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Article

Morphological and Physiological Traits of Greenhouse-Grown Tomato Seedlings as Influenced by Supplemental White Plus Red versus Red Plus Blue LEDs

by
Geng Zhang
1,†,
Zhixin Li
2,†,
Jie Cheng
2,†,
Xianfeng Cai
2,
Fei Cheng
2,
Yanjie Yang
2 and
Zhengnan Yan
2,3,*
1
Department of Agronomy and Horticulture, Jiangsu Vocational College of Agriculture and Forestry, Jurong 212400, China
2
College of Horticulture, Qingdao Agricultural University, Qingdao 266109, China
3
Shandong Qicai Manor Vegetable Food Base Co., Ltd., Shouguang 262700, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2022, 12(10), 2450; https://doi.org/10.3390/agronomy12102450
Submission received: 11 September 2022 / Revised: 5 October 2022 / Accepted: 5 October 2022 / Published: 10 October 2022
(This article belongs to the Special Issue Growth Control of Plants on the Light Environment)
Browse Figures
Versions Notes

Abstract

:
The relatively low light intensity during autumn–winter or early spring and inclement weather such as rain or fog may lead to extended production periods and decreased quality of greenhouse-grown tomato seedlings. To produce high-quality tomato seedlings rapidly, the influences of supplementary lights with different spectra on the morphological and physiological traits of tomato seedlings were measured in a greenhouse. Supplemental lighting with the same daily light integrals (DLI) of 3.6 mol m−2d−1 was provided by white (W) light-emitting diodes (LEDs), white plus red (WR) LEDs, and red plus blue (RB) LEDs, respectively, and tomato seedlings grown under only sunlight irradiation were regarded as the control. Our results demonstrate that raised DLI by supplementary light improved the growth and development of greenhouse-grown tomato seedlings, regardless of the spectral composition. Under conditions with the equal DLI, the tomato seedlings grown under supplementary WR LEDs with a red to blue light ratio (R:B ratio) of 1.3 obtained the highest values of the shoot and root fresh weights, net photosynthetic rate, and total chlorophyll content. The best root growth and highest root activity of tomato seedlings were also found under the supplementary WR LEDs. Supplementary WR LEDs remarkably increased the stem firmness of the greenhouse-grown tomato seedlings, and increased the starch content in the leaves of greenhouse-grown tomato seedlings compared to the control. However, statistically significant differences did not occur in the sucrose, carotenoid contents, superoxide dismutase (SOD), and catalase (CAT) activities among the different supplemental lighting treatments. In conclusion, supplemental LED lighting could promote the growth and development of greenhouse-grown tomato seedlings grown under insufficient sunlight conditions. In addition, WR LEDs could obtain tomato seedlings with a higher net photosynthetic rate, higher root activity, and higher starch content compared with other treatments, which could be applied as supplementary lights in greenhouse-grown tomato seedlings grown in seasons with insufficient light.

