAR values of 34.37 and 43.67%, respectively. In contrast, plants treated with 10 mM sucrose exhibited a significant increase in NAR, rising by 61.62%. Meanwhile, control plants maintained stable low NAR values throughout the experiment.
In general terms, plants exposed to treatments at 10 mM and 100 mM displayed higher photosynthetic leaf areas to support plant respiration than those from the control group, surpassing them by percentages that range from 6.51 to 11.55% (Figure 6b). Similar LAR values were registered for both sucrose treatments over time. LAR values of plants under treatment at 1 mM were comparable to those observed in the control group, with an exception for period 1 (days 32–36), where the latter group presented lower values. Furthermore, plants from all treatments reduced LAR values across time. However, in relative terms, the relative reduction was slightly different among treatments and ranged from 9.81 to 11.88%.
Alternatively, when considering the entire growth period, it is evident that increasing sucrose concentrations led to higher RGR values (Figure 7a). Nevertheless, these increments were not significant among sucrose-treated plants. The RGR values of plants under sucrose treatments surpassed, by 3- to 3.9-fold, those observed in plants from the control group. When RGR was factorized into NAR and LAR, it was determined that NAR follows the same pattern as RGR (Figure 7a,b). Sucrose-treated plants expressed higher NAR values, which exceeded, by 2.81- to 3.65-fold, those from the control group. On the other hand, LAR showed a trend of an increase with higher sucrose concentrations (Figure 7b). Larger and similar LAR values were observed in plants from treatments at 10 and 100 mM, which were 7% superior to the registered in treatments at 0 and 1 mM.

