Spathiphyllum floribundum (Linden & André) N.E.Br. is an important indoor flower species. Thus, optimizing its growth by regulating the light quality under indoor low-light conditions may be critical for generating high-quality flowers. In this study, the effects of the following six light-quality treatments on peroxidase and superoxide dismutase activities in two S. floribundum cultivars (“Sweet Chico” and “Queen”) were analyzed: monochromatic light comprising 100% red (R, 657 nm) or 100% blue (B, 450 nm) light, a combination of R and B lights [80% R + 20% B (8:2), 70% R + 30% B (7:3), and 60% R + 40% B (6:4)], and white light. The light treatments were performed using light-emitting diodes. The light intensity and photoperiod were set to 45 ± 2 µmol·m−2·s−1 and 14 h·day−1, respectively. The results of this study revealed that an appropriate R:B light ratio may lead to increased pigment contents, thereby increasing the synthesis and accumulation of photosynthetic products, which will result in increased stress resistance and enhanced growth. These findings provide the basis for future investigations on the growth and production of S. floribundum and other indoor ornamental plants.
Spathiphyllum floribundum (Linden & André) N.E.Br. est une importante plante à fleurs d’intérieur. En optimiser la croissance en contrôlant la qualité de la lumière sous un éclairage tamisé pourrait donc être crucial si l’on veut obtenir des fleurs de belle allure. Les auteurs ont analysé les effets des six traitements lumineux qui suivent sur l’activité de la peroxydase et de la superoxyde dismutase chez deux cultivars de S. floribundum (« Sweet Chico » et « Queen ») : éclairage monochrome rouge (R, 657 nm) ou bleu (B, 450 nm), éclairage rouge et bleu [80 % R + 20 % B (8:2), 70 % R + 30 % B (7:3) et 60 % R + 40 % B (6:4)], éclairage blanc. La lumière venait de diodes électroluminescentes. L’intensité de l’éclairage et la photopériode ont été respectivement fixées à 45±2 µmol·par m2·par seconde et à 14 h par jour. Selon les résultats de l’étude, le rapport R:B adéquat pourrait augmenter la quantité de pigments, dont accentuer la synthèse et l’accumulation des produits de la photosynthèse, ce qui améliorerait la résistance de la plante au stress ainsi que sa croissance. Ces observation paveront la voie aux recherches futures sur la croissance et la production de S. floribundum et d’autres plantes ornementales d’intérieur. [Traduit par la Rédaction]
Introduction
Light is indispensable for plant growth. Specifically, its regulatory effects on the growth of plants are mainly associated with three factors, light intensity, light quality, and photoperiod (Johkan et al. 2010; J.F. Li et al. 2021). Light quality strongly influences plant development–related processes, including photosynthesis, morphogenesis, biomass accumulation, and phytochemical synthesis (Vänninen et al. 2010; Yang et al. 2017; Roso et al. 2020). Regarding light quality, red (R), blue (B), far red, and ultraviolet light are important for plant growth and development (Brown et al. 1995; Y. Li et al. 2021). They can regulate plant growth through photoreceptors (Sager and Wheeler 1992; Lim and Kim 2021; Li et al. 2022) and strongly affect the activity of antioxidant systems in different plants (Kim et al. 2013; Nascimento et al. 2013; Xu et al. 2014; Deng et al. 2017; Yu et al. 2017). Plant leaves absorb light in the R and B spectra. Therefore, R and B lights are most commonly used in studies on the effects of environmental light on plant physiology (Hogewoning et al. 2010). Red light is recognized by phytochromes (Urbonaviciute et al. 2007; Li et al. 2017), induces seed germination and hypocotyl elongation, and promotes cotyledon deployment and pigment synthesis (Hoenecke et al. 1992; Tripathy and Brown 1995). The leaf area of tomato seedlings is significantly promoted by R light, but the root growth is inhibited (Li et al. 2017). However, the hypocotyl elongation is more under R light than under combined R and B lights or B alone (Pay et al. 2022). R light shows a capacity to stimulate the antioxidant synthesis and increases the content of antioxidant phytochemical compounds (Arriaga et al. 2020). Blue light is sensed by cryptochrome, phototropin, and zeaxanthin (Blaauw and Blaauw-Jansen 1970; Thomas and Dickinson 1979; Schwartz and Zeiger 1984). Then it induces biomass production (Park and Kim 2010; Wu et al. 2014; Wang et al. 2017a), while also promoting chlorophyll biosynthesis, stomatal opening, root formation, and photomorphogenesis (Heo et al. 2002; Urbonaviciute et al. 2007; Lim and Eom 2013; Wang et al. 2017a). B light could upregulate auxin signaling and stimulate rooting (Gil et al. 2021). Plants grown under B light are characterized by higher chlorophyll a/b ratios, photosynthetic rate, and stomatal conductance (Eskins et al. 1991; Zheng and Van Labeke 2017a, 2017b; Lim and Kim 2021). And a higher activity of antioxidant enzymes is detected in B light (Aalifar et al. 2020). However, monochromatic R or B light might inhibit stomatal opening and the accumulation of intercellular CO2 more than combined R and B lights, thereby reducing the photosynthetic rate and plant growth (Azmat 2013; Wang et al. 2016). Combining R and B lights can significantly increase the plant photosynthetic rate, leaf nitrogen accumulation, and plant cell division and morphogenesis (Matsuda et al. 2004; Ohashi et al. 2006; Kwon et al. 2015; Wang et al. 2017b). Moreover, a proper proportion of R and B lights can enhance plant growth and development more than monochromatic R or B light to some extent (Matsuda et al. 2004).
