Effects of Light Supplementation on Lettuce Growth, Yield, and Water Use During Winter Season in North Mississippi

Abstract

Most vegetable crop production in Mississippi (MS) occurs during the summer, characterized by high temperature and relative humidity. Lettuce yield and harvest quality are significantly affected by heat stress. To avoid the heat stress of the summer months, lettuce production in MS is either produced in controlled environments or during the winter months with cooler temperatures. This period, however, coincides with months with low solar radiation and shorter day length, resulting in a longer growing season and poor harvest quality. Therefore, this study was conducted to determine the optimum duration of light supplement on the growth, yield, and water use of greenhouse (GH) lettuce during the winter season in north Mississippi. In this study, three daily supplemental light duration regimes, 0 h, 4 h, and 8 h, starting at sunset, were evaluated across two lettuce cultivars, Green Forest (GF) and Ruby (RB). The study indicated that supplemental lighting significantly increased lettuce growth, yield, and water use. Although day length extension from 4 to 8 h of supplemental light had no yield benefits on the RB cultivar, extending day length from 4 to 8 h increased GF yield by 42%. It was also observed that the effects of light supplementation during low natural light quality at early or later growth stages differ between cultivars. Based on the results obtained from this study, a 4 h and 8 h post-sunset light supplementation is considered optimum for RB and GF lettuce cultivars, respectively, during the winter growing season in MS.

1. Introduction

Lettuce is one of the most consumed vegetable crops and a staple food in the human diet. Lettuce is widely produced and consumed all around the world. In 2023, the total global cultivated area of lettuce was approximately 1.26 million ha, corresponding to a total production of 28.08 million tons [1]. Lettuce production in the U.S. is predominantly in the West Coast, with 75–80% and 20–25% of the total US production from California and Arizona, respectively [2]. Although the state of Mississippi is not typically known for its lettuce production, lettuce is rapidly becoming an important crop among commercial vegetable growers in the state. In terms of planted acres, MS lettuce production increased by 119% between 2017 and 2022, while the number of lettuce farms increased by 58% over the same period [3].

Temperature is a critical environmental factor that affects crop growth and development [4,5]. Lettuce is a cool-season vegetable that grows well under a temperature range of 7–24 °C [6]. Light supplementation has been reported to enhance CO₂ uptake and consequently improve lettuce growth at air temperatures between 22 and 25 °C [7]. Similarly, light supplementation promotes vegetative growth and reduces water stress, which triggers stomata closures at relative air humidity between 70 and 80% [7].

Most vegetable crop production in Mississippi (MS) occurs during the summer, with typical air temperatures and relative humidity exceeding 32 °C and 90%, respectively. At high temperatures (mainly at temperatures above 30 °C), lettuce leaves become narrow with elongated internodes, thereby decreasing their commercial value [8]. Previous studies have also demonstrated that the effects of high temperatures on lettuce include rib discoloration, tip burn, and premature bolting [9,10]. Heat stress not only affects market value but has also been associated with low nutritional quality in lettuce [11].

To avoid the effects of heat stress on lettuce productivity and quality during the summer months, lettuce in MS is either produced in controlled environments or during the winter months with cooler temperatures. Although lettuce production during the cooler months in MS will enhance productivity and harvest quality, this period coincides with months with low solar radiation and shorter day length throughout the state (Figure 1). These conditions have been reported to reduce lettuce growth rate by up to 30% and require a longer growing season [12]. The use of light supplementation under low light and/or shorter day length conditions has been reported to enhance lettuce productivity and nutritional quality [13,14,15]. Light supplementation is not a common production practice among commercial lettuce growers in MS, especially under protected environments; hence, its application is limited. Therefore, it is anticipated that light supplementation during the winter season could enhance lettuce growth rate and yield. Therefore, this study was conducted to determine the optimum daily duration of light supplementation on the growth and yield of greenhouse (GH) lettuce during the winter season in north Mississippi.

2. Materials and Methods

2.1. Seedling Preparation and Growing Environment

Lettuce cultivars Green Forest (GF) and Ruby (RB) seeds were sown during the winter of 2023 (11 December 2023) in rockwool trays and regularly watered to ensure optimum conditions for germination. At 4 weeks after seeding (10 January 2024), lettuce seedlings were transplanted into 10 L deep-water culture hydroponic systems in a greenhouse at North Mississippi Research and Extension Center in Verona, MS. The environment in the greenhouse was maintained at 20–25 °C (night and day temperatures, respectively) with the relative humidity (RH) between 75 and 80%. The study was arranged using a randomized complete block design (RCBD) with 4 replications per treatment. Each replicate had 12 plants per cultivar, corresponding to a total of 48 plants per cultivar per treatment.

