Effect of Different Monochromatic LEDs on the Environmental Adaptability of Spathiphyllum floribundum and Chrysanthemum morifolium

Light-emitting diodes (LEDs) can be programmed to provide specialized light sources and spectra for plant growth. UV-A (397.6 nm), blue (460.6 nm), green (520.7 nm), and red (661.9 nm) LED light sources were used to study the effects of different monochromatic lights on the growth, antioxidant system, and photosynthetic characteristics of Spathiphyllum floribundum ‘Tian Jiao’ (a shade-loving species) and Chrysanthemum morifolium ‘Huang Xiu Qiu’ (a sun-loving species). This research revealed that green and blue light could enhance the morphological indicators, Chl a/b, photosynthetic electron transfer chain performance, and photosystem activity of S. floribundum, blue and red light could enhance the solution protein, Chl a, and photosynthetic electron transfer chain performance of C. morifolium, red and UV-A light viewed the highest SOD and CAT activities of S. floribundum (275.56 U·min·g−1; 148.33 U·min·g−1) and C. morifolium (587.03 U·min·g−1; 98.33 U·min·g−1), respectively. Blue and green light were more suitable for the growth and development of the shade-loving plant S. floribundum, while red and blue light were more suitable for the sun-loving plant C. morifolium. UV-A light could be used for their stress research. The research revealed the different adaptation mechanism of different plants to light environmental conditions.


Introduction
Light is one of the most important environmental factors in plant growth and development [1,2]. As an important signal and energy source, light regulates plant photomorphogenesis and photosynthesis [3][4][5]. The fluorescent, metal halide, high-pressure sodium, or incandescent lamps often used to plant growth in commercial production, are costly, inefficient, and cannot be set to a single spectrum of light quality, so they produce light wavelengths that interfere with plant growth and differentiation [6]. The advancement and wider availability of next-generation energy-efficient, solid-state lighting offers the opportunity to use this technology in large-scale applications. Light-emitting diode (LED) technology has proven to be ideal in plant lighting according to its small mass/volume ratio, low energy consumption, long life, and various monochromatic spectrum [7,8]. Concomitantly, there is increasing research on plant growth and development both in monochromatic and polychromatic light environments by LED technologies [7][8][9][10].

Plant Growth
After 60 days of growth, 15 plants were selected from each treatment for determination of fresh weight, dry weight, plant height, leaf number, and maximum root length. The plant height and root length were measured by the vernier scale. After the fresh weight was determined, the material was dried in an oven at 105 °C for 30 min, then

Plant Growth
After 60 days of growth, 15 plants were selected from each treatment for determination of fresh weight, dry weight, plant height, leaf number, and maximum root length. The plant height and root length were measured by the vernier scale. After the fresh weight was determined, the material was dried in an oven at 105 • C for 30 min, then at 60 • C for 48 h, and then the dry weight was determined [38], both using a 1/10,000 balance scale (FA2104B, Shanghai Precision Scientific Instrument Co., Ltd., Shanghai, China). The fresh weight/dry weight ratio was calculated.

Photosynthetic Pigments
Photosynthetic pigments from leaf samples were determined according to the method of Holm [39], 0.2 g of chopped leaves were placed in 20 mL of a 1:1 (v/v) mixture of 80% acetone (acetone:water = 80:20, v/v) and absolute ethyl alcohol in a 25 mL stoppered test tube in the dark for 24 h. Using 80% acetone as the blank, the absorbance (OD) was measured at λ = 663, 645 and 470 nm using a UV spectrophotometer (UV-1900, Shimadzu, Tokyo, Japan). The chlorophyll a/b value was calculated.

Soluble Sugar and Soluble Protein
Soluble sugars were extracted using the method of Fairbairn [40] with slight modifications. Leaves (0.2 g) were put into a test tube, to which 10 mL of distilled water was added and mixed. After 30 min in a water bath at 100 • C, the supernatant was collected, and distilled water was added to a volume of 25 mL. The soluble sugar content was determined with the sulfuric acid anthrone method at a wavelength of 620 nm using a UV spectrophotometer (UV-1900, Shimadzu, Tokyo, Japan).
Leaves (0.2 g) were placed in a mortar, flash frozen with liquid nitrogen, and ground to a powder. Phosphate buffer (5 mL; 50 mM, pH 7.0) was added, and the homogenate was centrifuged at 12,000 rpm for 4 min at 4 • C. The supernatant was used for determination of soluble protein content according to the method of Bradford [41], using 0.1 mL of the supernatant and 4.9 mL of Coomassie Brilliant Blue G-250 (0.1 g·L −1 ). After 2 min, the absorbance was measured at 595 nm using a UV spectrophotometer (UV-1900, Shimadzu, Tokyo, Japan), and the soluble protein content was calculated using a standard curve (bovine serum albumin was used to make the standard curve).