Light-emitting diodes (LEDs) are accurate, practical, and relevant indoor light sources that may be used to improve the quality of economic crops and horticultural plants (Morrow 2008; Gómez and Mitchell 2015; Hernández and Kubota 2016; Calderon 2020). These light sources have been specially manufactured for plantlet research or for cultivating plants under specific monochromatic wavelengths (Zheng and Van Labeke 2017a). Many reports have verified the effects of LEDs that vary in terms of light quality on the developmental stages of various plants, including rapeseed (Li et al. 2013), tobacco (Yang et al. 2017), rosa hybrida (Bergstrand et al. 2016), chrysanthemum (Zheng and Van Labeke 2017a), lettuce (Chen and Yang 2018), and Gerbera jamesonii Bolus (Meng et al. 2019).
Spathiphyllum floribundum (Linden & André) N.E.Br. (peace lily), which is one of the most popular indoor ornamental plants and cut-flower species, is cultivated and grown commercially worldwide. In vitro studies on S. floribundum have been conducted to obtain virus-free and genetically modified plants (Silva et al. 2006; Zhao et al. 2012; Han et al. 2016; Yu et al. 2016). Additionally, several studies involving the cultivation of S. floribundum in fields have focused on pest management (Blagojević et al. 2016; Migotto et al. 2017), cultivation physiology (Heemers et al. 2003; Dewir et al. 2005; Paredes and Quiles 2017), and stress physiology (Dewir et al. 2005; Soto et al. 2014;Wang et al. 2020). Thus, there has been relatively little research on the photosynthetic characteristics of S. floribundum, especially under simulated low-light environmental conditions during indoor growth.
In this study, tube-stock S. floribundum plants (“Sweet Chico” and “Queen”) after transplanting domestication were hydroponically grown under low-light environmental conditions provided by different R and B LED sources. The objectives of this research were as follows: (i) analyze the physiological acclimation of plantlets; (ii) select the optimal light proportions using R and B LEDs for improving commodity plantlet qualities and indoor ornamental plant growth; and (iii) expand the theoretical and technical basis of S. floribundum production.
Materials and methods
Materials, treatments, and cultural conditions
Two cultivars of S. floribundum plantlets (“Sweet Chico” and “Queen”, purchased from Dezhou Shijifeng Horticulture Scientific and Innovation CO., LTD, China) were used in the experiment. The plantlet roots were washed with distilled water and transplanted into planted aperture disks with the substrate (perlite: vermiculite = 1:1). The nutrient solution formula was used as nutrient solutions similar to the Japanese garden test formula (Table 1). There were 30 plantlets in each treatment, and each treatment was repeated three times.
Table 1.
Nutrient solution formula.
Six different light quality treatments were provided by LEDs in this research, which were white (W) as a control (CK), 100% blue (B, peak at 450 nm), 100% red (R, peak at 657 nm), 80% R + 20% B (8:2), 70% R + 30% B (7:3) and 60% R + 40% B (6:4). The relative spectral distributions are shown in Fig. 1.
The plant materials were cultured in the nursery cultivation room; the cultivation environment temperature and humidity were set as 25 ± 1 °C and 80 ± 5%, respectively. The photoperiod was 14 h·day−l, and photosynthetic active radiation was 45 ± 2 µmol·m−2·s−1 (simulated indoor low light conditions). Plantlets were cultured for 75 days prior to sampling.
Morphological indicators and root vigor
The morphological characteristics (i.e., plant height, number of leaves, leaf length and width, number of roots, root vigor, as well as root length and fresh/dry weight) of S. floribundum were measured randomly in this research. The third leaf from the top was used as the test material for assessing physiological indicators. The root vigor of plantlets was detected by the triphenyl tetrazolium chloride reduction methods described by Ryssov-Nielson (1975) and Trevors and Roger (1984). To measure Dry weight (DW), shoots and roots were dried separately at 105 °C for 30 min, then at 60 °C for 48 h in a thermostat, or until constant weight. Each index was repeated ten times.
