Far-Red Light Inhibits Soybean Biomass and Yield by Modulating Plant Photosynthesis

Abstract

Alterations in the light environment can significantly influence soybean morphology and yield formation; however, the effects and mechanisms of different light qualities on these aspects require further investigation. Consequently, we selected soybean cultivars with marked differences in light sensitivity as test materials, conducted experiments with red, blue, and green light qualities against a blue light background, and analyzed parameters related to leaf photosynthetic capacity, chlorophyll fluorescence, morphological characteristics, biomass, and yield variations following different light quality treatments. The results showed that following far-red light treatment, soybean plants exhibited significant shade avoidance syndrome, internode elongation, increased plant height, and a marked reduction in both root and leaf biomass, as well as total biomass. Furthermore, there was a substantial reduction in photosynthetic capacity. This indicated that far-red light exerts an inhibitory effect on soybean growth and yield formation. Red light has basically no regulatory effect on plant morphology and yield, while green light has a yield-increasing effect, but there was a cultivar effect. This study not only enhances our understanding of the mechanisms through which light quality regulates plant photosynthesis but also lays a scientific foundation for future crop light environment management and for the further exploration of light quality’s regulatory potential on crop growth.

1. Introduction

Shade avoidance syndrome (SAS) is a plant-specific phenotypic plasticity response to intraspecific competition and changes in light conditions. Research on the SAS has primarily focused on model dicot species like Arabidopsis thaliana, with limited knowledge in agronomically important grasses [1]. SAS involves a series of responses to lower photosynthetically active radiation (PAR) and the red to far-red light ratio (R:FR), leading to increased plant height and other agronomically relevant traits. The SAS is regulated by phytochrome signaling and other aboveground neighbor detection cues, which plants use to detect the presence and proximity of neighboring competitors [2]. Studies have shown that shade avoidance syndrome is influenced by phytochrome A and B, with phyB deactivation inducing the SAS and phyA antagonizing this response in Arabidopsis [3]. Additionally, phytochrome-interacting factors (PIFs) directly suppress MIR156 expression to enhance shade avoidance syndrome in Arabidopsis, highlighting the complex regulatory network involved in this response [4]. Furthermore, long noncoding RNAs (lncRNAs) have been identified as key regulators of auxin-related genes that control SAS in Arabidopsis, suggesting a role for epigenetic regulation in this process [5]. The key factors influencing SAS in plants, particularly in soybeans, are primarily driven by changes in light quality and quantity [6,7]. Here are some of the central factors: (1) Light Quality: The reduction in the red to far-red (R:FR) light ratio is a crucial trigger for SAS. This change in light quality occurs when plants are shaded by other plants, which absorb more red light and transmit or reflect far-red light. (2) Light Intensity: a decrease in overall light intensity due to shading from other plants or environmental structures can also initiate SAS responses. (3) Plant Hormones: Phytohormones like auxin and gibberellins play significant roles in mediating the SAS response. Auxin is often redistributed within the plant, promoting stem elongation toward light sources. Gibberellins further facilitate this growth elongation. (4) Genetic Factors: Specific genes regulate the SAS, including those encoding phytochrome photoreceptors that perceive changes in light conditions. Variations in these genes can affect the intensity and nature of the SAS response in different soybean varieties. (5) Environmental and Contextual Factors: temperature, soil quality, and moisture availability can modulate the extent to which SAS influences plant growth and development. Overall, research on shade avoidance syndrome has provided insights into the molecular components and regulatory mechanisms underlying this plant response. Future studies may focus on further elucidating the genetic pathways and environmental factors that modulate the SAS in different plant species to enhance our understanding of plant adaptation to changing light conditions and intraspecific competition.

Soybean (Glycine max (L.) Merr.) is a pivotal legume crop, providing food for humans, feed for animals, and raw materials for industries globally. Soybeans account for 59% of global oilseed production, 70% of the protein meal, and 28% of the vegetable oil consumed worldwide [8]. Given the substantial and growing global demand, soybean production must increase by approximately 2.4% annually despite limited arable land, presenting a significant challenge for breeders to develop higher-yielding cultivars continuously. Moreover, soybeans are a crucial crop that exhibit the classic shade avoidance syndrome. In soybeans, SAS can manifest in several ways, impacting both growth dynamics and agricultural productivity. One of the most notable effects of SAS in soybeans is increased stem elongation. When soybean plants perceive that they are in shaded conditions—typically signaled by an increase in far-red light relative to red light—they respond by elongating their stems. This rapid growth is aimed at outgrowing competitors and reaching stronger light sources. While this may be advantageous in natural settings, in dense agricultural plantings, excessive stem growth can lead to weaker plants that are more prone to lodging, which complicates harvesting and reduces yield. Along with promoting stem elongation, SAS often causes a reduction in leaf size and number. For soybeans, leaves are the primary sites of photosynthesis, so smaller and fewer leaves mean less surface area for photosynthesis to occur, directly affecting biomass accumulation and ultimately yield. SAS can also delay flowering in soybeans. The plant invests more energy in growing taller rather than in reproductive development. Delayed flowering can desynchronize the crop with the optimal pollination period and shorten the time for seed development, which can adversely affect yield. Energy and resources that could be used for developing seeds are diverted to stem growth during SAS. This diversion can result in less energy being available for pod filling, leading to smaller seeds and fewer pods per plant. The increased proportion of far-red light in shaded conditions can also affect the efficiency of photosynthesis. Phytochromes, which regulate light responses, are less active in these light conditions, potentially reducing the plant’s overall photosynthetic rate and energy production. Taller, thinner stems with less robust overall growth can make soybean plants more susceptible to diseases and pest infestations. The microclimate around elongated, dense foliage can favor the development of fungal diseases, which thrive in less ventilated and moister conditions. Previous research on SAS has primarily concentrated on the molecular mechanisms of seedling hypocotyl elongation under shaded conditions, while studies on how light quality regulates plant morphology, material accumulation, yield, and flowering are sparse, and their molecular mechanisms remain largely unclear.