Antioxidant Enzyme Activities
Activities of four antioxidant enzymes were determined by spectrophotometric methods. Leaf samples (0.2 g) were ground in a mortar with liquid nitrogen, to which 5 mL of phosphate buffer (50 mM, pH 7.8) containing 1% polyvinyl pyrrolidone (PVP), 0.2 mM ethylene diamine tetraacetic acid (EDTA), and 5 mM ascorbic acid (ASA) was added. The slurry was centrifuged at 12,000 rpm at 4 • C for 20 min. The supernatant was used for determination of the enzyme activities.
Superoxide dismutase (SOD) activity was measured by photochemical reduction of nitrotetrazolium blue chloride (NBT) [42]. The enzyme extract from above (0.05 mL) was mixed with 1.5 mL phosphoric acid buffer (50 mM, pH 7.8), 0.3 mL Met solution (130 mM), 0.3 mL NBT solution (0.75 mM), 0.3 mL EDTA solution (0.1 mM), 0.3 mL riboflavin solution (0.02 mM), and 0.25 mL distilled water in the reaction tube. The control reaction contained enzyme extract and only 0.05 mL phosphoric acid buffer. After mixing, one reaction tube was placed in the dark as a positive control, and the rest of the reaction tubes were centrifuged at 12,000 rpm for 20 min. At the end of the reaction, the absorbance value was measured at 560 nm wavelength, and 1 unit (U) of SOD activity was the amount of enzyme that inhibited the photochemical reduction of NBT by 50%.
The analysis of peroxidase (POD) activity was based on the oxidation of guaiacol using H 2 O 2 according to the method described by Zhang and Kirham [43]. The reaction mixture consisted of buffered pyrogallol in 0.1 M potassium phosphate buffer (pH 7.0), 1% H 2 O 2 , and the enzyme extract. The reaction rate was calculated by monitoring the changes in absorbance at 430 nm (ε = 12 mM −1 ·cm −1 ), with the activity expressed as U·mg −1 (protein)·min −1 .
Catalase (CAT) activity was measured according to the method described by Cakmak and Marschner [44] by reading the absorbance value of the reaction solution containing phosphate buffer (25 mM, pH 7.8), H 2 O 2 (10 mM), and enzyme solution (0.2 mL) at 240 nm. As such, 1 U CAT activity was the amount of enzyme that reduced the OD 240 value by 0.01 per min.
Ascorbate peroxidase (APX) activity was determined with reference to the Nakano and Asada [45] ascorbate (Asc) method. The reaction mixture consisted of 50 mM phosphoric acid buffer (pH 7.0), 5 mM Asc, 20 mM H 2 O 2 , and 5 mM EDTA. After the enzyme extract was added to the above mixture, the absorption values were immediately measured at 240 nm over 2 min at 20 s intervals.

Chlorophyll Fluorescence
The fast chlorophyll fluorescence induction kinetic curves (OJIP curve) of C. morifolium and S. floribundum were assayed by a multifunctional plant efficiency instrument (m-pea-1, Hansatech Instruments Ltd., Norfolk, UK), the plants should be dark-adapted for 30 min before they are measured. The OJIP curve was induced by a 3000 mol·m −2 ·s −1 pulse of light, and the fluorescence signal was detected from 10 positions for 1 s, with an initial rate of 105 data points per second. Then, we analyzed the OJIP curve using the jip-test [46]. The following fluorescence parameters were obtained: the light energy absorbed by the unit reaction center (ABS/RC); the energy (TR o /RC) captured by the unit reaction center for the reduction of QA; the energy captured by the unit reaction center for electron transport (ET o /RC); the energy dissipated by unit reaction center (DI o /RC); the probability that the captured exciton would transfer electrons to other electron receptors in the electron transport chain that exceed QA (ψ o ); the quantum yield of the terminal electron acceptor phi Ro (ϕ Ro ); the quantum ratio for heat dissipation (ϕ Do ); the quantum yield for electron transfer (ϕ Eo ); the number of reactive reaction centers per unit area (RC/CS o ); the performance index based on absorption of light energy (PI abs ); and the comprehensive performance index (PI total ). The measurements were repeated 5 times per species for each light condition.