1. Introduction

Tomato (Solanum lycopersicum L.) is one of the most extensively cultivated and widely consumed vegetable crops worldwide, which is a focus of the agricultural industry [1,2]. Tomatoes are full of nutrients and antioxidants that provide numerous health benefits to humans [3]. Because of its notable economic values and potentially beneficial health effects, the global harvested area of tomato and the total tomato production have tremendously increased over the past decades. The harvested area of tomato was 5.05 million hectares in 2020, which increased by 14.10% from 2010 (http://www.fao.org/faostat/en/#data/QCL, accessed on 7 September 2022). The global production of tomato reached up to 186.82 million tons in 2020, which increased by 21.88% compared to those in 2010 (https://www.fao.org/faostat/en/#data/QCL, accessed on 7 September 2022). China is the world’s largest producer and consumer of tomatoes with a cultivation area and yield of 1.11 million hectares and 64.87 million tons, respectively [4].
The production of tomato seedlings is a pretty vital operation, particularly for the production of tomato all year round [5]. In China, greenhouses have been widely used for the commercial production of tomato seedlings. The seedling nursery greenhouse provides a relatively suitable growing condition for tomato seedlings to remove the abiotic and biotic stresses. However, greenhouses usually receive low ambient light conditions during the autumn–winter and early spring. Vegetable seedlings also have to cope with the low light conditions in greenhouses caused by the influences of undesirable weather conditions such as heavy rain, snow, or fog. The slow growth and deterioration in the quality of tomato seedlings caused by insufficient light will not only upset the cultivation management of tomato plants in the middle- and late-growth stages, but also have immediate negative effects on the fruit ripening period and fruit quality. Previous studies have indicated that maximized yield and quality potential cannot be achieved without using high-quality, healthy, and vigorous seedlings [6]. Therefore, it is necessary to incorporate supplemental lighting to improve the growth and development of greenhouse-grown tomato seedlings during the time periods with low levels of solar radiation.
Light is one of the most vital environmental variables regulating plant growth and development as the energy source for photosynthesis and an external signal [7]. Light imposes a vast range of regulatory influences on plants, particularly on their morphological traits, various physiological processes, yield formation, and the final product quality [8,9]. Artificial light sources such as fluorescent lamps, high-pressure sodium lamps, and metal-halide lamps are generally applied for greenhouse cultivation to increase DLI [10]. However, greenhouses utilizing the aforementioned artificial light sources are often inefficient because of the high operation temperatures, high energy consumptions, low luminous efficiencies, and inappropriate spectra for optimal growth. LEDs now have a great potentiality due to their long operating times, low heat dissipation, high energy conversion efficiency, and greater wavelength specificity [11,12,13] compared to traditional artificial light sources.
It is widely known that red (600 to 700 nm; R) and blue light (400 to 500 nm; B) are more efficient than other wavebands in driving photosynthesis [14] as red light has the maximum quantum yield, and blue light plays an important photomorphogenic role and influences photosynthetic function [15,16]. LEDs allow the manipulation of spectra, thus facilitating the investigation of the optimal light conditions for plant cultivations. Many researchers have found that proper combinations of red and blue LEDs could enhance plant growth, promote nutritional quality, and even increase health-promoting compounds. For example, a combination of red and blue LEDs (R:B ratio of 1) enhanced the photosynthetic rate and the number of stomata in tomato seedlings, and the chloroplasts and palisade tissue cells were well-developed [17]. In cucumber seedlings, LEDs with the R:B ratio of 1 could increase the photosynthetic rate and decreased hypocotyl length [18]. The LEDs with an R:B ratio of 2 increased the stem diameter and proline content in pepper seedlings [19]. The combination of blue and red LEDs with a ratio of R:B of 8 induced the accumulation of vitamin C content in the leaves of non-heading Chinese cabbage [20]. A combination of red plus blue LEDs with a ratio of R:B of 1 increased the chlorophyll and carotenoid contents and photosynthetic rates in the rice seedlings [21]. The total phenolic concentration and antioxidant capacity in the leaves of lettuce seedlings were increased under the combination of red plus blue LEDs [22]. Thus, red and blue light combinations are often used in protected horticulture to regulate the light conditions to produce better plant growth [23]. However, plants cultivated under mixed red and blue light appear purplish to the human eye, so it is difficult to observe growth abnormalities and the disease symptoms of plants [24,25]. Furthermore, accumulating evidence has indicated that plant growth over wide spectra (e.g., white LEDs, fluorescent lamps) has advantages over those grown under narrow spectrum LEDs [26,27,28,29]. Since the plant has certain responses to a specific spectral band of light, the suitable spectral composition of full spectrum LEDs for plant production differs among species. Recent findings have revealed that white light combined with red light improved plant growth compared to white light in lettuce [23,30], spinach [31], and pepper [32], but few studies have been conducted on tomato. Thus, it is difficult to figure out precisely what kind of spectral composition is suitable for tomato seedlings in a greenhouse. Therefore, the present study evaluated the effectiveness of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on the growth and physiological traits of greenhouse-grown tomato seedlings. Our investigations could provide guidelines for the commercial production of tomato seedlings with high quality via using supplementary light in greenhouses.

2. Materials and Methods

2.1. Plant Materials

Tomato (Solanum Lycopersicum cv. Fenbeibei) seedlings were grown in 72-cell plastic plug trays filled with mixed vermiculite, peat, and perlite (3:1:1, v/v/v). Plug trays were kept in a Venlo-type greenhouse with a floor area of 2736 m2 at Qingdao Agricultural University, Qingdao, Shandong Province, China, at a temperature of 25 ± 1 °C during the day and 18 ± 1 °C at night, and the relative humidity was maintained at 60–70% for 30 days. Hoagland’s nutrient solutions were applied for the cultivation of tomato seedlings and the management of tomato seedlings was based on a previous study [33].

2.2. Treatment Design

The average DLI of solar light inside the greenhouse was 9.0 mol m−2 d−1 during the experimental period. Based on the suitable daily light integral and supplementary duration for the growth of tomato seedlings [34], the tomato seedlings in the current study were grown under supplemental DLI at 3.6 mol m−2 d−1 with a light intensity and photoperiod at 125 μmol m−2 s−1 and 8 h d−1, respectively, created by white (W) LEDs (Weifang Hengxin Electric Appliance Co., Ltd., Weifang, China), white plus red (WR) LEDs (Zhongshan Aierzhiguang Lighting Technology Co., Ltd., Zhongshan, China), and red plus blue (RB) LEDs (Zhongshan Aierzhiguang Lighting Technology Co., Ltd., Zhongshan, China), respectively. Additionally, tomato seedlings grown without supplementary light were regarded as the control (DLI at 9.0 mol m−2 d−1 from sunlight only). The spectral properties (Table 1) and spectral distribution of the LEDs (Figure 1) applied in the present study as supplementary light were measured with a spectrometer (PG100N, United Power Research Technology Corporation, Miaoli, China) at the plant canopy. Three replications were applied for each treatment, and 72 tomato seedlings were used per replication.