Soluble sugar and protein content
The content of soluble sugar was determined with the sulfuric acid anthrone method by Clegg (1956). Each sample (30 mg leaf fresh weight (FW)) was extracted in 4 mL 80% ethanol (v/v) at 80 °C for 40 min and then 10 mg activated carbon was added, and the mixture was incubated at 80 °C for 30 min. The supernatant was reduced to 10 mL, and then the soluble sugar content was determined using the anthrone method with absorbance measured at 620 nm. Each index was repeated five times.
Soluble proteins were measured by Coomassie's brilliant blue method by Bradford (1976). Each sample (0.2 g leaf FW) was ground with liquid nitrogen in a mortar and pestle and then extracted in 3 mL phosphate-buffered solution (pH 7.0). The extract was centrifuged at 6000g at 4 °C for 15 min. The supernatant (0.5 mL) was combined with 4.5 mL Coomassie brilliant blue G-250 solution (0.1 g·L−1). After 2 min, the absorbance at 595 nm was measured. The soluble protein content was calculated from a standard curve. Each index was repeated five times.
Superoxide and peroxidase activity
Superoxide dismutase (SOD) activity and peroxidase (POD) activity were assessed based on the photochemical method and the rate of oxidation method described by Zhao et al. (1998). Each index was repeated five times. Leaf samples (1 g) were ground using phosphate buffer (0.05 mmol·L−1, pH 7.8, 10 mL), then centrifuged at 13 000g for 20 min at 4 °C. The supernatant was used for assays of antioxidant enzyme activities.
POD activity was determined according to the change in absorption at 470 nm caused by guaiacol oxidation. One milliliter of enzyme extract liquid and 3 mL of the reaction liquid were mixed evenly; the reaction liquid was composed of 28 µL guaiacol, 19 µL H2O2, and 50 mL of potassium phosphate buffer (0.05 mol·L−1, pH 6.0). Then, the absorbance was recorded at 470 nm.
One unit of SOD was the amount of enzyme that caused a 50% inhibition of nitroblue tetrazolium (NBT) reduction. A total of 0.1 mL of enzyme extract liquid was added to the reaction liquid (2 mL methionine (15 mmol·L−1) + 0.1 mL ethylenediaminetetraacetic acid (EDTA, 0.003 mmol·L−1) + 0.1 mL riboflavin (0.1 mmol·L−1) + 2 mL NBT(0.1 mmol·L−1) ), fully mixed evenly, and then was placed in the sunlight for 15 min. The absorbance was then recorded at 560 nm.
Chlorophyll and carotenoid contents
The chlorophyll (Chl) and carotenoid contents were assessed with the third leaf from the apex by the acetone method (Holm 1954). Leaf samples (100 mg leaf FW) were ground in a mortar and then extracted in 20 mL of a 1:1 (v/v) mixture of 80% acetone and absolute ethyl alcohol in a 25 mL stoppered vial in the dark for 24 h. Using 80% acetone and absolute ethyl alcohol as the blank, the optical density was measured at λ = 663, 645, and 470 nm by a UV spectrophotometer (Shimadzu-UV2400, Japan). Each index was repeated five times.
Chlorophyll fluorescence and photosynthetic characteristics
Chlorophyll fluorescence was measured with the third leaf by a fluorescence monitoring system (FMS, Hansathech Instruments, King's Lynn, UK). Leaves were dark adapted for 20 min before measuring relevant parameters. The maximum quantum yield of PSII photochemistry (Fv/Fm) was determined as (Fm − Fo)/Fm. The quantum yield of PSII (ФPSII) was calculated by the method of Genty et al. (1989): ФPSII = (Fm − Fs)/Fm'. The non-photochemical quenching (NPQ) was calculated as NPQ = (Fm − Fm')/Fm' (Bilger and Bjorkman 1991). Each index was repeated 10 times.
The second fully expanded leaf was used for the determination of photosynthetic characteristics (CO2 assimilation rate (Pn), transpiration rate (E), stomatal conductance (gs), and intercellular CO2 concentration (Ci)) using an infrared gas analyzer (LI-6400, Li-COR, USA) in a growth chamber with a constant temperature of 25 °C, saturated CO2 concentration, and 70% relative humidity. Each index was repeated 10 times. All materials randomly were selected uniform plantlets per replicate.
Statistical analysis
The design of morphological indicators, root vigor, chlorophyll fluorescence, and photosynthetic characteristics was completely randomized with 10 replications. The design of soluble sugar and protein content, superoxide and peroxidase activity, and chlorophyll and carotenoid contents was completely randomized with five replications. All data in this research underwent a two-way analysis of variance (ANOVA), and significant differences between the means were tested using Duncan's post hoc test (p ≤ 0.05). All statistical analyses were conducted using the Microsoft Office 2010 and SPSS 19.0 (SPSS, Chicago, USA) programs for Windows.