4. Discussion

As mentioned previously, morphological parameters are influenced by light quantity and quality. Even though plants of all treatments were exposed to the same light conditions, it can be noticed that the morphological parameters differed among treatments as an effect of sucrose applications.
High dry matter accumulation of individual plant organs consistently supported a higher total dry matter in sucrose-treated plants. Sucrose is the primary product of photosynthesis in higher plants, serving as the main form of carbon transport and a crucial substrate for sink metabolism [17,23]. In addition, this molecule stimulates plant growth through cell division in apical meristems [24]. However, sucrose also acts as a signaling molecule. For instance, low sucrose concentrations induce genes involved in photosynthesis and the mobilization of energy reserves [24]. These facts may explain the positive responses of plants to exogenous sucrose. Moreover, it seems that exogenous sucrose is more effective when environmental conditions limit photosynthesis [16,25] and promote respiration [18]. In these situations, plant necessities for sucrose are higher, so exogenous sucrose might be more opportune. On the other hand, when photosynthesis is not impaired, sucrose is available, and the growth of plant organs is restricted to their ability to use this source of carbon. If sucrose utilization is slow (sink limitation), sugars will be accumulated, and photosynthetic rates will be reduced [10,16,25]. In this research, the concentration of sucrose solutions and their frequency of application did not impair seedling growth, as observed by [17] in yields of soapberry trees. It means that tomato seedlings were in situations of source limitation, which corresponds to environmental conditions of low light intensities and daily maximum temperatures that commonly rise to 35 °C (Figure 1).
Additionally, although exogenous sucrose has an important effect on dry matter accumulation, it might not be in the same proportion as the one promoted by optimum light intensities. In fact, the dry matter of tomato seedlings grown under optimum light intensities was 55 to 95% higher than that registered for seedlings grown under low light intensities [3,4,5], while in the current research, the increments caused by sucrose sprays ranged from 28 to 42%. It is important to note that light intensities, evaluated by the previous authors, varied from low (50 μmol m−2 s−1) to optimum (300 μmol m−2 s−1), which had a strong effect on photosynthesis, explaining the variation of the relative increase in total dry matter. This fact is corroborated by [9], who found that sucrose sprays augmented the plant dry weight of tomato seedlings exposed to low-light conditions, though this increment was not equivalent to the one obtained by plants subjected to optimum light conditions. In contrast, the infusion of sucrose solutions (438 mM) in maize plants allowed them to reverse 100% of the dry weight losses, a consequence of impaired photosynthesis caused by shade [16] or drought [26]. This compensatory effect might vary depending on the concentration of sucrose solutions and their frequency of application, the methods used to provide it to the plants, the environmental conditions, and, of course, the sucrose-mediated signaling functions.
Although limited information is available on the specific effects of exogenous sucrose on leaf dry matter and area, this study suggests that the higher values observed in these variables may be attributed to the supplemental carbon provided by sucrose applications. Interestingly, despite the similar leaf dry matter among sucrose treatments, variations in leaf area resulted in different SLA values. Notably, plants treated with 100 mM sucrose exhibited a higher SLA, enabling them to maximize light capture and enhance growth. In contrast, control plants had similar SLA values to sucrose-treated plants but developed less leaf area, leading to reduced light interception and slower growth.
On the other hand, low-light conditions [3,5] and high planting densities [2,7] are conducive to shade avoidance phenomenon, which results in taller plants and thinner stems. It is tempting to assume that the higher leaf areas registered in sucrose-treated plants would induce taller plants and thinner stems due to stronger light competition situations; however, our results do not show this trend in all sucrose treatments. In fact, control plants had similar plant heights to plants under the 10 mM treatment. Alternatively, sucrose treatments did not cause plant height or hypocotyl length reductions in relation to the control treatment, indicating that sucrose sprays did not repress far-red light-induced phenotypes, as mentioned by Yoon et al. [24]. Supplementary to this, only the highest sucrose dose led to significantly longer measurements for hypocotyl length. This finding contrasts with the results obtained by García-González et al. [27], where no significant differences were observed between sucrose-treated and non-treated plants. It might be that there is a threshold of sucrose concentration to induce higher hypocotyl lengths; for instance, the previous authors evaluated a sucrose concentration of 29 mM, which is much lower than the highest sucrose concentration used in the current study. Furthermore, the application methods were different in both investigations. In addition, stem diameters were not reduced with increasing plant heights. Indeed, taller plants showed thicker stems. This behavior contrasts with that observed in plants that have been subjected to low light intensities or high planting densities. Our results suggest that gains in plant height and stem diameter were induced by the extra carbon provided by the exogenous sucrose.
With respect to roots, it is known that conditions that favor photosynthesis can lead to positive morphological features. In fact, Freixes et al. [28] registered higher elongation rates of primary and secondary roots and higher secondary root densities in Arabidopsis thaliana seedlings when subjected to increasing light intensities (120 to 460 μmol m−2 s−1). On the other hand, under greater CO2 levels (900 μmol mol−1), plants of Arabidopsis thaliana displayed more roots with superior length and diameter [29]. Alternatively, exogenous sucrose, irrespective of the supply method, may mimic the effects of higher photosynthetic rates on root morphology. Actually, MacGregor et al. [30] encountered that depositing exogenous sucrose (29 mM) with agar in the aerial tissue of plants of Arabidopsis thaliana promoted longer primary roots. In addition, García-González et al. [27] found that exogenous sucrose (29 mM) caused root elongation in etiolated seedlings of Arabidopsis thaliana. Longer roots were also observed by González-Hernández et al. [31] in tomato seedlings treated with 87 mM sucrose. In turn, Lee-Ho et al. [29] revealed that increasing concentrations of sucrose (0 to 146 mM) led to more roots with superior lengths, mainly if CO2 concentrations were increased to 900 μmol mol−1. Additionally, exogenous sucrose could compensate for the negative effects of low light levels on root morphology. Certainly, plants under low-light conditions exhibited poor root elongation rates; meanwhile, sucrose-treated plants under the same conditions had similar elongation rates to plants exposed to superior light levels [28]. The previous literature consolidates our results. Indeed, our research demonstrates that sucrose-treated plants had a trend to be longer than those