Statistical Analysis
Data underwent a one-way analysis of variance (ANOVA), and significant differences between the means were tested using Duncan's post-hoc test (p ≤ 0.05) with the SPSS 19.0 software (IBM, Inc., Chicago, IL, USA).

Growth Characteristics
After 60 days of cultivation under different monochromatic light spectra, LED light quality did result in significant differences in total fresh weight, total dry weight, plant height, leaf number, and maximum root length in both S. floribundum and C. morifolium (Table 1, Figure 2). The total fresh weight values for both species were significantly greater under blue LED than under green (by 81.80% for S. floribundum and 27.20% for C. morifolium), red (97.11% and 39.16%), or UV-A (272.64% and 309.84%). The total dry weight values of S. floribundum and C. morifolium were also significantly greater under blue LED than under either green (by 95.56% and 32.37%), red (by 57.14% and 33.33%), or under UV-A (by 282.61% and 360.00%). The results showed that blue light was more favorable to S. floribundum and C. morifolium biomass accumulation than other monochromatic light, while UV-A light was not favorable to biomass accumulation. The height of S. floribundum was greatest under blue LED, followed by green LED, which both yielded plants significantly taller than UV-A or red LEDs. The maximum height value for C. morifolium also occurred under blue LED, followed by the green and red spectra. The results showed that blue light was more favorable to stem elongation in both S. floribundum and C. morifolium than other monochromatic light. The number of C. morifolium leaves was greatest in both red and blue LED. The results showed that blue light significantly promoted the number of leaves in S. floribundum, while red light and blue light increased the number of leaves in C. morifolium. UV-A light resulted in the lowest number of leaves in S. floribundum and C. morifolium among these monochromatic light treatments. The maximum root length values for both S. floribundum and C. morifolium occurred when the plants were grown under the green LED, followed by the blue and red LEDs, with the minimum values under the UV-A LED.

Soluble Sugars, Soluble Proteins, and Chlorophyll
Growth under different wavelengths of monochromatic light affected the synthesis and accumulation of soluble sugar and protein in S. floribundum and C. morifolium plants ( Table 2). The soluble sugar content in S. floribundum was significantly different under each monochromatic light, with significantly higher content in plants grown under red or blue LED than under green (by 140.24% and 84.62%) or UV-A (by 301.98% and 208.91%) The soluble sugar content in C. morifolium under different light showed the opposite effect, that is, the soluble sugar content under green or UV-A LED were higher than those

Soluble Sugars, Soluble Proteins, and Chlorophyll
Growth under different wavelengths of monochromatic light affected the synthesis and accumulation of soluble sugar and protein in S. floribundum and C. morifolium plants ( Table 2). The soluble sugar content in S. floribundum was significantly different under each monochromatic light, with significantly higher content in plants grown under red or blue LED than under green (by 140.24% and 84.62%) or UV-A (by 301.98% and 208.91%). The soluble sugar content in C. morifolium under different light showed the opposite effect, that is, the soluble sugar content under green or UV-A LED were higher than those under blue or red LED. It indicated that the synthesis and accumulation of soluble sugar in the two plants are different in response to monochromatic radiation. The highest soluble protein level in S. floribundum was under red LED, followed by blue LED. The soluble protein level in C. morifolium was the highest under blue LED, followed by red LED, which were significantly higher than the levels under green LED. The results showed that red and blue light better promoted the synthesis of soluble proteins in both S. floribundum and C. morifolium than green or UV-A light. Table 2. Content of basic metabolites and photosynthetic pigments in S. floribundum and C. morifolium grown under UV-A, blue, green, or red single-spectrum LEDs for 60 days.

Species
Light Quality  The content of chlorophyll a and chlorophyll b in S. floribundum was higher under red or green LED than under blue or UV-A spectra. Red and green light promoted the synthesis of chlorophyll a and b in S. floribundum, while blue light promoted the synthesis of chlorophyll a and b in C. morifolium. UV-A light inhibited the synthesis of both chlorophyll pigments in both S. floribundum and C. morifolium compared with the other monochromatic lights. In S. floribundum, there were no significant differences in the content of carotenoids or the chlorophyll a/b ratio under the different light treatments, while red light did yield a significantly higher carotenoid content and chlorophyll a/b ratio in C. morifolium.