2.3. Growth Measurements

2.3.1. Plant Morphology and Growth Characteristics

The shoot and root fresh weights, plant heights, hypocotyl lengths, stem diameters, and total leaf areas of greenhouse-grown tomato seedlings were measured at 30 days after treatment. The total leaf areas were measured with a leaf area meter (LI-300; Li-Cor Inc., Lincoln, NE, USA). The root length, root surface area, and root volume were determined and analyzed with WinRHIZO software (Version 2016a, Regent Instruments Inc., Quebec, QC, Canada). The shoot and root dry weights were obtained after oven-drying at 80 °C for at least 72 h. The seedling quality index and specific leaf area of the tomato seedlings were analyzed according to the description of Yan et al. [33].

2.3.2. Gas Exchange and Photosynthetic Pigments

The leaf gas exchange parameters were recorded on fully-expanded leaves just prior to the harvest with a portable photosynthesis system (LI-6400XT, Li-Cor Inc., Lincoln, NE, USA) equipped with a 6400-02 LED light source. The measurement conditions of light intensity, gas flow rate, CO2 concentration, relative humidity, and leaf temperature were 400 μmol m−2 s−1, 500 μmol s−1, 400 μmol mol−1, 70 ± 5%, and 25 °C, respectively. The chlorophyll a, chlorophyll b, and carotenoid contents were determined spectrophotometrically according to the method of He et al. [7].

2.3.3. Root Activity

The root activity of tomato seedlings was determined using the triphenyl tetrazolium chloride (TTC) method as described by Li [35]. The absorbance of the test samples was measured at the wavelength of 485 nm at room temperature using a spectrophotometer (1810, Shanghai Yoke Instrument Co., Ltd., Shanghai, China).

2.3.4. Antioxidant Enzymes Activities

The leaf fresh tissues (0.5 g) were homogenized with 5 mL of phosphate buffer saline (50 mM, pH 7.8) and the homogenates were centrifuged at 4 °C for 20 min at 12,000× g. The supernatants were collected for the determination of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activity as described previously [36]. The specific activity of all enzymes was expressed as units per gram of fresh weight.

2.3.5. Stem Firmness, Starch, and Sucrose Contents

Stem firmness was determined using a texture analyzer (TMS-Pro, Food Technology Corporation, Sterling, VA, USA) equipped with a metal probe 2 mm in diameter, and was expressed as Newtons (N). The starch content of the tomato leaf samples was measured according to the method of Takahashi et al. [37]. The sucrose concentration of the tomato leaf samples was evaluated using the method of Jing et al. [38].

2.4. Statistical Analysis

The results are reported as the means ± standard deviations (SD) of the three replications of each treatment. One-way analysis of variance (ANOVA), followed by the least significant difference (LSD) test, were carried out using SPSS 19.0 software (IBM, Inc., Chicago, IL, USA) to compare the means between treatments. Differences were considered significant at p < 0.05.

3. Results

3.1. Influences of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Plant Morphology of Tomato Seedlings

The morphology of tomato seedlings influenced by supplementary light with different light spectra with the same DLI is shown in Table 2. Compared with the control, supplementary light with WR LEDs displayed the highest plant height of the tomato seedlings followed by supplementary light with W and RB LEDs. Supplemental lighting with WR and RB LEDs significantly decreased the hypocotyl length of tomato seedlings, but no significant difference was observed between the control and W LED treatment. The stem diameters and total leaf areas of the tomato seedlings with supplementary light regardless of light quality were significantly higher than those in the control treatment. The stem diameters of the tomato seedlings with supplementary RB LEDs were significantly higher than those of the WR LED treatment. The total leaf areas of seedlings with supplementary WR LEDs were significantly larger than those of the W LED treatment. The specific leaf areas of seedlings with supplementary light were remarkably lower than those of the controls, and no significant changes in the specific leaf areas were observed among all supplemental lighting treatments.