Shade avoidance syndrome in soybean is a crucial phenomenon that can significantly impact yield by affecting essential carbon resources reserved for yield [9]. Extensive research conducted in model plants has provided a framework for understanding SAS in soybean [9]. Soybean, as an important legume crop, exhibits classic SAS characteristics, such as exaggerated stem elongation, in response to reduced blue light [7]. The shade avoidance response in soybean includes attributes like hypocotyl and stem elongation, the upward orientation of leaves, the alteration of flowering time, and more [10]. Additionally, genetic and transcriptome analyses have identified candidate genes and pathways involved in the inactive shade avoidance response, enabling the high-density planting of soybean [11]. Furthermore, studies have highlighted the role of GmCRY1s in modulating gibberellin metabolism to regulate soybean shade avoidance [7]. The blue light receptor GmCRY1s has been found to mainly mediate low blue light-induced shade avoidance syndrome in soybean [12]. Additionally, PIFs have been identified as a novel target in soybean to modulate shade avoidance syndrome. RNA-seq analysis has revealed weed-induced PIF3-like genes as potential targets for manipulating weed tolerance in soybean, further emphasizing the importance of understanding the molecular mechanisms underlying shade avoidance syndrome [13]. In conclusion, shade avoidance syndrome in soybean is a complex phenomenon influenced by various genetic and environmental factors. Understanding the molecular pathways and candidate genes involved in this response is crucial for developing strategies to mitigate the negative effects of SAS on soybean yield and growth. Currently, few studies have explored the effects of light quality on soybean morphology and yield. However, the underlying principles and mechanisms require further analysis.

The soybean cultivars Zhonghuang 13 (ZH13) and Williams 82 (Wm82) were selected for study due to their distinctive traits and relevance in genetic research, making them valuable for specific scientific inquiries. Here are some reasons why these cultivars are chosen: (1) Genetic Background and Traits: Both ZH13 and Wm82 might possess specific agronomic traits that are of interest in soybean research, such as resistance to diseases, tolerance to environmental stresses, or particular yield characteristics. These traits can provide insights into genetic responses to various conditions or treatments in experiments. (2) Reference Genome Availability: Wm82 is particularly notable for being the source of the reference genome for soybeans. Its genome was one of the first to be fully sequenced, which makes it a standard for genetic studies. Researchers use it extensively to explore genetic variations and gene functions and to compare other genotypes for molecular breeding purposes. (3) The Representation of Genetic Diversity: The selection of two different cultivars can provide a broader understanding of soybean genetics and physiology. ZH13 is an early-maturing cultivar with weak photosensitivity, while Wm82 is a late-maturing cultivar with strong photosensitivity. By studying diverse genetic backgrounds, researchers can draw more general conclusions about the species or identify unique traits and genes that might be beneficial for breeding new cultivars. (4) Historical and Research Significance: Both cultivars might have a long history of use in agricultural research, making them well documented and familiar to the scientific community. These historical data can enrich current studies, providing a deeper context for new findings.

Using hydroponic cultivation in experiments with controlled conditions offers several significant advantages, particularly when studying the effects of different light treatments: (1) Control Over Nutrient Delivery: Hydroponics allows precise control over the nutrient mix and concentration that plants receive. This is critical because it eliminates the variability that often comes with soil-based growing, where factors such as soil composition, moisture levels, and microbial activity can affect nutrient availability and uptake. (2) Consistency Across Experimental Conditions: By using hydroponics, researchers can ensure that all plants in an experiment are subject to the same environmental conditions aside from the variable being tested (e.g., light quality). This homogeneity makes it easier to attribute observed differences in plant growth or physiology directly to the light treatment rather than to variations in soil nutrients or water availability. (3) Enhanced Growth and Faster Results: Hydroponic systems typically promote faster plant growth compared to soil due to the more efficient delivery of nutrients to the roots. This efficiency can be particularly advantageous in experiments where quick results are desirable or where multiple generations of plants are needed within a short timeframe. (4) The Reduced Risk of Soil-Borne Diseases and Pests: By eliminating soil, hydroponics also reduces the risk of diseases and pests that can complicate experimental outcomes. This can improve the reliability and reproducibility of experimental results. (5) Improved Accessibility to Roots: With hydroponics, researchers have easier access to plant roots without disrupting the plant’s growth. This accessibility is beneficial for studies focusing on root development or for sampling root tissues without harming the plant, which is more challenging in soil-based setups. Overall, the use of hydroponic cultivation with controlled conditions in experiments involving different light treatments ensures a high degree of experimental control. This control is crucial for accurately assessing how specific light wavelengths affect plant growth, development, and physiology, enabling more precise scientific inquiries and conclusions.

Intercropping is the predominant soybean planting mode currently promoted on a large scale in China. However, significant differences exist in the morphology and yield between soybean monoculture and intercropping with a change in the light quality environment (the R/Fr/G ratio is altered) [14,15]. Therefore, clarifying the regulatory effects and mechanisms of these three light qualities on soybean plant morphology and yield is crucial to improving soybean lodging resistance and yield under intercropping. However, there are still few studies on the regulatory effects of the three light qualities on soybean morphology and yield. Determining whether the critical light quality responsible for this phenomenon in soybeans involves one, two, or all three of the light qualities requires further research. This study hypothesized that far-red light will suppress shade avoidance syndrome more significantly than red or green light in soybean plants, potentially leading to increased biomass accumulation and yield. Therefore, we examine the regulation of soybean morphology and yield by red, green, and far-red light, investigate the regulatory effects of far-red light on these parameters, and identify the key photosynthetic response factors involved in this regulation. This study systematically studied the regulatory effects of these three light qualities on soybean morphology and yield to provide a theoretical basis for using supplementary lighting technology in production to improve the lodging resistance and yield of intercropped soybeans.