Activity Levels of Antioxidant Enzyme
In both S. floribundum and C. morifolium, SOD, CAT, POD, and APX showed varying degrees of activities under the different bands of monochromatic light (Figure 3). In S. floribundum, every light condition except blue LED induced the highest activity of at least one enzyme. SOD activity was highest under red LED and lowest under blue LED. CAT activity was highest under red LED and lowest under green LED. POD activity was significantly higher under green LED than under any of the other monochromatic light treatments, while APX activity was highest under UV-A treatment, and lowest in red treatment. In C. morifolium, SOD and CAT activities were highest under UV-A LED, and lowest under red or blue LED. POD activity was highest under red LED, and lowest with UV-A treatment. Green LED yielded the highest activity APX, while blue LED the lowest. In C. morifolium leaves, UV-A LED best enhanced SOD and CAT activity, red LED best promoted POD activity, and green LED most increased APX activity. treatments, while APX activity was highest under UV-A treatment, and lowest in red treatment. In C. morifolium, SOD and CAT activities were highest under UV-A LED, and lowest under red or blue LED. POD activity was highest under red LED, and lowest with UV-A treatment. Green LED yielded the highest activity APX, while blue LED the lowest. In C. morifolium leaves, UV-A LED best enhanced SOD and CAT activity, red LED best promoted POD activity, and green LED most increased APX activity.

Photosynthetic Characteristics
In both S. floribundum and C. morifolium, monochromatic light had differential effects on the net photosynthetic rate (Pn), intercellular CO 2 concentration (Ci), transpiration rate (Tr), stomatal conductance (Gs), vapor pressure deficit (VPD), and water use efficiency (WUE) (Figure 4). The lack of significant effect of different LED spectra on vapor pressure deficit was the only similar response between the two species ( Figure 4E). These results showed that the monochromatic light had no significant effect on the vapor pressure deficit of S. floribundum and C. morifolium. All other measured photosynthetic parameters showed different effects under the different light spectra between the two species.
In S. floribundum, the Pn was significantly higher under blue light than under the other monochromatic light treatments ( Figure 4A). But the Ci was higher under UV-A and red light ( Figure 4B). The Tr and Gs were highest under blue light ( Figure 4C,D), similar to that of the Pn. Together, these parameters indicate that blue light has a significant effect on photosynthesis in the leaves of S. floribundum, and that as the Pn increases, so does the Tr through opening the stomata, which leads to a decrease in Ci. The maximum WUE in S. floribundum was significantly higher under blue and green light, as compared to UV-A light, but was still the highest under blue light treatment. While blue and green light had a stronger effect on WUE, blue light was still more effective at promoting photosynthesis, possibly because shade plants like S. floribundum prefer blue light.
In C. morifolium leaves, the Pn was significantly higher under green light than that of blue, and green and blue light significantly promoted photosynthesis, while red light increased it compared to UV-A light ( Figure 4A). Again, the Ci response under the different light qualities was opposite to the Pn ( Figure 4B). The highest Tr and maximum Gs appeared under red light treatment, followed by green treatment, both of which were significantly higher than blue and UV-A treatment. Again, the WUE in C. morifolium was similar to the Pn, with the largest value under green light treatment, followed by blue light treatment. In the sun-loving C. morifolium, green light had a stronger effect on Pn and WUE, red and green light could promote Tr and Gs, and red light increased the Ci, again an opposite response compared to the Pn. ts 2023, 12, 2964 9 of efficiency (WUE) (Figure 4). The lack of significant effect of different LED spectra vapor pressure deficit was the only similar response between the two species ( Figure 4 These results showed that the monochromatic light had no significant effect on the vap pressure deficit of S. floribundum and C. morifolium. All other measured photosynthe parameters showed different effects under the different light spectra between the tw species. In S. floribundum, the Pn was significantly higher under blue light than under t other monochromatic light treatments ( Figure 4A). But the Ci was higher under UV and red light ( Figure 4B). The Tr and Gs were highest under blue light ( Figure 4C,D similar to that of the Pn. Together, these parameters indicate that blue light has significant effect on photosynthesis in the leaves of S. floribundum, and that as the increases, so does the Tr through opening the stomata, which leads to a decrease in The maximum WUE in S. floribundum was significantly higher under blue and gre light, as compared to UV-A light, but was still the highest under blue light treatme While blue and green light had a stronger effect on WUE, blue light was still mo effective at promoting photosynthesis, possibly because shade plants like S. floribund prefer blue light. In C. morifolium leaves, the Pn was significantly higher under green light than that blue, and green and blue light significantly promoted photosynthesis, while red lig increased it compared to UV-A light ( Figure 4A). Again, the Ci response under t different light qualities was opposite to the Pn ( Figure 4B). The highest Tr and maximu Gs appeared under red light treatment, followed by green treatment, both of which we significantly higher than blue and UV-A treatment. Again, the WUE in C. morifolium w