3.2. Effects of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Photosynthesis and Photosynthetic Pigments of Tomato Seedlings

Compared to the control, the net photosynthetic rate, stomatal conductance, and intercellular CO2 concentration were considerably higher in the leaves of tomato seedlings grown under supplementary light with WR LEDs (Table 3). The intercellular CO2 concentration and transpiration rate in the leaves of tomato seedlings grown under supplementary light with RB LEDs were significantly higher than those of the control treatment (Table 3). The chlorophyll (Chl) contents in the leaves of the tomato seedlings showed distinct responses to the different supplemental lighting (Figure 2). The Chl a, Chl b, and total Chl contents in the leaves of the tomato seedlings grown with supplementary light were significantly higher than those of the controls (Figure 2). The Chl a and total Chl contents in the leaves of tomato seedlings with supplementary WR LEDs were considerably higher than those of the W LED treatment, and the Chl b content in the leaves of the tomato seedlings with supplementary WR LEDs were significantly higher than those of the W and RB LED treatments (Figure 2). No significant difference in Chl a, Chl b, and total Chl contents was found between the W and RB LED treatments (Figure 2). In addition, the carotenoid contents in the leaves of the tomato seedlings with supplementary light were significantly higher than those of the control treatment (Figure 2). No pronounced differences in the carotenoid content were observed among the different supplemental lighting treatments.

3.3. Effects of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Plant Growth of Tomato Seedlings

Supplemental lighting significantly promoted the plant growth and biomass accumulation of greenhouse-grown tomato seedlings compared to the control (Table 4). The fresh and dry weights of the tomato seedlings grown under supplementary light were significantly greater than those of the control, regardless of light quality. The shoot and root fresh weights of the tomato seedlings grown under supplementary light with WR LEDs were significantly higher than those of the RB LED treatment. The shoot and root dry weights of the tomato seedlings grown under supplementary light with RB LEDs were significantly higher than those of the W LED treatment. The seedling quality index of the tomato seedlings grown under supplementary light were significantly higher compared to those of the control, but no pronounced difference was observed in the seedling quality index among the different supplemental lighting treatments.

3.4. Effects of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Root Characteristics of Tomato Seedlings

The root characteristics of the tomato seedlings were influenced by the supplementary LED light (Figure 3). The root length, root surface area, root volume, and root activity of tomato seedlings grown under supplementary light with WR LEDs were significantly greater than those of the controls. No pronounced difference was found in the aforementioned root characteristics between the W LEDs or the RB LED treatment and the control treatment.

3.5. Effects of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Antioxidant Enzymes Activities of Tomato Seedlings

Supplemental lighting significantly increased the activities of POD and CAT of the tomato seedlings compared to the control (Table 5). The POD activity in the tomato seedlings grown under supplementary light with RB LEDs was significantly higher than those of the W and WR LED treatments, while no significant change was found between the W and WR LED treatments (Table 5). No significant differences were observed in the activities of SOD and CAT of the tomato seedlings grown with the different supplemental lighting (Table 5).

3.6. Effects of Supplementary White LEDs, White Plus Red LEDs, and Red Plus Blue LEDs on Stem Firmness of Tomato Seedlings, and Starch and Sucrose Contents in Tomato Leaves

The stem firmness of the tomato seedlings grown under supplementary light with WR LEDs was significantly harder than those of the control (Figure 4). No significant difference was found in the stem firmness between the W LEDs or the RB LED treatment and the control treatment (Figure 4). The starch content and sucrose content in the leaves of the tomato seedlings grown under supplementary light with WR LEDs were significantly higher than those of the control treatment (Figure 5). No significant change was found in the starch content between the W LEDs or the RB LED treatment and the control treatment (Figure 5). The sucrose contents of the WR and RB LED treatment were significantly higher than those of the control, and an insignificant difference in sucrose content was observed between the W LEDs and control treatments (Figure 5).