2. Materials and Methods

2.1. Plant Materials and Growth Conditions

The soybean cultivars selected included ZH13 and Wm82. Soybean seeds of uniform size and free from pests and diseases were selected, sterilized in a 3% H₂O₂ solution for 20 min, and then thoroughly rinsed with distilled water. The rinsed seeds were soaked in distilled water for eight hours and germinated via the paper roll method. During this period, daytime and nighttime temperatures were maintained at 26 ± 2 °C and 20 ± 2 °C, respectively, the photoperiod consisted of 12 h of light and 14 h of dark, with a light intensity of 300 μmol m⁻² s⁻¹, and the relative air humidity ranged between 65% and 85%. When the seedlings developed true leaves approximately 7 days later, they were thinned and transferred to a hydroponic tray filled with ½-strength Hoagland nutrient solution (EC = 1.2 ± 0.3 ms cm⁻¹, pH = 6 ± 0.1) for cultivation. After eight days, the light quality treatment experiment commenced. LED lamps manufactured by Guangdong Weizhaoye Optoelectronic Energy Saving Co., Ltd. (Guangzhou, China), served as the light source for the climate chamber. Four light treatments were established: white light at 300 μmol m⁻² s⁻¹ (W), blue and red light at 150 μmol m⁻² s⁻¹ each (BR), blue and green light at 150 μmol m⁻² s⁻¹ each (BG), and blue and far-red light at 150 μmol m⁻² s⁻¹ each (BFr). During the experiment, the spectral characteristics and intensity of the light source were measured and documented biweekly using a fiber optic spectrometer (AvaSpec-ULS2048x64TEC-EVO, Zwolle, The Netherlands) (Figure 1). Each treatment was independently and biologically replicated three times, and technical repeats were included twice in each independent biological replicate, with 12 plants per pot of technical repeat, and the plant spacing was 9 by 9.5 cm. The half-strength Hoagland nutrient solution was adjusted to optimal levels (EC = 2.1 ± 0.3 ms cm⁻¹, Ph = 6 ± 0.1) and refreshed weekly. During this stage, the photoperiod was set to 10 h of light and 14 h of dark, daytime, and nighttime temperatures were maintained at 26 ± 2 °C and 20 ± 2 °C, respectively, the light intensity remained at 300 μmol m⁻² s⁻¹, air humidity ranged from 65% to 85%, and the CO₂ concentration was maintained at 400 ppm. When the plants reached the full bloom stage, all treatments were transferred to a white light environment to continue cultivation. During this stage, the photoperiod was adjusted to 16 h of light and 8 h of dark, the light intensity was increased to 400 μmol m⁻² s⁻¹, daytime and nighttime temperatures were maintained at 26 ± 2 °C and 20 ± 2 °C, respectively, the relative humidity ranged from 65% to 85%, and the CO₂ concentration was maintained at 400 ppm.

2.2. Morphological Characteristics and Biomass Measurements

After 35 days of light quality treatment, the plant height, stem diameter, and total leaf area per plant in each treatment group were measured. After the morphological characteristics were measured, samples from each light quality treatment group were collected to assess the fresh and dry weights of the soybean roots, stems, and leaves.

2.3. Photosynthetic Performance Parameter Measurements

A portable photosynthetic meter (LI-6800, LI-COR, Inc., Lincoln, NE, USA) was utilized to assess the third fully expanded leaf from the top of the soybean plant. After 26 days of light quality treatment, the photosynthetic parameters and light response curves were quantified once. During the measurements, the environmental parameters within the leaf chamber were configured as follows: temperature 26 °C, CO₂ concentration 400 ppm, relative humidity 65%, and airflow rate 10,000 µmol s⁻¹. The light source within the leaf chamber consisted of a combination of red and blue light, with red light comprising 90% and blue light 10%. The data were recorded after the net photosynthetic rate had stabilized on the operation interface. To explore the response characteristics of photosynthetic gas exchange relative to light intensity, leaves subjected to various light treatments were placed in a leaf chamber. Initially, they acclimated to a saturated light intensity of 1500 µmol m⁻² s⁻¹ until both the stomatal conductance and net photosynthetic rate (A) had stabilized. Subsequently, the leaves were subjected to a light intensity gradient of 1500, 1200, 900, 600, 300, 100, 50, 20, 15, and 0 µmol m⁻² s⁻¹, and data were captured once the A value had stabilized under each light intensity. Finally, a non-rectangular hyperbolic model was employed to fit the A value.

2.4. Determination of Chlorophyll Fluorescence Parameters

At the third trifoliolate stage of soybean growth, all fully expanded leaves were selected for the measurement of chlorophyll fluorescence parameters. The maximum photochemical quantum yield (Fv/Fm), nonphotochemical quenching coefficient (NPQ), and actual quantum yield of photosystem II (YII) were measured by a Walz ultraportable modulated chlorophyll fluorescence meter (Walz Company, Nuremberg, Germany). During the determination process, the soybean leaves were completely dark-treated for 15 min, and Fv/Fm was measured.

2.5. Yield and Its Component Assays

At the full maturity stage, the yield and various yield components of soybean plants under different light quality treatments were evaluated, including 100-seed weight, pod number per plant, and lowest pod height.

2.6. Statistical Analysis

An analysis of variance (ANOVA) was performed using SAS 9.4 software, and means were tested using the least significant difference test (LSD 0.05) at the p < 0.05 level. Correlation and regression analyses were performed and plotted using Origin 9.0 software.

3. Results

3.1. Effects of Different Light Qualities on Soybean Morphological Characteristics

In this study, soybeans were treated with different light qualities under a blue light background to explore the effects of light quality changes on soybean photosynthesis and photomorphogenesis. Special attention was focused on shade avoidance syndrome induced by far-red (BFr) treatment. The results indicated that, compared with white light (W), soybean cultivars ZH13 and Wm82 exhibited pronounced morphological changes following BFr treatment. Specifically, plant heights increased by 23.62% and 47.96%, respectively, while stem diameters decreased by 26.06% and 19.79%, respectively. Additionally, the total green leaf area per plant declined significantly by 54.11% and 34.45%, respectively; the lowest pod heights rose substantially by 67.16% and 88.82%, respectively (Figure 2). Compared with white light treatments, following red light (BR) treatment, plant heights of ZH13 and Wm82 decreased by 33.00% and 29.16%, respectively, and the total green leaf area per plant for ZH13 declined by 31.46%. Under other light quality treatments, no significant differences were observed in morphological indicators. These results suggest that shade avoidance syndrome under far-red light and red light treatments significantly affects the plant height, lowest pod height, leaf area, and stem diameter of soybeans, while green light exerts a relatively minor effect on soybean plant morphology.