observed in the control group, with an exception for plants exposed to the treatment at 10 mM, which exhibited similar values to those registered in plants from the control group. Although our measurements focused solely on primary root lengths, it is plausible that the sucrose-treated plants developed more roots or enhanced their lengths and diameters, potentially due to a greater accumulation of dry matter in the roots. Even though the mechanisms of how exogenous sucrose induces better root morphologies in plants are not well elucidated, MacGregor et al. [30] suggested that sucrose metabolism, but not signaling, might be the regulatory factor that promotes lateral root primordium. This agrees with Freixes et al. [28], who found that higher hexose and sucrose concentrations in the apical region of the primary and secondary roots were induced by high light levels and high sucrose concentration in the root medium, which was also positively correlated with the root elongation rate. Contrastingly, increasing concentrations of sucrose (132 to 3512 mM) induced poor seedling growth (shoot and root) [32]. Corroborating these results, increasing sucrose concentrations from 29 to 132 mM reduced total root length (including all lateral roots) in seedlings of Arabidopsis thaliana [33]. Similarly, Baque et al. [34] registered lower root dry weights when Morinda citrifolia plantlets were exposed to sucrose concentrations that exceeded 146 mM. As mentioned above, there might be a threshold at which a dose of exogenous sucrose could impair growth due to metabolism; however, low water potentials in the medium might also be a cause for growth reduction. In fact, the growth performance of Arabidopsis thaliana seedlings was negatively affected by decreasing water potential in the medium; nevertheless, the root dry weights and the elongation rates of the primary roots were stimulated by moderate stress (−0.23 to −0.51 MPa) [35]. Nonetheless, the negative effects of low water potentials were discarded in our research because exogenous sucrose was applied to the leaves.
Additionally, little information has been developed regarding exogenous sucrose and its incidence on the shoot-to-root ratio. For instance, Wang et al. [13] encountered that exogenous sucrose (0.5 mM) reduced this ratio in triticale seedlings. This reduction was a consequence of a relatively higher root dry mass accumulated in the roots, as observed in the sucrose-treated plants in the current study.
Conversely, the high DQI values recorded for sucrose-treated plants in this study can be attributed to enhanced plant dry matter accumulation, thicker stems, and reduced shoot-to-root ratios, particularly observed in the plants subjected to the 10 and 100 mM treatments. All these variables improved plant vigor and suggest that exogenous sucrose contributes positively to higher plant quality. Due to technical constraints, we could not contrast sucrose-treated plants with plants grown under adequate light levels during the natural photoperiod; nevertheless, the beneficial effects of exogenous sucrose on the morphological parameters of tomato seedlings grown under non-optimal light conditions are evident.
For dry matter partitioning, it was not possible to detect a trend that explains the effect of increasing doses of exogenous sucrose over the allocation of photoassimilates toward the different organs. Nonetheless, the observations for plants under treatment at 100 mM totally contrast with those described by Wang et al. [9], who found that exogenous sucrose led to a higher allocation of photoassimilates to the leaves and lower to the stems. These differing results might be explained by the variation in the developmental stage of the evaluated material [36,37,38,39]. For instance, Wang et al. [9] evaluated mature tomato plants in the fruiting stage while this study focused on seedlings. Additionally, the previous authors did not report alterations in photoassimilate allocation to roots, which is in agreement with our results, particularly for plants under treatment at 100 mM. Curiously, plants under treatments at 1 and 10 mM led to a shift in the distribution of photoassimilates toward the roots but not to the leaves or stems.
Concerning the relation between exogenous sucrose and chlorophyll content, previous studies have determined that chlorophyll levels may increase with exogenous sucrose [13,40,41]; nonetheless, opposite effects may also occur [9,10]. In our study, the higher chlorophyll concentrations observed in sucrose-treated plants might be explained by a possible enhanced production of chlorophyll precursors accompanied by the up-regulation of the δ-aminolevulinate acid dehydratase gene, as shown by Guo et al. [12]. The augmented levels of chlorophyll might have induced higher photosynthetic rates under the predominant low-light conditions registered in our study (Figure 1), as determined by Liu et al. [6], helping plants to achieve higher dry matter.
As far as we know, this is the first report in which the effect of exogenous sucrose on the regular growth rate of tomato seedlings has been analyzed. In this sense, there is scarce literature concerning this topic. Concerning RGR over successive periods of time, our results (Figure 5) are similar to those from [42], in which the RGR of nimblewill plants decreased over time. In our study, the decay of RGR values might be a consequence of resource limitation, especially light, particularly for treatments at 1 and 100 mM which presented a decreasing NAR over time (Figure 6a). Curiously, the NAR of plants from treatments at 0 and 10 mM displayed distinct patterns. In fact, the NAR of plants under treatment at 0 mM was not affected by light competition, while an increasing NAR was observed for plants under treatment at 10 mM. About LAR, plants from all treatments showed decreasing values over time (Figure 6b). This is explained by the increasing dry matter accumulation in all organs and a reduction in leaf area during days 36 and 40 after sowing due to leaf abscission. Contrastingly, Sønsteby et al. [21] did not register significant changes in the RGR of strawberry plants subjected to different temperatures and photoperiods over successive periods of time. This discrepancy could be explained by the bigger size of the containers used to grow the plants and the invariable climate conditions provided by the phytotron in their study.
Considering the entire growth period, the increase in RGR with rising sucrose concentrations was linked to gains in both NAR and LAR, with NAR having a more significant influence than LAR on the RGR (Figure 7a,b). Interestingly, this finding resembles the outcomes obtained by Groot et al. [22], who found that under low or high light levels, NAR has a stronger effect than LAR on the RGR of tomato plants. Additionally, increasing RGR and NAR values were observed in young tomato plants under increasing daily light integrals [43]; nonetheless, these values were higher than those registered in the current study, possibly explained by lower plant densities and probable higher daily light integrals. On the other hand, the same authors recorded a contrasting trend for LAR; indeed, decreasing daily light integrals induced higher LAR values, meaning that plants prioritized light capture under low light levels. In our study, treatments at 10 and 100 mM may have facilitated plants to accomplish this objective, considering that all plants were subjected to low light levels. This fact is supported by higher leaf areas recorded in sucrose-treated plants.