4. Discussion

The seedling nursery facilities installed with the environmental control system could provide tomato seedlings with a high quality to meet the needs for year-round production. In China, greenhouses are widely applied in the production of tomato seedlings. However, the sunlight in the greenhouse varies widely across regions and latitudes, and solar light typically appears weak at certain times of the year, especially during winter, early spring, and the rainy seasons [39]. Insufficient light can seriously affect the quality of tomato seedlings, which in turn impacts the later plant cultivation and the overall benefit of tomato production. In these cases, it is necessary to apply supplementary light to ensure the quality of tomato seedlings. Plants have evolved diverse photoreceptors that perceive the light of specific wavelengths and individually or synergistically trigger a series of responses that influence growth and development [40,41]. The data obtained in the current study indicated that higher DLIs via applying the supplementary light led to increases in the plant height, stem diameter, leaf area, and plant biomass, regardless of the different composition of the spectra (Table 2 and Table 4). Similar results were observed in sweet basil [42], sweet potato [7], cucumber [9], and lettuce [23], demonstrating that the plant yields increased with the increase in DLI. Light deficiency inhibits plant growth and development [43], whereas excess light has detrimental effects on the photosynthetic apparatus of plants [5], and even causes a reduction in the carbohydrate accumulation [44]. As the excessive light energy cannot be dissipated quickly, it will induce photoinhibition and protein degradation of photosystem I [45], and also inhibit the repair of photosystem II [46]. The present results showed that the supplementary light could promote the growth of the greenhouse-grown tomato seedlings compared with those grown without supplemental lighting.
The responses of tomato seedlings to the different composition of the spectra were different to the same supplemental DLI. The supplementary WR LEDs efficiently increased the plant height of the greenhouse-grown tomato seedlings in comparison with the supplementary W and RB LEDs (Table 2). Park and Runkle [25] observed that the primary stem of petunia seedlings elongated and had flower buds earlier under white or white plus red LEDs compared to under blue plus red LEDs. It is interesting to further study whether the flowering time of tomato plants are affected by white plus red LEDs during the seedling stage. The dry weights of the tomato seedlings cultivated under the supplementary RB LEDs with a 66.5% red light fraction were significantly higher than those cultivated under the W LED treatment with a 22.4% red light fraction (Table 4). Prior research found that a higher red light fraction could promote plant growth [22,47,48], which may be due to the red light being effective in promoting plant biomass [49,50]. However, Liu et al. [51] reported that the dry weight of the cherry tomato seedlings decreased as the red light fraction exceeded 50%, which was not consistent with the result in our study. Furthermore, green light accounted for a large fraction of the light spectrum in the W LED (49.1%) and WR LED (41.3%) treatments (Table 1). Kim et al. [24] observed that the addition of 24% green light in the mixed red and blue light increased lettuce growth. Chang and Chang [52] also showed that 14.9% of green light supplemented to the mixed red and blue light led to a large leaf area, high shoot dry weight, and low nitrate content of lettuce. These results may be because green light can penetrate further into the leaf compared with red or blue light, thus increasing the speed of photosynthesis [53]. Similar results were observed in the current study, indicating that the plant height, shoot and root fresh weights of the greenhouse-grown tomato seedlings grown under supplementary WR LEDs were significantly higher than those under supplementary RB LEDs (Table 2 and Table 4). The net photosynthetic rate of the tomato seedling leaves of the WR LED treatment was also remarkably higher compared with those cultivated under the RB LED treatment (Table 3). However, no significant change in the shoot and root fresh and dry weights was observed between the W and WR LED treatments (Table 4). Snowden et al. [54] also reported that the increase in green light from 0% to 30% did not affect the dry mass accumulation in lettuce grown under a lower light level. This was presumably due to the excessive green light (over 50% fraction) being energetically wasteful and might cause a reduction in plant yield [24,28].
Plants can change their morphological and physiological properties such as SLA, leaf size, and chlorophyll content to acclimate for different light conditions [55,56]. SLA is estimated as the leaf area per unit of dry weight, which is a vital plant functional feature as it is an indicator of ecophysiological traits [57]. A lower SLA indicates the increased leaf thickness of plants [58], which was beneficial for maximizing the capture of the available light to meet the demands for the photosynthesis of plants grown under low light conditions [59]. Our results showed that supplemental lighting decreased the SLA of the tomato seedlings, which was similar to that in previous studies [5,60]. Combined with the biomass data analysis of the tomato seedlings, it indicates that the light condition in the greenhouse might not be sufficient for seedling growth, whereas the supplemental lighting alleviated weak light stress in tomato. Both the light quantity and quality could affect the SLA of plants [22,61,62], but no significant change was observed among the supplementary LEDs with different compositions of spectra (Table 2). A leaf with a larger surface area allows for greater light interception, often leading to an increase in the biomass [63]. Similar results were found in our results (Table 2 and Table 4), indicating that supplemental lighting could increase the biomass by enlarging the leaf area. Morphological traits are usually measured to estimate the quality of the seedlings, but it is difficult to comprehensively characterize the quality of the seedlings by a single index [9]. Therefore, the seedling quality index, a composite index of the morphological trait, is applied to evaluate the seedling quality [64]. Previous studies have reported that the seedling quality index rises with the increased light intensity [9,33,60,65]. Similarly, our results showed that the supplemental lighting was suitable for the growth of tomato seedlings compared to the controls grown without supplemental lighting (Table 4).