3.2. Response of Soybean Biomass to Light Quality

The relationship between plant morphology and biomass accumulation constitutes a core issue in plant physiology. To determine the effects of different light qualities on the fresh and dry weights of various soybean organs, biomass measurements were taken under different light quality treatments. The results revealed that the response of fresh and dry weights to different light qualities followed consistent patterns across various organs. Under far-red light (BFr) treatment, there was a significant reduction in dry matter accumulation in soybeans after 35 days of light treatment (Figure 3). Compared with white light (W), dry matter accumulation in the roots and leaves of ZH13 was reduced by 47.83% and 56.71% after BFr treatment, respectively, while the reductions for Wm82 amounted to 4.55% and 38.06%, respectively; however, the stem dry weight of Wm82 measured a significant increase of 36.36%. Furthermore, red–blue light (BR) and blue–green light (BG) treatments resulted in reductions in dry matter accumulation in the leaves of ZH13 by 24.52% and 22.99%, respectively, with no significant differences noted among other treatment groups. These results suggest that Fr light can significantly inhibit dry matter accumulation in soybean leaves and roots, while R and G light exhibit weaker inhibitory effects on dry matter accumulation in leaves, with significant variation across different cultivars.

3.3. Effects of Different Light Qualities on Net Photosynthetic Rate and Transpiration Rate of Soybean

This study additionally investigated the effects of different light qualities on photosynthesis and related physiological parameters in soybeans. Given that the products of photosynthesis constitute the primary source of dry matter accumulation in plants, changes in this accumulation are intrinsically linked to their photosynthetic capacity. In this study, detailed measurements of photosynthetic-related parameters were conducted for soybean cultivars ZH13 and Wm82 under various light quality treatments. The results indicated that BFr treatment significantly influenced the photosynthetic physiological responses of the leaves. Specifically, the net photosynthetic rate of leaves in the tested soybean cultivars exhibited a significant reduction following BFr treatment. When compared with the white light group, the net photosynthetic rates of ZH13 and Wm82 showed decreases of 24.91% and 28.43%, respectively. Additionally, the transpiration rates demonstrated a corresponding decline, with those of ZH13 and Wm82 falling by 28.32% and 24.55%, respectively (Figure 4). These findings suggest that BFr light treatment inhibits both photosynthesis and transpiration in soybean leaves. This result not only unveils the complexity of plant physiological functions under light quality regulation but also offers an important physiological basis for further exploring the adaptation mechanisms of plants to different light quality environments.

3.4. Effect of Light Quality on Soybean Light Energy Utilization Ability

In this study, we aimed to explore the effects of different light qualities on soybean photosynthesis and light energy conversion capacity. We analyzed changes in the relative electron transfer rate (ETR) and actual electron transfer efficiency of photosystem II (PSII) in soybean leaves, including non-photochemical quenching (NPQ). The experimental results demonstrated that after far-red light (Fr) treatment, the parameters exhibited a marked downward trend in both soybean cultivars (ZH13 and Wm82). Specifically, the ETR values for ZH13 and Wm82 decreased by 26.36% and 34.34%, respectively; PSII activity by 20.75% and 34.33%; qP by 57.73% and 49.40%; and NPQ by 29.20% and 39.64% (Figure 5). These results elucidated the negative regulatory effect of Fr on the light energy conversion capacity of soybean leaves, signifying that Fr can significantly diminish the relative electron transfer rate, actual electron transfer efficiency, photosynthetic system efficiency, and heat dissipation ratio of PSII. In contrast, red light (R) and green light (G) exhibited negligible effects on the light energy conversion capacity of soybean leaves.

3.5. Far-Red Light Inhibit Gas Exchange in Leaves

We further explored how gas exchange parameters are intricately related to transpiration and photosynthesis, with a specific focus on changes in intercellular CO₂ mole fraction (Ci) and stomatal conductance. These parameters are essential for understanding the physiological responses of plants under various light qualities. By subjecting soybean cultivars ZH13 and Wm82 to diverse light quality treatments, we observed that stomatal conductance was significantly inhibited following BFr treatment; specifically, stomatal conductance in ZH13 and Wm82 was reduced by 39.04% and 29.19%, respectively. However, while the intercellular CO₂ mole fraction exhibited a downward trend, these changes were not statistically significant (Figure 6). These results suggest that Fr irradiation significantly impairs the stomatal conductance and CO₂ assimilation rate in soybean leaves yet exerts a minimal impact on the Ci. Furthermore, red light (R) and green light (G) treatments demonstrated no apparent regulatory effects on the gas exchange parameters of soybean leaves.

3.6. Photosynthetic Pigment Synthesis in Response to Light Quality in Cultivars

To investigate the close relationship between the photosynthetic capacity of leaves and the type and relative contents of pigments, we conducted a detailed chlorophyll fluorescence analysis on soybean leaves following various light quality treatments to assess the impact of light quality on pigment composition. Growth and physiological responses vary depending on the cultivar were observed in the responses to light quality. Following far-red light (BFr) treatment, a significant decrease in pigment content was observed in Wm82, whereas ZH13 exhibited no notable change in pigment levels. Under the blue plus red light (BR) treatment, both cultivars demonstrated a significant increase in pigment content. For the blue plus green light (BG) treatment, ZH13 exhibited a significant increase in pigment content, whereas Wm82 displayed a marked decline. Furthermore, the maximum quantum efficiency of photosynthetic system II (Fv/Fm) was not significantly different among the treatments, suggesting that while light quality significantly influenced pigment accumulation, it did not markedly affect the photosynthetic potential of PSII (Figure 7). This result indicates that despite varietal differences, soybean leaves under various light quality treatments did not exhibit significant stress.