Results
Morphological characteristics
Plant growth and morphology
There were distinct differences in growth and morphological characteristics among the S. floribundum plantlets treated with six different light conditions (Table 2). With the increase in the ratio of red light, the morphological indicators of S. floribundum plantlets showed an increase and then a decrease. The leaf length and leaf width of “Sweet Chico” had peak values with the 7:3 treatment (70% R + 30% B, same below); therefore, its leaf area was the largest. At the same time, leaf and root number, plant height, and root length were also the largest, and they were significantly higher than the others. It could be seen that “Sweet Chico” had the highest light use efficiency under this treatment. The morphological index of “Queen” also showed a similar trend; the leaf number in the 7:3 treatment was slightly less than that in the 6:4 treatment. Most of parameters showed the largest values, and the light use efficiency was the highest, in the 7:3 treatment, but its light use efficiency was lower than that of “Sweet Chico” on the whole.
Table 2.
Effects of different light qualities on the growth of Spathiphyllum “Sweet Chico" and "Queen”.
The fresh and dry weight of “Sweet Chico” obtained the best value under the 7:3 treatment, followed by 8:2, 6:4, B, and CK, and showed the smallest value at R. “Queen” indicated a peak value of shoot fresh and dry weight under the 7:3 treatment, the largest value of root and total fresh and dry weight in the 6:4 treatment, and had lesser values at R and CK. The dry mass rate of “Sweet Chico” and “Queen” was the largest under the B treatments. The fresh and dry weight of “Sweet Chico” was better than that of “Queen”; they were better in B than R (Table 3). The photosynthetic product synthesis and accumulation efficiency of “Sweet Chico” and “Queen” were higher under the 7:3 treatment, and the water and nutrient metabolism were rapid during plant growth.
Table 3.
Effects of different light qualities on the fresh and dry weight of Spathiphyllum “Sweet Chico" and "Queen”.
Root vigor
As shown in Fig. 2, the root vigor of “Sweet Chico” and “Queen” showed similar change trends, and they fell with the increase in the red light ratio. “Sweet Chico” had the largest root vigor under 6:4 (395.60 µg·g−1·h−1, 2.16 times that of CK treatment), followed by those under 7:3, 8:2, CK, B, and R. “Queen” showed the largest root vigor in the 7:3 treatment (130.63 µg·g−1·h−1, 3.72 times that of CK treatment), followed by 6:4, B, 8:2, CK, and R. They all gained the lowest root vigor in the R treatment. The root vigor in B was higher than in R.
Biochemical attributes
Soluble sugars
The soluble sugar content of “Sweet Chico” and “Queen” had similar change trends, first increasing and reaching the largest value (10.92 and 7.12 µg·g−1, 1.76 and 1.34 times that of CK treatment, respectively) under the 7:3 treatment then decreased with the increasing proportion of red light. The CK and B treatments were lower than others, and they all had the lowest value in B (5.37 and 5.51 µg·g−1). The soluble sugar content of “Sweet Chico” was higher than “Queen” on the whole (Fig. 3).
Fig. 3.
Effects of different light qualities on soluble sugar (A) and soluble protein (B) of Spathiphyllum “Sweet Chico" and "Queen”. Vertical bars indicate standard error (n = 5). Different letters represent significant differences at p < 0.05 among treatments by the Duncan's multiple range test.
Fig. 4.
Effects of different light qualities on POD (A) and SOD (B) activity of Spathiphyllum “Sweet Chico" and "Queen”. Vertical bars indicate standard error (n = 5). Different letters represent significant differences at p < 0.05 among treatments by the Duncan's multiple range test.
Fig. 5.
Effects of different light qualities on leaf chlorophyll content of Spathiphyllum “Sweet Chico" and "Queen”. Vertical bars indicate standard error (n = 5). Different letters represent significant differences at p < 0.05 among treatments by the Duncan's multiple range test. (A) Chl a. (B) Chl b. (C) carotenoid. (D) Chl a/b.
Fig. 6.
Effects of different light qualities on chlorophyll fluorescence parameters of Spathiphyllum “Sweet Chico" and "Queen”. Vertical bars indicate standard error (n = 5). Different letters represent significant differences at p < 0.05 among treatments by the Duncan's multiple range test. (A) Fv/Fm. (B) ФPSII. (C) qP. (D) NPQ.
Fig. 7.