Fast Chlorophyll Fluorescence
Different monochromatic light qualities had different effects on the fast chlorophyll fluorescence parameters of S. floribundum and C. morifolium (Table 3, Figure 5). Under red LED, the electron transfer probability (ψ o ) of the Q A -downstream and the quantum yield for electron transfer (ϕ Eo ) in the electron transfer chain of S. floribundum leaves was greatly reduced, and that was used to reduce the quantum yield of terminal electron acceptor at the PS I receptor side (ϕ Ro ) and the quantum ratio (ϕ Do ) used for heat dissipation was significantly increased, eventually resulting in a decrease in maximum photochemical efficiency (ϕ Po ). This indicated that, in S. floribundum under red light, electron transfer is severely inhibited, the quantum yield and photochemical efficiency are significantly decreased, and the relative activity of PS I is significantly affected.
The rapid chlorophyll fluorescence parameters of S. floribundum leaves under green and blue lights were exactly the opposite of those under red light treatment, that is, ψ o , ϕ Eo , and ϕ Ro at the PS I receptor side were significantly increased, while ϕ Do was significantly reduced. Finally, ϕ Po was increased. The results showed that the electron transfer of S. floribundum under green and blue light were severely promoted, and the quantum yield and photochemical efficiency were significantly increased. quantum yield and photochemical efficiency were significantly increased. In C. morifolium, ψo and ϕEo was significantly lower under UV-A treatment than under the other monochromatic light bands. ϕRo was the lowest, ϕDo was the highest. Under blue light treatment, ϕEo, ψo, and ϕPo were the highest, while ϕDo was lowest. Table 3. Quantum yields and efficiencies/probabilities in the leaves of S. floribundum and C. morifolium grown under UV-A, blue, green, or red LEDs for 60 days.

Species
Light  In C. morifolium, ψ o and ϕ Eo was significantly lower under UV-A treatment than under the other monochromatic light bands. ϕ Ro was the lowest, ϕ Do was the highest. Under blue light treatment, ϕ Eo , ψ o , and ϕ Po were the highest, while ϕ Do was lowest.
In S. floribundum, red light was significantly higher than the light energy absorbed by the unit reaction center (ABS/RC), the light energy dissipated per unit reaction center (DI o /RC), and the light energy captured by the unit reaction center (TR o /RC) was compared to that under blue and green light (Table 4). However, under blue and green light, the light energy used for electron transfer in the unit reaction center (ET o /RC) and the light energy delivered to the PS I in the unit reaction center (RE o /RC) of S. floribundum were not significantly different with that under red light. Under blue and green light, the performance index (PI abs ) and the comprehensive performance index, based on the absorption of light energy (PI total ) of S. floribundum were significantly higher than under UV-A or red light treatment.