5. Conclusions

This study demonstrates that exogenous sucrose plays an important role on the morphology, quality, and growth of tomato seedlings grown under low light conditions. Exogenous sucrose acted as an extra source of carbon which induced higher leaf areas and chlorophyll contents, favoring light absorption under light-limiting conditions. Moreover, the higher leaf areas facilitated the absorption of exogenous sucrose over time. This had a positive impact, mainly on NAR, which promoted a strong effect on the RGR. Therefore, sucrose-treated plants presented a higher dry matter accumulation in individual plant organs, consistently supporting a higher plant dry matter. Consequently, plants subjected to sucrose treatments displayed better DQI. Finally, although plants subjected to the sucrose treatment at 10 mM shared the highest DQI with plants from the treatment at 100 mM, plants from the latter treatment displayed the highest plant and stem dry matter, leaf area, stem diameter, and plant height. Nevertheless, more research is needed to understand if these differences represent significant improvements in field conditions.

Author Contributions

Conceptualization, M.G.-C.; methodology, M.G.-C. and Á.E.; formal analysis, M.G.-C.; investigation, M.G.-C. and Á.E.; resources, M.G.-C. and Á.E.; data curation, M.G.-C. and Á.E.; writing—original draft preparation, M.G.-C.; writing—review and editing, M.G.-C.; visualization, M.G.-C.; supervision, M.G.-C.; project administration, M.G.-C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. Publication funding was provided by the Universidad Técnica del Norte.

Data Availability Statement

The data presented in this study are openly available in Office 365 Onedrive cloud, https://utneduec-my.sharepoint.com/:x:/g/personal/magomez_utn_edu_ec/EVTBiBf-BOFHtB2ztihx01IBYB3yxwQxLezk_NgPvlgcLw?e=AcxsaP (accessed on 1 October 2024).