Effects of different light qualities on photosynthetic parameters of Spathiphyllum “Sweet Chico" and "Queen”. Vertical bars indicate standard error (n = 5). Different letters represent significant differences at p < 0.05 among treatments by the Duncan's multiple range test. (A) Pn. (B) E. (C) gs. (D) Ci.
Soluble proteins
The contents of soluble proteins showed a similar change trend as soluble sugars under different light qualities. “Sweet Chico” showed the maximum soluble protein content at 7:3 (188.65 µg·g−1, 1.48 times that of CK treatment), followed by 6:4. “Queen” had the peak value of protein content under the 8:2 treatment (162.71 µg·g−1, 1.26 times that of the CK treatment), followed by 6:4. They all had the lowest protein contents in the R treatment (100.00 and 76.06 µg·g−1). The soluble protein contents of “Sweet Chico” were overall higher than those of “Queen” (Fig. 3).
Antioxidant enzyme activity (POD, SOD)
Overall, the POD and SOD activities were significantly higher in “Sweet Chico” than in “Queen” (Fig. 4). The POD activity in “Queen” initially increased and then decreased as the R light proportion increased. More specifically, the POD activity was highest (195.81 U·mg−1) during the 8:2 treatment, followed by the 6:4, 7:3, B, and R treatments, which resulted in activities that were respectively 36.82%, 36.24%, 30.31%, 24.5%, and 6.23% higher than those during the CK treatment. Similar trends were observed for “Sweet Chico”, in which POD activity was highest (349.22 U·mg−1) during the 6:4 treatment (26.44% higher than that during the CK treatment). The POD activities during the B and R treatments were, respectively, 22.37% and 26.81% lower than those during the CK treatment.
The changes in the SOD activities in “Sweet Chico” and “Queen” were similar to the POD activity trends. The peak SOD activities in “Sweet Chico” and “Queen” were observed during the 8:2 treatment (251.01 and 361.74 U·mg−1, respectively) and were 40.01% and 25.43% higher than those during the CK treatment, respectively. The lowest SOD activity (261.01 U·mg−1), which was detected during the B treatment, was 9.5% lower than the corresponding activity during the CK treatment.
Chlorophyll contents
The Chl a, Chl b, and Car contents were higher in “Sweet Chico” than in “Queen” (Fig. 5). In contrast, Chl a/b was lower in “Sweet Chico” than in “Queen”. The changes in the leaf chlorophyll contents of “Sweet Chico” and “Queen” were consistent with the changes in the growth and morphological characteristics. They increased and then decreased as the R light ratio increased. The photosynthetic pigment (Chl a, Chl b, and Car) contents peaked during the 7:3 treatment and were lowest during the CK treatment (except for the Car content during the B treatment). For both “Sweet Chico” and “Queen”, the Chl a content was highest during the 7:3 treatment (27.7963 and 23.836 mg/g, respectively). The Chl b and Car contents were highest in “Sweet Chico” during the 8:2 and 6:4 treatments, respectively, whereas they were highest in “Queen” during the 7:3 treatment. Additionally, Chl a/b was higher during the B treatment than during the R treatment for “Sweet Chico” and “Queen” and it was lower during the polychromatic light treatments (8:2, 7:3, and 6:4) than during the monochromatic light treatments (R and B). The composite (R and B) light treatments had a greater effect than the monochromatic light treatments. Thus, regarding the light effects on the chlorophyll content, the composite light treatments (8:2, 7:3, and 6:4) were better than the monochromatic light treatments (R and B), which were superior to the CK treatment.
Chlorophyll fluorescence
There was no significant difference in the Fv/Fm of “Sweet Chico” and “Queen”, but ΦPSII and qP were generally higher in “Sweet Chico” than in “Queen” (Fig. 6). “Sweet Chico” and “Queen”, respectively, had the lowest ΦPSII and qP and the highest NPQ during the B and R treatments. The polychromatic light treatments (8:2, 7:3, and 6:4) resulted in better chlorophyll fluorescence parameters than the R, B, and CK treatments. The Fv/Fm of “Sweet Chico” and “Queen” was highest during the R and 6:4 treatments, respectively. In “Sweet Chico”, ΦPSII was highest (0.1906) during the R treatment, followed by the 7:3, 6:4, 8:2, CK, and B treatments, whereas qP peaked (0.3243) during the 7:3 treatment. However, in “Queen”, ΦPSII was highest (0.15) during the 8:2 treatment, followed by the 6:4, B, 7:3, CK, and R treatments. Similarly, qP peaked during the 8:2 treatment, with a trend that was similar to that of ΦPSII. For both cultivars, Fv/Fm was lowest during the CK treatment. In this study, ΦPSII and qP were lowest in “Sweet Chico” during the B treatment, whereas they were lowest in “Queen” during the R treatment. In “Sweet Chico”, NPQ was highest during the B treatment, followed by the CK, R, 8:2, 6:4, and 7:3 treatments. In contrast, in “Queen”, NPQ was highest during the R treatment, followed by the CK, B, 7:3, 6:4, and 8:2 treatments.