Correlation Analysis
The correlation analysis of physiological indicators in S. floribundum was shown in Figure 6A. The morphological indicators had a significant positive correlation with Chl a/b, Pn, Tr, Gs, WUE, ψ o , ϕ Eo , ϕ Ro , ET o /RC, RE o /RC, PI abs , and PI total , but they showed a significant negative correlation with SOD and Ci. Soluble sugar and soluble protein had a significant positive correlation with Chl a, Chl b, CAT, ϕ Do , ABS/RC, and DI o /RC, and they were significantly negatively correlated with carotenoid, APX, and ϕ Po . Chl a and Chl b showed a significant positive correlation with VPD, ϕ Do , ABS/RC, and DI o /RC, and they were significantly negatively correlated with carotenoid, APX, and ϕ Po . The carotenoid was significantly positively correlated with APX and Ci. The SOD activity was significantly positively correlated with Ci, VPD, ABS/RC, DI o /RC, and TR o /RC, and significantly negatively correlated with Chl a/b, Pn, Tr, Gs, WUE, ψ o , ϕ Eo , ϕ Ro , ET o /RC, RE o /RC, PI abs , and PI total . The CAT showed a similar correlation trend with SOD, but the POD overall showed an opposite trend. The Pn, Tr, and Gs were significantly positively correlated with Chl a/b, WUE, and RE o /RC, and they significantly negatively correlated with SOD and Ci. The correlation analysis of physiological indicators in C. morifolium was shown in Figure 6B. The morphological indicators (excluding root length) were significantly positively correlated with soluble protein, Chl a, Chl b, POD, Pn, ψ o , ϕ Eo , ϕ Ro , RE o /RC, PI abs and PI total , but they showed a significant negative correlation with SOD, CAT, Ci, ϕ Do , ABS/RC, DI o /RC, and TR o /RC. Soluble sugar had a significant positive correlation with WUE, ϕ Do , ABS/RC, DI o /RC, TR o /RC, and ET o /RC, and significantly negatively correlated with Ci, ϕ Po , ϕ Ro , PI abs and PI total . The correlation between soluble protein and various indicators overall showed a opposite trend with soluble sugars. Chl a and Chl b were significantly positively correlated with soluble protein, POD, Pn, ϕ Po , ψ o , ϕ Eo , ϕ Ro , RE o /RC, PI abs and PI total , but they showed a significant negative correlation with SOD, CAT,