Chlorophylls are essential molecules that catch light energy to drive photosynthetic electron transfer [66], while carotenoids serve as accessory light-harvesting pigments, extending the spectral range over which light drives photosynthesis [67]. The total chlorophyll and carotenoid contents of tomato seedlings with supplemental lighting were significantly higher compared to the tomato seedlings cultivated with solar light only (Figure 2), indicating that increased light intensity could increase the chlorophyll and carotenoid contents. Similar results have been found in previous studies, and it might be due to the increase in the leaf thickness with the increase in light intensity [68,69]. The chlorophyll contents of the tomato seedlings were different among the supplementary LEDs with different light spectra, while the highest values of chlorophyll a, chlorophyll b, and total chlorophyll contents were observed in the supplementary WR LEDs (Figure 2). Excessive red light causes a reduction in the chlorophyll contents, and green light at an appropriate level may promote the accumulation of chlorophylls when combined with red and blue lights [30,70,71]. The high contents of chlorophylls and carotenoids enable plants to absorb more light to enhance the photosynthetic rate, which might result in the highest photosynthetic rate in the supplementary WR LEDs (Table 3).
The plant root system of plants was an important organ that absorbs and transports water and nutrients to the growing plant [72]. A deep and robust root system supports shoot growth by supplying adequate water and nutrients to the plant [63]. The root length, root volume, root surface area, and root activity of the greenhouse-grown tomato seedlings grown with supplementary WR LEDs were significantly higher than those grown without supplementary light (Figure 3), indicating that a high DLI led to a vigorous root system [73,74]. Previous studies have indicated that root development is dependent on the light qualities and was affected by the ratios of red and blue lights [22,27,75,76]. In addition, the root activities of various plants exhibited differences in their response to the different light spectra. For example, the root activity of the peanut plant was significantly higher under R:B = 1:1 LEDs than those under fluorescent lamps, red LEDs, blue LEDs, R:B = 7:3 LEDs, and R:B = 3:7 LEDs [77]. The root activity of cotton plantlet was significantly higher under red LEDs than those under R:B = 1:1 LEDs, R:B = 1:3 LEDs, R:B = 3:1 LEDs, and fluorescent lamps [78]. In our case, the root architecture showed better in the WR LED treatment than those of the RB and W LED treatments (Figure 3) through synthetically comparing the root system characteristics of each treatment. It has been reported that the supplemental LED inter-lighting could modulate the root activity of tomato by increasing the availability of the assimilate for the root [79]. Considering the significantly higher net photosynthetic rate in the WR LED treatment, it seems that supplementary WR LEDs could obtain more assimilation, thus leading to the good root development of tomato plants.
Insufficient light causes considerable stress (e.g., accumulation of reactive oxygen species (ROS)) for plant growth and development, especially during the seedling stage [33,80,81]. The ROS sources include superoxide anion radical (O2−), hydrogen peroxide (H2O2), and hydroxyl radical (·OH), which may lead to oxidative-stress-induced toxic effects in plants [82]. SOD can convert O2− to H2O2 and O2, and POD, CAT, and ascorbate peroxidase subsequently detoxify H2O2 [83,84]. The SOD activity in the WR LED treatment was significantly higher than those of the controls (Table 5), indicating more conversion of O2− to H2O2. It has been proven in many plants (e.g., cucumber, amaranth, carnation) that the antioxidant enzyme activities of plants are related to the light intensity and the proportion of blue light [85,86,87]. For example, Zhang et al. [85] reported that the activities of the SOD and POD of cucumber seedlings were enhanced by increasing the supplementary light intensity or blue light proportion. The activities of SOD and POD of freshly cut amaranth ascended with supplementary blue LEDs [86]. Similarly, the activities of POD and CAT in tomato seedlings with supplemental lighting were remarkably higher than those of the control (Table 5). No significant differences were observed in the CAT activities among all of the supplementary LED treatments with different light spectra (Table 5). Similar results were found in rice seedlings [88] and cucumber seedlings [33], suggesting that the CAT activity was less affected by the light quality.
High-quality tomato seedlings should exhibit compact morphological traits such as thick leaves, firm stem, and large white roots [33,89]. Our investigations revealed that the stem firmness of the tomato seedlings grown with supplementary WR LEDs was significantly higher compared with those cultivated under solar light only (Figure 4). Our results indicate that tomato seedlings grown under supplementary WR LEDs could conduct photosynthesis better and had more sugars for building up tissues. The stem firmness of cucumber seedlings grown under supplementary white LEDs was over 30% harder than the cucumber seedlings grown without supplementary light [33]. However, the stem firmness of the tomato seedlings with the supplementary W LEDs was not significantly different from those of the controls without supplementary light (Figure 4). These differences could be due to the different species, R:B ratio, and spectral composition.
Starch is the basic carbohydrate reserve that accumulates in the chloroplasts of photosynthesizing leaves [63]. The high rate of photosynthesis observed in the tomato seedlings cultivated under the supplementary WR LEDs might result in starch accumulation through a possible increase in starch synthesis (Figure 5). Red light has been proven to enhance starch accumulation in several species [20,77,90,91], but excess red light may inhibit the translocation of photosynthetic products out of leaves, subsequently causing the inhibition of photosynthesis. Thus, the supplementary WR LEDs with an R:B ratio of 1.3 may be the preferred light source for tomato seedling growth compared to the supplementary WR or W LEDs. Sucrose is an important substrate involved in various development and physiological events of plants [92]. A prior study found that the combinations of red and blue light (ratio of 3) increased melon resistance to powdery mildew, which might be due to the accumulated sucrose acted as the signal for inducing the defense-related pathway [93]. Moreover, the active sucrose metabolism provided enough precursor and energy for the secondary metabolites synthesis, leading to a high strength of the cell wall [94]. Tomato seedlings grown with supplementary WR or RB LEDs had a remarkably higher level of sucrose than that of the control treatment, suggesting that supplementary WR or RB LEDs could increase the resistance ability of tomato seedlings.