2.2. Treatment Specification and Nutrient Management

Both cultivars were tested under three light treatment regimens at 0 h, 4 h, and 8 h, corresponding to 0, 4, and 8 h of daily supplemental light. Since the study was designed to evaluate the effects of day length extension using light supplementation, the corresponding light regime was programmed to start daily at sunset. All light treatments started at 2 weeks after transplanting, using high-pressure sodium lights (HPS, Luminaire Series, by Phantom, Houston, TX, USA) at a constant Photosynthetic Photon Flux Density (PPFD) of 1000 µmol m⁻² s⁻¹. The time frame was used to ensure seedlings were fully established before treatment application. Light blackout curtains were installed to prevent cross-contamination or light infiltration between treatments.

Hydroponic nutrient solution was prepared using 5-11-21 (N-P-K) fertilizer (Peter Group, Needham, MA, USA) and 15.5-0-0 (N-P-K) YaraLiva CALCINIT greenhouse/solution grade (Yara, Tampa, FL, USA). The nutrient solution was prepared at an electric conductivity (EC) concentration of 150 ppm. The pH of the nutrient solution was adjusted with diluted sulfuric acid and maintained between 5.7 and 5.8 throughout the study period. Each hydroponic system was constantly aerated using an air stone connected to an electric pump.

2.3. Data Collection

Each plant was treated as a sampling unit; hence, data were collected from all 12 plants per treatment replicate. Data collection started 1 week after treatment (WAT) started and continued until harvest. Plant height, leaf number, head diameter, and water use were collected weekly, while yield data were collected at harvest on 23 February 2024. Leaf numbers were collected by counting the number of fully formed leaves per plant. The head diameter (cm) of each sampling unit was determined as the average head length and breadth on the same horizontal plane. Lettuce height was measured from the base of the plant and recorded in cm, while yield was measured as fresh weight and reported in grams per plant. A known volume of nutrient solution was added to the hydroponic system of each treatment replicate, and the weekly water use was determined by the difference between the initial known volume and the volume at sampling. Water use efficiency (WUE) was estimated as the ratio of lettuce fresh weight and the corresponding total water used.

2.4. Data Analysis

Data were preprocessed using Microsoft Excel 365 and analyzed using R statistical software v. 2024.12.1 (R Foundation for Statistical Computing, Vienna, Austria). All data were tested for outliers using the identify_outliers function in R. No extreme outliers were detected; hence, all outliers were ignored for all datasets. All data were subjected to Shapiro–Wilk’s and Levene’s tests to determine normality and homogeneity of variance before analysis. Once assumptions were met, a two-way ANOVA test was performed to evaluate the effects of light supplementation and cultivar on height, leaf number, head diameter, yield, and water use. A Bonferroni post-hoc test was conducted to determine differences among treatments. All data were analyzed at a 5% significance level.

3. Results and Discussions

3.1. Effects of Light Supplementation on Lettuce Growth: Head Diameter

The ANOVA results indicate that light supplementation significantly affected lettuce head diameter at 1 and 3 WAT, while cultivar effects were only significant at 3 WAT (

Table 1. ANOVA table for the effects of light supplementation on lettuce head diameter.

Sampling Date	Effect	DFn	DFd	F	p	p < 0.05
1 Week After Treatment	Treatment	2	6	12.731	0.007	*
	Cultivar	1	3	0.100	0.773
	Treatment–Cultivar	2	6	3.546	0.096
2 Weeks After Treatment	Treatment	2	6	1.870	0.234
	Cultivar	1	3	0.170	0.708
	Treatment–Cultivar	2	6	2.596	0.154
3 Weeks After Treatment	Treatment	2	6	35.110	0.0004	*
	Cultivar	1	3	155.71	0.001	*
	Treatment–Cultivar	2	6	0.41	0.568

* Indicate statistical differences at p value < 0.05.