Discussion
Recent studies have used the LED light technologies to test how plants respond to monochromatic light, because low costs and wide availability of these technologies open up the possibility of culturing an entire greenhouse under augmented light spectra [29,47]. For instance, in cherry tomato seedlings, the fresh and dry weight of plant buds and roots, the leaf area, and the stem diameter were greater under single-spectrum green light than under other monochromatic light treatments [48]. Alvarenga et al. [27] studied the growth and development of tissue culture plantlets of Achillea millefolium by red light, blue light, white light, green light, and far-red light. The fresh and dry weights, plant height, and leaf number were greater for plants grown under blue light compared with the other monochromatic spectra. In this experiment, the fresh weight, dry weight, and plant height of S. floribundum and C. morifolium were the greatest under blue light. These qualities were significantly higher in plants grown under blue light than under the other monochromatic light treatments, with UV-A treatment producing the smallest plants by all measures. Seeing the best growth under blue light was similar to results for lettuce [49], Achillea millefolium [27], and Rehmannia glutinosa [50]. This phenomenon was similar to related research in Nicotiana tabacum and Pisum sativum [51,52]. It may be due to differences in the effects of different wavelengths of light on the functional status of photosynthetic organs and pigment protein synthesis, while blue light can participate in and actively coordinate the functional relationship between the nucleus and plastids. The growth quality of S. floribundum and C. morifolium under green light is only inferior to blue light, indicating that green light plays an important role in promoting the morphogenesis of the two plants. It may be that green light is also involved in the synergistic regulation of chlorophyll, which resulted in a higher photosynthetic efficiency.
Light quality directly affects the synthesis of photosynthetic pigments, which affect plant photosynthesis, synthesis, and the accumulation of metabolites [53,54]. Some studies have shown that blue light affects the content of chlorophyll in leaves [12,55]. The contents of chlorophyll a and chlorophyll b in C. morifolium leaves in this experiment were higher under blue light, which is consistent with the result on hemerocallis [56]. The contents of chlorophyll a and chlorophyll b in S. floribundum leaves were lower under blue light than that of red or green light. But the net photosynthetic rate was higher under blue light. This phenomenon was similar to the research on Lactuca sativa [57], it may be due to the fact that plants with less chlorophyll content are more effective at using chlorophyll than plants with excess chlorophyll. S. floribundum showed more chlorophyll under red and green light, possibly to compensate for the decrease in photosynthetic rate caused by insufficient chlorophyll activity. The content of soluble protein and soluble sugar in S. floribundum is the highest under red light. C. morifolium achieve their maximum values under blue and green light, respectively. Both the changing trends chlorophyll, soluble protein, and soluble sugar contents in C. morifolium and S. floribundum all perfectly interpret the dynamic regulation mechanism of photoresponse systems under different monochromatic light conditions.
Plants have complex and dynamic photoresponse systems, involving reactive oxygen and hormonal signals, for optimizing light adaptation and defense systems [58]. As a stimulating factor, light activates antioxidant defense systems in plants and synthesizes antioxidants [13]. As the first line of defense in antioxidant enzyme systems, SOD specifically converts superoxide anion radicals into hydrogen peroxide and oxygen molecules, and several studies have shown that elevated SOD activity is associated with plant tolerance to environmental stress [59,60]. In C. morifolium grown under UV-A light, SOD activity was higher, creating a higher active oxygen scavenging ability. It may be that UV-A light stimulates plants to produce excess superoxide anion, which has been recognized as a signaling factor induced by antioxidant enzymes that stimulates the production of SOD. The UV-A light also induced high CAT activity in C. morifolium. This high CAT activity is likely in response to the higher SOD activity under UV-A light, because the conversion of superoxide anion radicals by SOD produces a large amount of hydrogen peroxide, which is removed by CAT. But S. floribundum viewed the maximum value of SOD and CAT under red light. As a highly active, adaptive enzyme, POD can reflect the characteristics of plant growth and development, the metabolic state, and the adaptability to the external environment [61]. The POD activity in S. floribundum was stronger under green light, while the POD activity in C. morifolium was stronger under red light. This indicates that S. floribundum was more adaptable to the environment under green light (such as what filters through the canopy of plants above this shade-tolerant species), while C. morifolium was more suitable for growth under red light (which it would be exposed to as a sun-tolerant species). This phenomenon could be interpreted using the research of cowpea [62], which showed that an increase in POD activity may be primarily involved in regulating plant growth, rather than protecting plant tissues from oxidative damage caused by hydrogen peroxide. APX is a key enzyme in the ascorbate-glutathione cycle, using electrons from photosynthetic organs or NADPH as a reducing force to remove H 2 O 2 from chloroplasts and to modulate the signaling of reactive oxygen intermediates [60]. APX activity was higher under UV-A light in S. floribundum, but higher under green light in C. morifolium, indicating that S. floribundum had a greater ability to scavenge ROS in chloroplasts under UV-A light, while C. morifolium had a stronger ability to clear ROS under green light.