Acknowledgments

The authors acknowledge the Universidad Técnica del Norte for the facilities provided to realize this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Environmental conditions during this study. (a) Photosynthetic active radiation registered on a clear sky day, inside and outside the greenhouse, on day 22 after sowing. Light intensity inside the greenhouse was measured just above the terminal apex of the seedlings with a quantum sensor (model LI-190R; LI-COR, Lincoln, NE, USA). (b) Daily maximum, minimum, and average (dashed line) temperatures.
Figure 1. Environmental conditions during this study. (a) Photosynthetic active radiation registered on a clear sky day, inside and outside the greenhouse, on day 22 after sowing. Light intensity inside the greenhouse was measured just above the terminal apex of the seedlings with a quantum sensor (model LI-190R; LI-COR, Lincoln, NE, USA). (b) Daily maximum, minimum, and average (dashed line) temperatures.
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Figure 2. Dixon quality index of tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same plant organ indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
Figure 2. Dixon quality index of tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same plant organ indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
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Figure 3. Dry matter partitioning in tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same plant organ indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
Figure 3. Dry matter partitioning in tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same plant organ indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
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Figure 4. Chlorophyll content measured in tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
Figure 4. Chlorophyll content measured in tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
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Figure 5. Relative growth rate over 4 successive periods of time in tomato seedlings grown under sucrose applications. Data are mean ± standard error, n = 3.
Figure 5. Relative growth rate over 4 successive periods of time in tomato seedlings grown under sucrose applications. Data are mean ± standard error, n = 3.
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Figure 6. Relative growth rate components over 3 successive periods of time in tomato seedlings grown under sucrose applications. (a) Net assimilation rate. (b) Leaf area ratio. Data are mean ± standard error, n = 3.
Figure 6. Relative growth rate components over 3 successive periods of time in tomato seedlings grown under sucrose applications. (a) Net assimilation rate. (b) Leaf area ratio. Data are mean ± standard error, n = 3.
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Figure 7. Growth analysis parameters for tomato seedlings grown under sucrose applications. The results represent growth over the period between days 32 and 44 after sowing. (a) Relative growth rate. (b) Net assimilation rate (circumferences) and leaf area ratios (triangles). Data are mean ± standard error, n = 3.
Figure 7. Growth analysis parameters for tomato seedlings grown under sucrose applications. The results represent growth over the period between days 32 and 44 after sowing. (a) Relative growth rate. (b) Net assimilation rate (circumferences) and leaf area ratios (triangles). Data are mean ± standard error, n = 3.
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Table 1. Morphological parameters of tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same parameter indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
Table 1. Morphological parameters of tomato seedlings grown under sucrose applications. Results correspond to day 44 after sowing. Different letters for the same parameter indicate significant differences (p < 0.05). Data are mean ± standard error, n = 3.
VariableTreatment
0 mM1 mM10 mM100 mM
Plant DM (mg)61.679 ± 2.344 c83.110 ± 3.170 ab79.246 ± 3.530 b87.500 ± 2.878 a
Leaf DM (mg)34.688 ± 1.267 b47.065 ± 1.703 a44.290 ± 2.048 a46.725 ± 1.454 a
Stem DM (mg)23.481 ± 1.064 c31.792 ± 1.419 b29.827 ± 1.530 b35.713 ± 1.327 a
Root DM (mg)3.510 ± 0.237 c4.254 ± 0.254 b5.129 ± 0.441 a5.063 ± 0.274 a
Leaf Area (cm2)13.955 ± 0.572 c18.089 ± 0.574 b17.324 ± 0.766 b19.325 ± 0.548 a
Stem Diameter (mm)1.988 ± 0.023 c2.088 ± 0.018 b2.005 ± 0.028 c2.180 ± 0.022 a
Hypocotyl Length (cm)5.031 ± 0.068 b5.185 ± 0.097 ab5.110 ± 0.070 b5.281 ± 0.069 a
Root Length (cm)5.123 ± 0.227 b5.956 ± 0.253 a5.448 ± 0.298 ab5.669 ± 0.237 a
Plant Height (cm)9.965 ± 0.233 c11.165 ± 0.309 b10.377 ± 0.258 c12.131 ± 0.256 a
SLA (m2 g−1)0.041 ± 0.001 ab0.039 ± 0.001 b0.040 ± 0.001 b0.042 ± 0.001 a
Shoot-to-Root Biomass Ratio18.990 ± 1.042 ab20.034 ± 0.728 a17.688 ± 1.057 c17.553 ± 0.686 c
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Gómez-Cabezas, M.; España, Á. Exogenous Sucrose Enhances Growth and Physiological Performance of Tomato Seedlings Under Suboptimal Light Conditions in Passive Greenhouses. Horticulturae 2024, 10, 1337. https://doi.org/10.3390/horticulturae10121337

AMA Style

Gómez-Cabezas M, España Á. Exogenous Sucrose Enhances Growth and Physiological Performance of Tomato Seedlings Under Suboptimal Light Conditions in Passive Greenhouses. Horticulturae. 2024; 10(12):1337. https://doi.org/10.3390/horticulturae10121337

Chicago/Turabian Style

Gómez-Cabezas, Miguel, and Ángelo España. 2024. "Exogenous Sucrose Enhances Growth and Physiological Performance of Tomato Seedlings Under Suboptimal Light Conditions in Passive Greenhouses" Horticulturae 10, no. 12: 1337. https://doi.org/10.3390/horticulturae10121337

APA Style

Gómez-Cabezas, M., & España, Á. (2024). Exogenous Sucrose Enhances Growth and Physiological Performance of Tomato Seedlings Under Suboptimal Light Conditions in Passive Greenhouses. Horticulturae, 10(12), 1337. https://doi.org/10.3390/horticulturae10121337

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