). The individual effects of treatment and cultivar on lettuce head diameter were not significant at 2 WAT. Both cultivars responded differently to light treatment for head diameter at 1 WAT (Figure 2). At this stage, light supplementation had no significant effects on GF head diameter; however, head diameter for RB increased in the order of 4 h > 8 h > 0 h. While the effects of light treatment on lettuce head diameter at 1 WAT were only significant for RB, both cultivars showed a significant response at 3 WAT. At this stage, the head diameter was the lowest under the 0 h treatment compared to the 4 h and 8 h treatments. However, there were no significant differences in head diameter at the 4 h and 8 h light durations, regardless of cultivar. Hence, these results indicate that lettuce’s response to extended day length using light supplementation differs among cultivars and growth stages.

3.2. Effects of Light Supplementation on Lettuce Growth: Plant Height

The ANOVA results suggest significant differences at both the treatment and cultivar levels, with no interactions, at all sampling dates (

Table 2. ANOVA table for the effects of light supplementation on lettuce height.

Sampling Date	Effect	DFn	DFd	F	p	p < 0.05
1 Week After Treatment	Treatment	2	6	7.951	0.021	*
	Cultivar	1	3	24.701	0.016	*
	Treatment–Cultivar	2	6	1.965	0.221
2 Weeks After Treatment	Treatment	2	6	5.475	0.044	*
	Cultivar	1	3	291.093	0.000439	*
	Treatment–Cultivar	2	6	0.578	0.589
3 Weeks After Treatment	Treatment	2	6	63.909	9.01 × 10−5	*
	Cultivar	1	3	1113.29	5.92 × 10−5	*
	Treatment–Cultivar	2	6	1.851	0.236

* Indicate statistical differences at p value < 0.05.

). The effects of light supplementation on lettuce height were similar to those observed for head diameter. Lettuce height was significant only for RB at both 1 and 2 WAT, where lettuce height under 0 h light treatment was consistently lower compared to 4 h and 8 h (Figure 3). A similar response was observed for both cultivars at 3 WAT, where lettuce height under the 0 h light treatment was significantly lower compared to those observed under 4 h and 8 h. As observed for head diameter, the RB cultivar showed an early plant height response (at 1 WAT) to light treatment; however, no significant response was observed for GF until near harvest at 3 WAT. At this stage, increasing the duration of light supplementation from 4 h to 8 h after sunset did not increase lettuce height regardless of cultivar (Figure 3C).

3.3. Effects of Light Supplementation on Lettuce Growth: Leaf Number

Light treatment had significant effects on leaf number at all sampling dates, while cultivar was only significant at 2 WAT (

Table 3. ANOVA table for the effects of light supplementation on lettuce leaf number.

Sampling Date	Effect	DFn	DFd	F	p	p < 0.05
1 Week After Treatment	Treatment	2	6	5.93	0.038	*
	Cultivar	1	3	7.76	0.069
	Treatment–Cultivar	2	6	0.99	0.424
2 Weeks After Treatment	Treatment	2	6	6.55	0.031	*
	Cultivar	1	3	206.54	0.00073	*
	Treatment–Cultivar	2	6	0.27	0.773
3 Weeks After Treatment	Treatment	2	6	6.52	0.031	*
	Cultivar	1	3	7.38	0.073
	Treatment–Cultivar	2	6	0.19	0.828

* Indicate statistical differences at p value < 0.05.

). Both genotypes showed a contrasting response to light treatment at all sampling dates. Light treatment had no significant effects on GF’s leaf number, regardless of sampling time, while the leaf number for RB was significantly different among treatments, irrespective of sampling dates (Figure 4). Generally, the leaf number for RB was consistently lower under 0 h compared to other light treatments. A visual comparison of the effects of light supplementation on all growth parameters for both cultivars is presented in Figure 5.

Light is an important factor in crop growth, productivity, and harvest quality [16,17,18]. This is because the quantity and quality of light intercepted by a crop canopy are directly related to its photosynthetic activities and the synthesis of biochemical compounds. Increasing photoperiod has been reported to enhance chlorophyll content, photosynthesis, and CO₂ assimilation in addition to the increase in lettuce biomass, dry weight, and leaf area [19,20,21,22]. Therefore, the observed increase in lettuce height and head diameter could be attributed to the enhanced photosynthesis and CO₂ assimilation effects of day length extension using light supplementation.

3.4. Effects of Light Supplementation on Lettuce Yield

The ANOVA results indicated that lettuce yield was significantly affected by light treatment and cultivar (

Table 4. ANOVA table for the effects of light supplementation on lettuce leaf yield.