Plant photosynthetic regulation mainly includes regulating photosynthetic pigments, chloroplast structure, and stomatal movement [1,63]. It was found that blue light can affect plant photosynthesis by inducing the opening of stomata [54,64,65]. In this experiment, blue light significantly increased the net photosynthetic rate, transpiration rate, and stomatal conductance of the leaves of S. floribundum, mainly because blue light induced stomatal opening. The chlorophyll content in S. floribundum leaves was the highest under red light, but the net photosynthetic rate was significantly decreased. This may be because the soluble sugar content of S. floribundum leaves was higher under red light treatment, and the photosynthetic products were inhibited from being exported from leaves, resulting in a significant decrease in photosynthetic rate through negative feedback. In addition, the stomatal conductance in S. floribundum leaves decreased significantly under red light, while the intercellular CO 2 concentration increased significantly, indicating that the utilization efficiency of carbon dioxide was lower under red light and that the limiting factors for photosynthetic rate decline are non-stomatal factors. This phenomenon was similar with the research by Farquhar [66], and it may be due to a decrease in the photosynthetic activity of mesophyll cells. Most studies have found that plant leaves absorb very little green light, so green light is often considered ineffective for plant photosynthesis and not conducive to plant growth [16,[24][25][26]. However, relevant studies have shown that green light can affect the proportion and state of light distribution in plant leaves, participating in the regulation process of photosynthesis [67][68][69]. In this experiment, the net photosynthetic rate of C. morifolium under green light was significantly higher than under other monochromatic light, probably because red light and blue light were effectively absorbed by the plant surface, while green light may have reduced the potential light gradient inside the leaf and provided the energy for photosynthesis in deeper layers. Researchers have shown that chloroplast decline would occur because part of the membrane system was destroyed and because stomatal conductance was decreased, which would increase the intercellular carbon dioxide concentration, especially because carbon dioxide fixation was blocked, and a large amount of reactive oxygen species would accumulate [19,22,70]. Compared with other monochromatic light, the net photosynthetic rates of S. floribundum and C. morifolium were the lowest under UV-A light, which may be caused by UV-A light irradiation destroying the chloroplast system, causing the chlorophyll a, chlorophyll b, and chlorophyll a/b values to decrease, and ultimately the number of chloroplasts to decrease. Although the activity of POD and CAT in S. floribundum and C. morifolium leaves was higher under UV-A light, it was likely not enough to remove the excess active oxygen species in the plant, resulting in blocked photosynthetic electron transport and decreased net photosynthetic rate, which makes the plants unable to grow normally.
The photoreaction phase of photosynthesis mainly absorbs light energy and converts it into active chemical energy through photosystem II (PSII) and photosystem I (PSI). Photosystem II (PSII) is located on the thylakoid membranes and is the primary site for photosynthesis. Its performance directly determines the light energy utilization efficiency and photosynthesis of leaves [45,71]. The rapid chlorophyll fluorescence kinetics curve (JIP test) is based on the energy flow principle, and can quantitatively explain PSII light energy absorption, conversion, electron transport, PSII action center activity on the receptor side and donor side, and the dynamic changes of the redox state of the electron transporter [45,72]. In the research, it showed that ultraviolet light caused the PSI reaction center (ϕ Ro ) in S. floribundum and C. morifolium leaves to decrease, while the maximum photochemical efficiency (ϕ Po ) of the PSII reaction center was not significantly different among the monochromatic light treatments, indicating that the PSI reaction center of S. floribundum and C. morifolium leaves was more UV-sensitive than the PSII reaction center. In S. floribundum leaves under red light, the light energy and the captured light energy absorbed by the unit reaction center and the dissipated light energy increased, while the electron transport capacity of the PSII receptor side decreased and heat loss increased, indicating that red light suppresses PSII electron transfer from the primary receptor (Q A to Q B ), resulting in a large accumulation of electrons at Q A in S. floribundum. However, green and blue light could significantly improve the photosynthetic electron transfer chain performance and net photosynthetic rate of S. floribundum; blue and red light carried out the same for C. morifolium.
The correlation analysis results also showed that morphological indicators of S. floribundum and C. morifolium were all significantly positively correlated with Pn, Tr, Gs, ψ o , ϕ Eo , ϕ Ro , RE o /RC, PI abs and PI total . It indicated that the growth strategies of S. floribundum and C. morifolium under different monochromatic light conditions were closely related to their utilization efficiency of light energy. At the same time, the morphological indicators of S. floribundum showed a significant positive correlation with Chl a/b, but the morphological indicators of C. morifolium showed a significant positive correlation with Chl a and Chl b. It was consistent with the phenomenon that S. floribundum viewed the greatest overground morphological indicators, net photosynthetic rate, transpiration rate, and stomatal conductance of the leaves under blue light, C. morifolium showed the highest chlorophyll content and photosynthetic electron transfer chain performance under red light. This phenomenon might be due to the fact that S. floribundum was a shade-loving plant, C. morifolium was a sun-loving plant, and shade-loving plants could better utilize blue light, while sun-loving plants had a higher utilization rate of red light [73,74]. Meanwhile, there was also a significant positive correlation between indicators such as chlorophyll content, soluble sugar, soluble protein, antioxidant enzyme activity, and photosynthetic fluorescence parameters of S. floribundum and C. morifolium. This indicated the complexity of light utilization strategies and efficiency of S. floribundum and C. morifolium under different monochromatic light conditions.

Conclusions
This research revealed that green and blue light could enhance the morphological indicators, Chl a/b, photosynthetic electron transfer chain performance, and photosystem activity of S. floribundum, blue and red light could enhance the solution protein, Chl a, and photosynthetic electron transfer chain performance of C. morifolium, red and UV-A light could promote SOD and CAT enzyme activities of S. floribundum and C. morifolium, respectively. This result indicated that blue and green lights were more suitable for the growth and development of the shade-loving plant S. floribundum, while red and blue lights were more suitable for the sun-loving plant C. morifolium. UV-A light could be used for their resistance research. Overall, the shade-loving plant S. floribundum could fully utilize blue light for photosynthetic metabolism according to its own characteristics, while the sun-loving plant C. morifolium could also fully utilize its own advantages to regulate the growth and development process using red light. They could adjust indicators in a timely manner according to light environmental conditions to fully utilize light energy. The research could provide technical and theoretical support for plant factory and plant photo-physiological regulation, however, the specific mechanism was still unclear and needs to be further studied.