3.7. Yield and Its Components

To assess the effects of different light quality treatments on the yield of soybeans, we measured the 100-seed weight and pod number per plant. The results showed that the 100-seed weight of ZH13 experienced decreases of 10.27%, 3.30%, and 4.20% under BFr, BR, and BG treatments, respectively. Correspondingly, the 100-seed weight of Wm82 was significantly reduced by 25.35% and 7.58% under BFr and BR treatments, respectively, and increased by 26.17% under BG treatment (Figure 8). Furthermore, BFr treatment significantly reduced the pod number per plant, suggesting that far-red light could potentially lead to a yield reduction by inhibiting both the pod number per plant and the 100-seed weight. These results demonstrated the potential regulatory effects of different light quality treatments on soybean yield formation, with green light presenting the potential to enhance yield by increasing 100-seed weight.

4. Discussion

4.1. Soybean Biomass Accumulation, Allocation, and Plant Morphology Development Are Regulated by Light Quality

Far-red light has distinct effects on the morphological characteristics of soybean plants when compared to white light and other light treatments. Increased Stem Elongation: Far-red light is known to trigger a response in plants mediated by a group of light receptors called phytochromes. These receptors, when activated by far-red light, promote stem elongation. As a result, soybean plants exposed to far-red light often have longer stems compared to those grown under white light or other spectral treatments. This effect is a part of shade avoidance syndrome, where plants elongate their stems to outgrow competitors and reach more light. Reduced Leaf Development: Exposure to far-red light can also result in larger but thinner leaves, as the plant prioritizes vertical growth over expanding its leaf surface area. This can impact the overall photosynthetic efficiency of the plant, as leaf expansion is critical for capturing light and performing photosynthesis. Alterations in Leaf Chlorophyll Content: Far-red light can affect the chlorophyll content in soybean leaves. Typically, plants under far-red light might exhibit reduced chlorophyll concentration, which can affect the green coloring of the leaves and potentially decrease photosynthetic capacity. Root Development Variations: Although not as visually apparent as changes in aboveground parts, far-red light may influence root development. Depending on the overall light spectrum balance, root systems under far-red light might adapt differently, potentially impacting their ability to uptake water and nutrients.

Shade avoidance syndrome in plants, triggered by far-red light, is a complex adaptive response that directly influences morphology, particularly in aspects like stem elongation and leaf area in soybean plants. Soybean plants, like other plants, possess a phytochrome system, which consists of photoreceptors that can detect the ratio of red to far-red light. In natural environments, sunlight has more red light compared to far-red light. However, under a canopy or when shaded by other plants, this ratio shifts towards far-red light as red light is absorbed by the leaves above. This shift is detected by phytochromes, which switch from their active (Pfr) to inactive (Pr) form in response to increased far-red light, signaling the plant to initiate shade avoidance responses. The change in phytochrome status leads to alterations in hormonal balances, particularly an increase in gibberellin (GA) levels and a decrease in auxin transport control. Gibberellins promote cell elongation and division, particularly in the stems. Alongside hormonal shifts, changes in cell wall extensibility occur, facilitated by enzymes such as expansins, which are also regulated by the shade avoidance response. This contributes to rapid stem elongation under far-red-enriched light conditions. In essence, shade avoidance syndrome triggered by far-red light leads to significant morphological adaptations in soybeans, such as stem elongation and reduced leaf area. These changes are crucial for the plant’s survival in competitive environments, allowing it to reach more favorable light conditions, albeit at the cost of potential reductions in overall biomass and reproductive output under extreme conditions.

Under different light quality conditions, soybean plant morphology characteristics responded inconsistently to light quality. These differences can be attributed to the regulatory mechanisms of light quality on plant photosynthesis efficiency, hormone balance, and resource allocation strategies. The significant increase in plant height and lowest pod height under blue-far-red light treatment is likely due to the photomorphogenesis response induced by far-red light, whereby plants enhance light capture by elongating cells, particularly in environments with light competition [16]. Far-red light typically promotes cell elongation by influencing the levels of the plant hormone gibberellin [17]. The reduction in stem diameter and leaf area per plant suggests that under blue-far-red light, plants likely prioritize resource allocation to promote vertical growth (upward growth) at the expense of lateral growth and leaf area expansion. This change in resource allocation may represent an optimized response to photosynthetic activity and light capture efficiency (Figure 3, Figure 4 and Figure 5). Predecessors have found that supplementing with far-red light (low R/Fr ratio) can increase plant biomass [18]. In the blue-far-red light treatment, we found that the biomass of roots and leaves was significantly reduced compared to other light quality treatments, likely due to a shift in resource allocation towards stems and plant height growth (Figure 1 and Figure 2). This strategy is likely to reduce the expansion of the root system and the growth of leaves, the latter being the primary site of photosynthesis, potentially affecting the overall energy capture and utilization efficiency. Stem biomass remained relatively stable, indicating that despite the decrease in stem diameter, the total stem biomass was likely maintained due to increased plant height. In summary, light quality can alter plants’ growth patterns and biomass allocation by modulating hormone levels, affecting photosynthetic efficiency and plant respiration.