Effect	DFn	DFd	F	p	p < 0.05
Treatment	2	9	36.871	4.62 × 10−5	*
Cultivar	1	9	122.00	1.56 × 10−6	*
Treatment–Cultivar	2	9	6.472	1.80 × 10−2	*

* Indicate statistical differences at p value < 0.05.

). A significant interaction between these factors was also observed. The pattern of lettuce yield response for RB was consistent with those observed for height and head diameter. The yield for RB was significantly higher under both 4 h and 8 h light treatments compared to 0 h (Figure 6). However, there were no significant differences between the observed yields under 4 h and 8 h light treatments. The observed treatment effects on lettuce yield for GF were different from those observed for RB. Lettuce fresh weight was only significantly different at the highest light duration (8 h) compared to other treatments. Unlike RB, the observed yield for GF under 4 h of light treatment was significantly lower compared to 8 h, while 4 h post-sunset light supplementation did not improve lettuce yield compared to the 0 h treatment.

Crop yield is directly related to both the intensity and duration of light incident on the crop canopy [23,24,25]. The increase in photoperiod through light supplementation significantly increased lettuce yield. These results are in line with those reported in the literature for lettuce grown under different growing conditions [26,27,28,29]. Although light supplementation increased lettuce yield, the extent of light-induced yield increases may be different among lettuce cultivars. Compared to the observed yield at 0 h treatment, lettuce yield increased by 26% and 57% for GF at 4 h and 8 h, respectively, while an increase of 28% and 30% was observed in the same order for RB, hence suggesting that supplemental light requirements for lettuce production may be cultivar-specific. Based on the observations from this study, a 4 h day length extension during the winter months in MS could be considered optimum for RB, while the optimum day length extension for GF could be as high as 8 h after sunset.

3.5. Effects of Light Supplementation on Water Use and Water Use Efficiency

The results showed that lettuce water demand varies with treatment. Regardless of cultivar, the average daily water use (Figure 7) and average season total water use (Figure 7) were highest (p < 0.05) under the 8 h light treatment and lowest under 0 h. It was also observed that both average daily water use and average season total water use were not significantly different for both 0 h and 4 h treatments. Although the increase in photoperiod using light supplementation increased lettuce water demand, the observed results for water WUE indicated no significant differences among treatments, regardless of cultivar. Hence, based on the growing and treatment conditions in this study, increasing photoperiod does not affect WUE in lettuce. These results are similar to those reported in the literature [30], where water consumption for lettuce exposed to HPS lighting was significantly higher compared to natural light. However, our results on WUE were different from those reported by the same authors. Although the authors reported that HPS lighting significantly reduced WUE in lettuce compared to natural light, our results showed that HPS lighting had no significant effects on WUE compared to lettuce exposed to natural light. These differences could be attributed to the fact that lettuce plants in our study were exposed to a shorter daily length extension of 4 to 8 h compared to the 15 h described by Dannehl et al. [30]. To our knowledge, there are no other comparable studies that compared water use efficiency under HPS and natural light conditions. Another plausible reason could be that increasing light duration could have possibly enhanced metabolic efficiency in lettuce, especially under low natural light conditions. Therefore, studies that focus on the effects of extended light duration on lettuce metabolic activities could provide a new direction for the additional benefits of light supplementation in food production.

4. Conclusions

Based on the results observed from this study, optimizing day length extension using light supplementation could reduce lettuce water use without a reduction in plant growth and yield. Light supplementation of 4 h after sunset results in 21% and 20% for average daily and total season water savings, respectively, compared to 8 h of light duration. The results clearly indicated that a daily light supplementation beyond 4 h may not significantly increase lettuce growth rate. The results also indicated that the effects of day length extension on yield may vary among lettuce cultivars. While a daily day length extension from 4 to 8 h had no yield benefits on RB lettuce cultivar, day length extension from 4 to 8 h increased GF lettuce yield by 42%. Therefore, studies with a focus on understanding cultivar- and/or growth stage-specific responses to different light regimes will be considered essential to further enhance light and water optimization for lettuce production in indoor or controlled environments. This study did not include an economic analysis, which limits the ability to assess the impacts of extended day length on production cost. Future research should incorporate economic evaluations to provide a more comprehensive understanding of the practical implications for farmers.