Leaf gas exchange
The changes in Pn, E, gs, and Ci in “Sweet Chico” and “Queen” were in accordance with the changes in the morphological characteristics and chlorophyll contents (Fig. 7). They increased and then decreased in response to increases in the R light ratio. In “Sweet Chico”, Pn, E, gs, and Ci were highest during the 7:3 treatment, followed by the 6:4 and 8:2 treatments. In “Queen”, Pn was highest during the 6:4 treatment, E and gs were highest during the 7:3 treatment, and Ci peaked during the 8:2 treatment.
In both cultivars, Pn, E, and gs were lowest during the CK treatment, whereas Ci was lowest in “Sweet Chico” and “Queen” during the CK and R treatments, respectively. The R treatment was better than the B treatment for the Pn, gs, and Ci of “Sweet Chico”, whereas the opposite trend was observed for “Queen” (i.e., the B treatment was better than the R treatment). The photosynthetic characteristics of “Sweet Chico” were better than those of “Queen”.
Discussion
In this study, the growth of S. floribundum in hydroponic culture was significantly different under different LED light qualities. “Sweet Chico” and “Queen” had better leaf and root indexes and fresh and dry weight in the B treatment than in the R treatment. The results indicated that blue light had a relatively facilitating effect on the elongation of leaf and root growth, which was similar to the results reported for cucumber seedlings (Wang et al. 2009) and rosemary cuttings (Gil et al. 2021). Red light treatments were advantageous to the stem elongation on “Sweet Chico”, and a similar result was found on lettuce (Hoenecke et al. 1992), chrysanthemum (Kurilčik et al. 2008), and Arabidopsis (Pay et al. 2022); it was opposite for “Queen”, thus the phenomenon might depend on different spectrum absorption ranges because of the differences between different varieties of the same species (S. floribundum). However, the morphological indexes and light use efficiency of “Sweet Chico” and “Queen” showed a comparative advantage under composite light of R and B than under monochromatic R, B, and CK, and the indexes showed a trend rising first and then decreasing with the increase in the red light ratio, which was similar to the report by Kwon et al. (2015) and Hernández and Kubota (2016). The phenomenon showed that combined red and blue light might be regarded as a relatively good light quality to support the normal growth of S. floribundum (“Sweet Chico” and “Queen”).
Light quality had a significant impact on root induction, growth, and physiological activity (Lin et al. 2013; Y. Li et al. 2021). In previous studies on root growth and development, some researchers suggested that monochromatic red light could stimulate auxin translation from shoots to roots and promote root growth (Salisbury et al. 2007; Park and Kim 2010; Zeng et al. 2010; Wu et al. 2014); however, the view of Wang et al. (2017a) and Gil et al. (2021) was just the opposite; they reported that blue light accelerated root development. In this study, we found that monochromatic red light significantly inhibited the root activity of “Sweet Chico” and “Queen”; this result was similar to the study of butterfly orchids (Dai et al. 2010) and tomato seedlings (Li et al. 2017), but opposite to Salisbury et al. (2007), which may be due to differences between different species of plants. Zhang et al. (2015) also suggested that the combination of R and B lights could significantly improve plant biomass and plant activity. Similar to morphological indexes and photosynthetic pigments, the root activities of “Sweet Chico” and “Queen” under composite light of R and B were better than those under monochromatic light and CK, and they obtained the higher root activities under high red light ratio treatments. It manifested that composite light treatments by R/B could effectively mobilize the photoreceptors, mediate auxin, and other metabolic pathways and promote root growth and development.
Light quality could enhance the production of plants, especially red light and blue light could promote the biosynthesis and accumulation of plant carbohydrates (Gómez and Mitchell 2015; Hernández and Kubota 2016; Roso et al. 2020). The monochromatic red light was likely to accelerate the synthesis and accumulation of soluble sugar by increasing the content of chlorophyll and promoting the rate of photosynthesis, and monochromatic blue light might affect the photosynthetic carbon assimilation by impacting the chloroplast monochromatic structure, and then resulting in the lower soluble sugar content (Dai et al. 2004). We viewed a similar result in our study: “Sweet Chico” and “Queen” showed higher soluble sugar contents under monochromatic red light than under blue light. However, the soluble sugar contents of “Sweet Chico” and “Queen” in the composite light of R and B treatments were better than those in the monochromatic R and B treatments; they all obtained the largest value in the 7:3 treatment, and it showed a trend of rising first and then decreasing with the increase in the red light ratio, which was similar to the research on Nicotiana tabacum (Zhang et al. 2013). It is suggested that the appropriate proportion of red and blue light could increase the carbon accumulation.