4.2. The Relationship Between the Photosynthetic Production Capacity and the Light Quality

Under blue–far-red light irradiation, both the net photosynthetic rate and transpiration rate of soybean leaves were significantly reduced; furthermore, Fr substantially diminished the relative electron transfer rate, actual electron transfer efficiency, photosynthetic system efficiency, and heat dissipation ratio of PSII. In contrast, red light and green light exhibited negligible effects on soybean leaves’ light energy conversion capacity. This phenomenon pertains to the light-dependent reactions of photosynthesis, particularly affecting the function and efficiency of PSII. The photosynthetic physiological changes exhibited by soybean leaves under blue–far-red light irradiation closely correlate with the mechanisms induced by light of different wavelengths. Far-red light primarily influences the photomorphogenesis of plants, exerting a less direct impact on photosynthesis. The reduction in electron transfer rate, actual electron transfer efficiency, and photosynthetic system efficiency of PSII due to far-red light may stem from changes in plant hormone levels induced by far-red irradiation [19,20], indirectly impacting the function and structure of chloroplasts. Far-red light regulates cell elongation by altering plant hormone balances [21,22], such as gibberellins, potentially leading to fewer chloroplasts or alterations in their internal structure, consequently impacting photosynthetic efficiency. Far-red light has a distinct impact on the light energy utilization ability of soybean plants, particularly regarding the efficiency of PSII. Far-red light, which is at wavelengths typically around 700 nm to 750 nm, is not absorbed as efficiently by the chlorophyll pigments in PSII compared to red or blue light. PSII primarily absorbs light in the blue (around 450 nm) and red (around 680 nm) regions. Since far-red light is not effectively absorbed, it is less efficient in driving the photochemical reactions in PSII that are essential for the conversion of light energy into chemical energy. Plants can undergo state transitions, shifting the balance of excitation energy between PSII and photosystem I (PSI) based on the light environment. Under far-red light, there is a shift towards PSI activity because PSI is better at utilizing far-red light. This shift can lead to a relative decrease in PSII efficiency as the system rebalances the distribution of excitation energy to optimize overall light utilization under different spectral conditions. Exposure to far-red light can lead to an increase in cyclic electron flow around PSI, which helps in photoprotection by dissipating excess energy that cannot be efficiently used by PSII. This mechanism ensures plant survival under far-red dominant environments but does not necessarily contribute to increased PSII efficiency. In summary, while far-red light is less efficient for driving PSII directly, it triggers a series of adaptive responses in soybean plants that help optimize energy utilization under specific environmental conditions.

Stomatal opening is primarily regulated by light, specifically by the photoreceptors in plants, including phytochromes. Far-red light (wavelengths approximately 700–740 nm) plays a significant role in this process. When plants perceive far-red light, phytochromes undergo a conformational change that triggers signaling pathways affecting stomatal behavior. Stomata are surrounded by guard cells, which control their opening and closing. In response to light, guard cells accumulate potassium ions, leading to water influx and turgor pressure increase, causing the stomata to open. Far-red light can influence this mechanism by regulating the balance of auxin and abscisic acid, two hormones that modulate guard cell function. In natural environments, far-red light often signals the proximity of neighboring plants (as it is transmitted more than red light). This cue can prompt stomatal opening to optimize gas exchange while avoiding excessive water loss, enabling plants to adapt to their competitive environment. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is critical for carbon fixation in photosynthesis. Far-red light influences the expression of genes coding for Rubisco and its activase, particularly under low-light conditions. Enhanced Rubisco activity in the presence of far-red light may facilitate more effective carbon fixation, albeit indirectly.

Plant photosynthesis systems typically utilize red light (R) efficiently [23,24], particularly in capturing light energy that drives PSII. Although green light exerts a lesser direct effect on plant photosynthesis, it can penetrate deeper leaf layers and influence the photosynthesis of lower chloroplasts [23]. Our research indicates that green light enhances the light distribution within the leaves and boosts overall energy utilization efficiency. Therefore, the diminished photosynthesis in soybean leaves under blue–far-red light irradiation may result from the indirect adverse effects of far-red light on plant physiology and photosynthetic structure, coupled with a suboptimal ratio of blue to far-red light, thereby failing to maximize the potential promotive effects of blue light. Compared to red and green light, these two light qualities exhibit a negligible impact on the light energy conversion capacity of soybean leaves, demonstrating their crucial role in maintaining the stability and efficiency of photosynthesis. Blue and red lights are most effective for photosynthesis due to their absorption by chlorophyll pigments [23]. Red light generally enhances the photosynthetic rate more than green or far-red light. Blue light, although less efficient than red light in terms of photosynthesis, can enhance leaf morphology and stomatal opening, indirectly supporting higher photosynthesis rates. Light quality also affects transpiration by influencing stomatal behavior. For example, blue light is particularly effective at inducing stomatal opening [25,26,27], thereby increasing the transpiration rate. This is beneficial for the nutrient uptake and cooling of the plant but can lead to faster soil moisture depletion. The interplay between net photosynthetic rate and transpiration rate under various light treatments is complex, involving direct effects on leaf physiology and indirect effects mediated by changes in leaf temperature and humidity conditions. Understanding these dynamics is crucial for optimizing growth conditions in both natural and controlled environments, such as greenhouses or vertical farms, where precise control over light conditions can significantly impact plant health and productivity.

4.3. Relationship Between Light Quality and Gas Exchange Parameters

Far-red irradiation significantly inhibited the stomatal conductance and CO₂ assimilation rate of soybean leaves yet had minimal effect on the intercellular CO₂ concentration. Moreover, red light and green light treatments did not exhibit discernible regulatory effects on the gas exchange parameters of soybean leaves. The experimental outcomes involve the gas exchange behavior of plants under diverse light quality conditions, a complex physiological process influenced by numerous factors. This analysis examines the effects of far-red, red, and green lights on the gas exchange parameters of soybean leaves from multiple perspectives. Far-red light irradiation diminishes stomatal conductance, potentially due to the increased activation of abscisic acid (ABA), which induces stomatal closure and consequently reduces both the CO₂ entry rate and water evaporation [28,29]. Since far-red light leads to a reduction in stomatal conductance, the CO₂ assimilation rate of soybean leaves correspondingly diminishes. This is because the CO₂ assimilation rate is reliant on the diffusion of CO₂ within the leaves, and reduced stomatal conductance directly limits CO₂ supply [30], thereby impacting photosynthetic efficiency. Although far-red light inhibits both stomatal conductance and the CO₂ assimilation rate, the relatively stable intercellular CO₂ concentration likely results not from decreased CO₂ diffusion due to stomatal closure but from a synchronous reduction in CO₂ utilization. Red light generally enhances photosynthesis by activating photosystems and improving the efficiency of light energy capture and conversion. However, in this experiment, the intensity or proportion of red light may have been insufficient to significantly influence stomatal behavior or gas exchange parameters. While green light can penetrate deeper into the leaf structure, affecting the photosynthetic activity of lower chloroplasts, it appears to exert no direct or significant regulatory effect on stomatal conductance or large-scale gas exchange. In summary, far-red light regulates the behavior of stomata and key parameters of photosynthesis by affecting the hormone balance and photosensitive pigment activity of plants. However, red and green lights may not demonstrate significant regulatory effects under the specific environmental conditions of this study, possibly due to variations in light intensity, irradiation ratios, or experimental setups.