5. Conclusions

The present study indicated that supplemental LED lighting could promote the growth and development of the greenhouse-grown tomato seedlings when the natural light was not adequate. In addition, the supplemental lighting with different spectra had different effects on the leaf morphology, photosynthetic properties, growth, and the physiological characteristics of the greenhouse-grown tomato seedlings. Compared to the supplementary W LEDs and RB LEDs, supplementary WR LEDs could obtain tomato seedlings with a higher net photosynthetic rate, higher root activity, and higher starch content, which could be applied as supplementary lights in greenhouse-grown tomato seedlings grown in seasons with insufficient light.

Author Contributions

Conceptualization, G.Z. and Z.Y.; Methodology, G.Z., Z.L. and J.C.; Validation, F.C. and Y.Y.; Data analysis, J.C. and X.C.; Writing—original draft preparation, G.Z. and Z.L.; Writing—review and editing, Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Key Research and Development Program of Shandong Province (2021TZXD007-02); the Science and Technology of People-benefiting Project of Qingdao (20-3-4-24-nsh); the Natural Science Foundation of Shandong Province (ZR2021QC174); the Experimental Technology Research Program of Qingdao Agriculture University (SYJS202117); and the Innovation and Entrepreneurship Training Program for College Students of QAU.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The relative spectral photon flux of white LEDs, white plus red LEDs, and red plus blue LEDs.
Figure 1. The relative spectral photon flux of white LEDs, white plus red LEDs, and red plus blue LEDs.
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Figure 2. The photosynthetic pigment contents of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings cultivated under sunlight only were regarded as the control. Different letters on top of bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
Figure 2. The photosynthetic pigment contents of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings cultivated under sunlight only were regarded as the control. Different letters on top of bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
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Figure 3. The effects of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on the root length (A), root surface area (B), root volume (C), and root activity (D) of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
Figure 3. The effects of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on the root length (A), root surface area (B), root volume (C), and root activity (D) of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
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Figure 4. The impacts of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on the stem firmness of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of the bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
Figure 4. The impacts of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on the stem firmness of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of the bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
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Figure 5. The effects of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on starch content (A), and sucrose content (B) of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of the bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
Figure 5. The effects of supplementary white LEDs, white plus red LEDs, and red plus blue LEDs on starch content (A), and sucrose content (B) of the greenhouse-grown tomato seedlings, and tomato seedlings grown under sunlight only were regarded as the control. Different letters on top of the bars denote significant differences according to the LSD test (p < 0.05). Error bars represent the mean ± SD.
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Table
Table 1. The spectral properties of the supplementary white LEDs, white plus red LEDs, and red plus blue LEDs used in this experiment.
Table 1. The spectral properties of the supplementary white LEDs, white plus red LEDs, and red plus blue LEDs used in this experiment.
ParameterLighting Source
WWRRB
Fraction z (%)
PPFD y (400 to 700 nm)100.0100.0100.0
Blue light (400 to 499 nm)28.525.832.1