Soluble protein is the main material of a variety of metabolic enzymes in plants (Gómez and Mitchell 2015; Hernández and Kubota 2016). The soluble protein contents of “Sweet Chico” and “Queen” were higher in the B than R treatment, which was opposite to the soluble sugar content in monochromatic light treatments, and similar to the research on cucumber and tobacco (Tang et al. 2011; Zhang et al. 2013). This implied that monochromatic red light might modify the nitrogen metabolism of plants. However, the soluble protein content indicated a similar trend under composite light treatments as soluble sugar content. “Sweet Chico” and “Queen” had the largest values under 7:3 and 8:2 treatments, respectively. We revealed that a high red light ratio within limits was conducive to the synthesis of soluble protein under combined light treatments.
Different light qualities can affect the metabolic and photosynthetic processes in plants, while also modulating the contents and activities of antioxidant enzymes (Simlat et al. 2016; Aalifar et al. 2020). A previous study revealed that monochromatic R light promotes SOD activities and inhibits POD activities (Simlat et al. 2016), but these findings contradict those of Kim et al. (2013) and Manivannan et al. (2015), possibly because of the differences in the materials and treatments among studies. In the current study, the SOD and POD activities of “Sweet Chico” and “Queen” were differentially affected by monochromatic R and B lights. However, the antioxidant enzyme activities were greater during the composite light treatments than during the monochromatic R and B treatments, which was in accordance with the morphological index, chlorophyll content, and photosynthetic parameters. The SOD and POD activities increased with increases in the R light proportion during the composite light treatments. Moreover, the analyzed enzyme activities were significantly higher in “Sweet Chico” than in “Queen”. These observations may be related to differences in the mechanisms underlying the responses of different varieties of the same species to light quality (Chen et al. 2014; Matsuda et al. 2016).
Earlier research demonstrated that plant pigments primarily absorb R and B light wavelengths, making them important for the morphogenesis of the photosynthetic system in plants (Wang et al. 2009; Edward 2015). Some studies indicated that monochromatic R light can inhibit the accumulation of chlorophyll and carotenoids (Barta et al. 1992; Matsuda et al. 2008; Johkan et al. 2010), whereas other studies revealed that monochromatic B light can interfere with chlorophyll biosynthesis (Heo et al. 2002; Urbonaviciute et al. 2007; Wang et al. 2017a). In the current study, the photosynthetic pigment contents of “Sweet Chico” and “Queen” were significantly higher during the R treatment than during the B treatment, which was similar to the reported results for lettuce (Urbonaviciute et al. 2007). These observations may reflect the beneficial effects of R light on morphogenesis as well as the synthesis and accumulation of photosynthetic pigments via light-induced transformations of the phytochrome system. Many studies have shown that combining R and B light treatments may positively affect the development of the photosynthetic apparatus and strongly influence plant architecture (Giliberto et al. 2005; Shao et al. 2015; Bożena et al. 2018). In our study involving “Sweet Chico” and “Queen,” similar to the changes in plant morphological traits (e.g., height, leaf number, and root number), the photosynthetic pigment contents were higher during the composite light treatments than during the monochromatic light and CK treatments. Specifically, the pigment contents initially increased and subsequently decreased as the R light content increased. Similar results were obtained in previous studies on cucumber (Wen et al. 2013) and tobacco (Shi et al. 2013). Hence, a high proportion of R light appears to contribute to the synthesis and accumulation of photosynthetic pigments in S. floribundum “Sweet Chico” and “Queen”. However, if the R light ratio exceeds a certain threshold, the excess R light may adversely affect photosynthetic pigment synthesis and accumulation via negative feedback.
Chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, qP, and NPQ) reflect changes in photosynthesis and have been used as internal indicators of the relationship between plant photosynthesis and environmental conditions (Maxwell and Johnson 2000; Lichtenthaler and Babani 2004; Wang et al. 2009; Wang et al. 2016). Additionally, Fv/Fm has been used to characterize the efficiency of the conversion of light energy by PSII, which is an important index for assessing photoinhibition (Daood et al. 1989; Reay et al. 2015; Akcin and Yalcin 2016). In contrast, qP represents the ability of PSII to perform photochemical reactions under the light as well as the relationship between photochemical electron transport and the light energy absorbed by the PSII antenna pigment. Moreover, NPQ is a mechanism by which plants protect themselves against excess excitation energy through carotenoids (Wang et al. 2010; Cao et al. 2013; Sawicki et al. 2017). Yang et al. (2017) reported that monochromatic light is not conducive to enhancing the fluorescence parameters of tobacco. In this study, the ΦPSII, qP, and Fv/Fm of “Sweet Chico” were generally better under monochromatic R light than under B light, whereas the opposite light effects were observed for NPQ. These findings suggest that B light has detrimental effects on the photosynthetic electron transport system of PSII and can damage PSII, which is consistent with the results of an earlier investigation by Han et al. (2011). However, in “Queen,” ΦPSII, qP, and Fv/Fm were higher during the B treatment than during the R treatment. Moreover, NPQ was higher under R light than under B light. Accordingly, the effective use of photosynthetic electrons may be inhibited by R light and promoted by B light. This possibility was also reported by Hoffmann et al. (2015), Wang et al. (2016), and Yang et al. (2017). Trouwborst et al. (2016) suggested that PSII functions better under monochromatic R light than under composite light consisting of R and B wavelengths. We found that in “Sweet Chico” and “Queen” ΦPSII and qP were higher during the composite light treatments than during the monochromatic light and CK treatments. Furthermore, NPQ was lowest in “Sweet Chico” during the 7:3 treatment and it was highest in “Queen” during the 8:2 treatment. Similar findings were reported by Hoffmann et al. (2015). We observed that the “Sweet Chico” leaves had a deeper color and were smoother than the “Queen” leaves, possibly because of the differences in the wax layer and leaf pigment content between the two examined cultivars. The diversity in the chloroplasts (i.e., photosynthetic organelles) of “Sweet Chico” and “Queen” may be associated with the differences in the adaptability of the cultivars to monochromatic R and B light treatments. The results presented herein imply that combining R and B light treatments can significantly improve the chlorophyll fluorescence parameters of plants; this improvement is mediated by a complex mechanism and is not simply a result of a superposition effect.
Photosynthetic parameters are closely related to photosynthetic pigment and chlorophyll fluorescence parameters. Increases in the photosynthetic pigment and chlorophyll fluorescence parameters will lead to increased photosynthetic efficiency in plants (Azmat 2013). We detected a similar trend in the photosynthetic pigment and chlorophyll fluorescence parameters in “Sweet Chico” and “Queen”. The data for “Sweet Chico” revealed that Pn, gs, and Ci were higher under monochromatic R light than under B light, whereas E was higher during the B treatment than during the R treatment. This was in accordance with the observed high photosynthetic pigment content. The relatively high chlorophyll content may increase the light absorption rate, which will lead to an increase in Pn. Regarding “Queen”, E and gs were highest during the B treatment and Pn, E, gs, and Ci were lower under R light than under B light. These results reflect a decrease in chlorophyll contents or other physiological indicators. Similar results were obtained in earlier studies by Lin et al. (2013) and Liang and Labeke (2017). Additionally, previous research demonstrated that B light induces stomatal conductance, promotes the opening of stomata, and initiates the activation of phototropins (Inoue et al. 2010; Lim and Kim 2021). Monochromatic R light apparently has inhibitory effects on some photosynthetic indices. This is in contrast to the positive effects of the composite R and B light treatment, which were consistent with the changes in the chlorophyll fluorescence parameters. Therefore, R and B lights applied together are likely better for plant photosynthetic activities than monochromatic light treatments. The absence of R or B light results in photosynthetic inefficiencies. Notably, the beneficial effects of the composite light treatment on photosynthesis are associated with a complex mechanism and are not simply due to the additive effects of the R and B light treatments.
Conclusion
S. floribundum can grow normally under different low-light environmental conditions provided by LEDs. Regarding “Sweet Chico” and “Queen”, B light can improve root activity and facilitate the accumulation of soluble proteins, whereas R light may promote the synthesis and accumulation of photosynthetic pigments. In this study, the morphological index, soluble sugar and chlorophyll contents, and photosynthetic activities were best during the 7:3 treatment, with “Sweet Chico” performing better than “Queen”. A high R light ratio promoted the production of pigments. The resulting increase in the synthesis and accumulation of photosynthetic products led to improved stress resistance, light use efficiency, and growth. The differences between “Sweet Chico” and “Queen” may have been due to the diversity in their light absorption efficiency, which may be attributed to leaf structural differences between the two cultivars. The specific internal mechanism underlying the effects of light quality on S. floribundum growth and development has not been fully characterized. Future related research should focus on plant metabolism and molecular biology.
Acknowledgements
We thank William Yajima, Ph.D., from Liwen Bianji, Edanz Group China ( www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
Author contributions
ZW and SH conceived and designed the experiment; YS and WS performed the experiment, analyzed the data and wrote this paper; LS, YS, XL, and YS analyzed the data of this research.
Funding Information
This study was financially supported by the National Key R&D Program of China (grant no. 2020YFD1000503), the University-Industry Cooperation Foundation of Henan Province (grant no. 162107000068), and the Science and Technology Program of Shanghai (grant no. 21DZ1202000).