4.4. The Light Quality Regulates the Biosynthesis of Photosynthetic Pigments on Soybean

Light quality exerts a significant influence on the synthesis and accumulation of leaf pigments. Far-red, green, and red light represent three spectra with distinct effects on plant growth and development. Far-red light predominantly regulates plant growth by influencing the photosensitive pigment system, notably the activity of red/far-red light receptors, such as phytochromes. Phytochromes toggle between red and far-red light to regulate the shade responses of plants. Regarding photosynthetic pigments, far-red light can lead to a relative decrease in chlorophyll content as plants might sense a ‘shade’ signal under a higher proportion of far-red light, consequently reducing chlorophyll synthesis and promoting other physiological adaptations to shade, such as redistributing photosynthetic system components within chloroplasts [31,32]. The reduction in photosynthetic pigment synthesis under far-red light is a critical aspect of SAS in plants, and it is driven by several physiological mechanisms: (1) Plants have phytochromes that detect the ratio of red to far-red light. Under a canopy or in shaded areas, the red to far-red ratio decreases because the red light is mostly absorbed by leaves above, while far-red light penetrates more efficiently. This shift in light quality is detected by phytochromes, which undergo a conformational change from the active Pfr to the inactive Pr form. This switch alters the transcription of genes involved in chlorophyll synthesis. (2) Changes in hormone levels, particularly increases in gibberellin and ethylene under far-red light, can negatively influence chlorophyll biosynthesis. Gibberellins, while promoting stem elongation, can indirectly affect the synthesis of chlorophyll by modulating the expression of genes involved in its biosynthesis. Ethylene has been known to have a complex role in leaf senescence, which includes the breakdown of chlorophyll. (3) Exposure to far-red light leads to the downregulation of genes essential for chlorophyll production. This is mediated through the phytochrome system influencing transcription factors that either promote or inhibit these genes. The decreased chlorophyll content not only reduces the plant’s capacity for photosynthesis but is also a strategic adaptation to avoid wasteful investment in light-harvesting complexes under suboptimal lighting conditions. (4) With reduced exposure to red light, the efficiency of photosynthesis can decrease, leading to a reduced production of NADPH and ATP, crucial elements in chlorophyll synthesis. This can create a feedback loop where reduced chlorophyll levels lead to an even lower photosynthetic capacity. These physiological responses are integrated within the plant’s broader adaptation strategy to optimize survival and growth under varying light conditions. By reducing the synthesis of photosynthetic pigments under far-red light, the plant conserves energy and resources in conditions where the efficiency of photosynthesis would be compromised anyway. This is part of a suite of adaptive responses aimed at ensuring the plant can escape shade and reach light conditions more favorable for photosynthesis and growth.

Green light is less absorbed by the pigments within plant chloroplasts, enabling it to penetrate deeper layers of the leaf and influence the photosynthesis of lower chloroplasts [23]. Green light also positively impacts the synthesis of chlorophyll and carotenoids [33,34,35], especially under conditions of low photosynthetically active radiation (PAR). Red light (R) significantly promotes chlorophyll synthesis as it directly activates photosystem II [36], enhancing the efficient capture and conversion of light energy into chemical energy. In summary, while far-red light may reduce chlorophyll synthesis due to perceived shade signals, green and red light enhances the synthesis and accumulation of chlorophyll and carotenoids. The specific manifestations of these effects may depend on the parameters of light conditions, such as light intensity, the light quality ratio, and interactions with other environmental factors. In actual agricultural production, optimizing the growth state and photosynthetic efficiency of crops can be achieved by adjusting the light quality ratio. In this study, we also investigated the effects of various light qualities on the pigment content in different soybean cultivars. To gain a deeper understanding of these differences between cultivars, this study assessed the impact of light quality treatment on the levels of several major pigments, including carotenoids and chlorophyll. The results indicated significant differences in the patterns of pigment accumulation among soybean cultivars treated with various light qualities. These differences are likely primarily due to variety-specific physiological and genetic response mechanisms. This study elucidated the role of light quality treatment in regulating soybean pigment biosynthesis and its interactions with the genetic characteristics of soybean cultivars. This finding holds significant importance for understanding the adaptive mechanisms of plants to light environments and for enhancing crop production through future light quality management.

4.5. Relationship Between Light Quality and Yield

In crop growth and yield formation, light quality exerts a significant impact on the physiological and morphological development of plants. The primary reasons for the reduction in soybean yield due to far-red light include the activation of photosensitive pigments and shading effects. Far-red light induces a shade avoidance response in plants by activating the phytochrome system (PhyA and PhyB) [37]. Phytochromes oscillate between red light (660 nm) and far-red light (730 nm), with the transition from the PhyB to PhyA form stimulating shading signals. When plants detect a high ratio of far-red to red light, they interpret this as being shaded by other plants [38], typically resulting in resource diversion from reproductive to vegetative growth like stem elongation [37,39], potentially inhibiting pod numbers and reducing the yield [15]. Under the influence of far-red light, soybeans may favor stem and leaf growth over pod number and seed size as they modify their growth strategies to evade shading, prioritizing photosynthetic efficiency and competitive survival. The primary reasons of yield-increasing effect in green light is enhancing light penetration for photosynthesis. Green light penetrates deeper into the leaves, activating chloroplasts in the lower layers and increasing the photosynthetic efficiency across the entire leaf surface. Efficient photosynthesis boosts the accumulation of photosynthetic products, such as carbohydrates, essential for seed development and weight gain [40]. Green light can alter the balance of plant hormones, notably by enhancing the activity of gibberellins involved directly in seeding development and fruit ripening [41].