Green light (500 to 599 nm)49.141.31.4
Red light (600 to 700 nm)22.432.966.5
Light quality
R:B ratio x0.81.32.1
z Data are photon flux-based proportions of blue, green, and red lights. y PPFD, photosynthetic photon flux density. x R:B ratio, red to blue light ratio.
Table
Table 2. Morphological traits of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings cultivated under sunlight only were regarded as the control.
Table 2. Morphological traits of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings cultivated under sunlight only were regarded as the control.
TreatmentPlant Height
(cm)		Hypocotyl Length (cm)Stem Diameter
(mm)		Leaf Area
(cm2)		Specific Leaf Area (cm2/g)
Control17.7 ± 0.9c 3.9 ± 0.3a3.1 ± 0.3c50.8 ± 7.2c499.4 ± 31.4a
W25.4 ± 0.6b3.9 ± 0.1ab3.7 ± 0.1ab71.6 ± 5.1b365.4 ± 10.0b
WR28.7 ± 1.5a3.6 ± 0.2c3.6 ± 0.3b81.8 ± 8.0a344.0 ± 55.7b
RB25.6 ± 0.4b3.6 ± 0.1bc3.9 ± 0.1a74.0 ± 3.8ab354.7 ± 48.7b
p value0.00010.02340.00010.00010.0007
Different letters with in the same column indicate remarkable differences according to the least significant difference (LSD) test at p < 0.05.
Table
Table 3. The photosynthetic characteristics of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
Table 3. The photosynthetic characteristics of greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
TreatmentNet Photosynthetic
Rate
(μmol m−2 s−1)		Stomatal
Conductance
(mol m−2 s−1)		Intercellular CO2Concentration
(μmol mol−1)		Transpiration
Rate
(mmol m−2 s−1)
Control5.51 ± 0.80b 0.030 ± 0.014b142 ± 25b0.77 ± 0.32b
W5.80 ± 0.53b0.051 ± 0.012ab192 ± 45ab0.85 ± 0.26b
WR7.03 ± 0.75a0.058 ± 0.004a221 ± 46a1.31 ± 0.43ab
RB6.02 ± 0.26b0.044 ± 0.008ab239 ± 35a1.42 ± 0.40a
p value0.01570.01920.01340.0335
Different letters within the same column indicate significant differences according to the LSD test at p < 0.05.
Table
Table 4. Growth of the greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
Table 4. Growth of the greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
TreatmentShoot Fresh Weight
(g/Plant)		Root Fresh Weight
(g/Plant)		Shoot Dry Weight
(g/Plant)		Root Dry Weight
(g/Plant)		Seedling Quality
Index
Control2.54 ± 0.51c 0.39 ± 0.04c0.235 ± 0.011c0.029 ± 0.005c0.081 ± 0.020b
W4.72 ± 0.24ab0.53 ± 0.08ab0.343 ± 0.021b0.042 ± 0.008b0.111 ± 0.015a
WR5.26 ± 0.44a0.57 ± 0.04a0.352 ± 0.021ab0.047 ± 0.008ab0.114 ± 0.023a
RB4.62 ± 0.40b0.48 ± 0.03b0.385 ± 0.021a0.053 ± 0.001a0.131 ± 0.010a
p value0.00010.00230.00010.00140.005
Different letters within the same column indicate remarkable differences according to the LSD test at p < 0.05.
Table
Table 5. The activities of the antioxidant enzymes of the greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
Table 5. The activities of the antioxidant enzymes of the greenhouse-grown tomato seedlings grown under supplementary white LEDs, white plus red LEDs, and red plus blue LEDs, respectively, and tomato seedlings grown under sunlight only were regarded as the control.
TreatmentPOD (U/g FW)SOD (U/g FW)CAT (U/g FW)
Control154.8 ± 28.6c 856.2 ± 55.2b22.0 ± 3.8b
W213.0 ± 11.9b886.5 ± 26.1ab45.2 ± 11.0a
WR237.4 ± 27.7b923.9 ± 10.5a54.9 ± 12.2a
RB304.8 ± 31.7a889.9 ± 16.9ab50.1 ± 8.4a
p value0.00080.04850.0115
Different letters within the same column indicate remarkable differences according to the least significant difference (LSD) test at p < 0.05.
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Zhang, G.; Li, Z.; Cheng, J.; Cai, X.; Cheng, F.; Yang, Y.; Yan, Z. Morphological and Physiological Traits of Greenhouse-Grown Tomato Seedlings as Influenced by Supplemental White Plus Red versus Red Plus Blue LEDs. Agronomy 2022, 12, 2450. https://doi.org/10.3390/agronomy12102450

AMA Style

Zhang G, Li Z, Cheng J, Cai X, Cheng F, Yang Y, Yan Z. Morphological and Physiological Traits of Greenhouse-Grown Tomato Seedlings as Influenced by Supplemental White Plus Red versus Red Plus Blue LEDs. Agronomy. 2022; 12(10):2450. https://doi.org/10.3390/agronomy12102450

Chicago/Turabian Style

Zhang, Geng, Zhixin Li, Jie Cheng, Xianfeng Cai, Fei Cheng, Yanjie Yang, and Zhengnan Yan. 2022. "Morphological and Physiological Traits of Greenhouse-Grown Tomato Seedlings as Influenced by Supplemental White Plus Red versus Red Plus Blue LEDs" Agronomy 12, no. 10: 2450. https://doi.org/10.3390/agronomy12102450

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