Plants grown under red light demonstrate higher biomass production than those under green light in ZH13. This is attributed to the higher efficiency of red light in driving photosynthesis and its direct impact on growth-promoting phytochrome pathways. Red light tends to increase photosynthetic capacity more effectively than green light due to its optimal absorption by chlorophyll. Green light’s impact is more complex; it can stimulate photosynthesis under certain conditions but is generally less efficient on its own. However, in combination with red light, green light can lead to an overall increase in plant growth and photosynthetic efficiency, especially in dense plantings where lower leaves receive limited light. Overall, while red light is typically more beneficial for photosynthesis and biomass production, the incorporation of green light can be advantageous under specific growing conditions, suggesting that a balanced spectrum that includes both red and green light could be optimal for enhancing overall plant growth and health. In practical applications, optimizing the light quality ratio can enhance crop growth conditions and increase yield. Varying light quality treatments influence plant growth and development by modulating their morphological and physiological characteristics. Understanding these mechanisms is crucial for basic plant science research and offers a theoretical foundation for managing the light environment in agricultural production. Regulating light quality can optimize crop growth conditions, enhance light energy utilization efficiency, and ultimately improve both crop yield and quality.

4.6. Physiological Mechanisms Underlying the Observed Inhibitory Effects of Far-Red Light on Soybeans

Far-red light significantly influences plant growth, photosynthesis, and yield by interacting with the plant’s light perception mechanisms and hormonal signaling pathways. Here is how far-red light affects these aspects: (1) Phytochrome Regulation: Plants possess photoreceptors called phytochromes that detect different wavelengths of light, including far-red. Phytochromes exist in two forms: an inactive form that absorbs red light and an active form that absorbs far-red light. When far-red light is prevalent (as in shaded conditions), it converts the active form of phytochrome back to its inactive form, triggering physiological changes associated with shade avoidance, such as elongation growth. (2) Leaf Development and Photosynthesis: While far-red light can trigger stem elongation, it is less effective at driving photosynthesis compared to red and blue light. This is because photosynthesis primarily relies on red and blue light absorbed by chlorophyll. If a plant receives excessive far-red light, it may have larger, but thinner and less chlorophyll-dense leaves, which can reduce photosynthetic efficiency. (3) Yield Implications: In the long term, the changes induced by far-red light can affect yield negatively. Although plants may grow taller, the energy diverted towards height gain rather than developing flowers, fruits, or seeds can reduce reproductive success. Furthermore, thinner leaves with less chlorophyll mean lower photosynthetic rates, impacting the plant’s ability to produce carbohydrates and thus the overall biomass and yield. (4) Photomorphogenic Responses: Far-red light is also involved in photomorphogenic responses that include flowering and circadian rhythms. These responses can be crucial for the timing of flowering and fruit set, which are directly related to yield. In soybeans, as with many other plant species, light perception and response mechanisms are crucial for regulating growth and development. The perception of light is primarily mediated by photoreceptors, which include phytochromes, cryptochromes, and phototropins. Phytochromes are particularly sensitive to red and far-red light. When soybeans detect increased far-red light, they interpret it as being shaded. This triggers shade avoidance syndrome, where plants elongate their stems and reduce leaf expansion in an effort to outgrow and overtop their competitors to access more light intensity. Exposure to far-red light alters the balance of red to far-red light, leading to a reduction in the synthesis of chlorophyll and other photosynthetic pigments through changes in gene expression regulated by Pfr. This reduction can impair photosynthetic efficiency. The switch from Pfr to Pr under far-red light can affect other light-mediated developmental processes. It impacts not only stem elongation but can also delay flowering, alter biomass accumulation, and affect root development, all of which can contribute to overall reduced plant vitality and yield. Far-red light influences the levels of plant hormones such as gibberellins and auxins, which are involved in cell elongation and division. The reduction in Pfr levels under far-red light conditions can lead to changes in the levels of these hormones, further contributing to the inhibitory effects on growth and development. Understanding these mechanisms helps in developing agricultural strategies, such as manipulating light spectra using LEDs in controlled environment agriculture, to optimize plant growth and improve crop yields.

The variability in plant responses to far-red light is indeed species-specific. We find far-red light inhibits growth and yield in soybean, but in Viola cornuta and lettuce, far-red light promotes growth, enhances flowering, or improves physiological productions [42,43]. Future research on light quality management in crop production can build on existing results by exploring several key areas to optimize the growth conditions and yield of various crops. Here are some promising directions: (1) Investigate specific light conditions tailored to different crop species, intercropping systems, growth stages, and even particular growth production (such as leaf size, fruit sweetness, or nutrient content). This involves determining the precise combinations and ratios of different light wavelengths, including not just red and green light but also blue, far-red, and ultraviolet, that most effectively promote desired plant characteristics. (2) Conduct studies that not only focus on light quality but also integrate other environmental factors such as temperature, humidity, CO₂ levels, and soil nutrition. Understanding the interactions between these factors and light quality could lead to more holistic approaches to crop management. (3) Deepen the understanding of the molecular mechanisms by which plants perceive and respond to light. This could involve genetic studies to discover how different light wavelengths influence gene expression related to growth, development, and stress responses. By focusing on these areas, future research can provide innovative solutions and refined strategies for managing light quality in crop production, ultimately enhancing productivity and sustainability in agriculture.

5. Conclusions

This study primarily investigated the effects of different light qualities on soybean growth, photosynthesis, and yield. Through experiments on different soybean cultivars (ZH13 and Wm82), it was determined that light quality can significantly regulate the morphological characteristics, photosynthetic system functions, and yield components of soybeans. Different light qualities have various impacts on the growth and yield of soybeans through the regulation of the morphological characteristics, photosynthetic system functions, and yield components. BR had no significant effect on plant morphology and yield, and the BG treatment was more effective in enhancing photosynthetic efficiency and increasing yield, whereas the BFr treatment partially inhibited growth and yield. These findings offer a scientific basis for light quality management in agricultural production, facilitating the optimization of crop growth and yield through adjustments to the light environment. Understanding these light quality effects is crucial for managing light environments in agriculture, particularly in controlled settings such as greenhouses where light spectra can be manipulated to influence crop growth and productivity.