Effects of light quality on pod elongation in soybean (Glycine max (L.) Merr.) and cowpea (Vigna unguiculata (L.) Walp.)

ABSTRACT

Soybean pods are located at the nodes, where they are in the shadow, whereas cowpea pods are located outside of the leaves and are exposed to sunlight. To compare the effects of light quality on pod growth in soybean and cowpea, we measured the length of pods treated with white, blue, red or far-red light. In both species, pods elongated faster during the dark period than during the light period in all light treatments except red light treatment in cowpea. Red light significantly suppressed pod elongation in soybean during the dark and light periods. On the other hand, the elongation of cowpea pods treated with red light markedly promoted during the light period. These results suggested that the difference in the pod set sites between soybean and cowpea might account for the difference in their red light responses for pod growth.

Introduction

Pods encapsulate the developing seeds and protect them from pests and pathogens.¹ Pod length and width are positively correlated with final seed size.² In soybean, cultured seeds continue to grow compared with seeds in pod, which suggests that soybean seed growth is physically suppressed by the pod wall.³ Decreasing of soybean pod size by brasinosteroid biosynthesis inhibitor resulted in reduction of 100-seed weight.⁴ These data suggest that seed weight could be controlled by restriction of pod growth. However, the mechanisms of pod growth have not yet been clarified even though 100-seed weight is one of the crucial yield components in legume.

Light is an important environmental signal that regulates plant growth and development. Blue light (BL), red light (RL), and far red light (FRL), which are sunlight components, regulate plant growth. Plants perceive BL via the photoreceptors cryptochromes (CRY1 and CRY2) and phototropins (phot1 and phot2), and RL and FRL via phytochromes (phyA–phyE).^5-7 Arabidopsis hypocotyl elongation is induced in the dark and is inhibited by light.⁸ BL and RL suppress Arabidopsis hypocotyl growth through the CRY1 and phyB photosensory pathways, respectively.^9,10

In soybean, RL increases plant height,¹¹ and BL suppresses stem elongation.¹² Soybean pods are located at the nodes and receive weakened sunlight due to shading, whereas cowpea pods are located outside of the leaves and receive sunlight directly (Fig. 1). However, the effects of light on pod growth of soybean and cowpea has never been studied so far. Therefore, we investigated the effects of light quality on pod growth in these species.

Figure 1.

Soybean and cowpea plants. White arrows show the pod set sites.

Results and discussion

To investigate the effects of light on pod growth in soybean and cowpea, we treated the plants with WL or monochromatic light (BL, RL or FRL). The elongation rate of soybean and cowpea pods was higher during the dark period than during the light period in all light treatments (Figs. 2A, 3). However, the elongation rate of cowpea pods treated with RL was nearly identical during the dark and light periods (Fig. 3). Plants can perceive not only RL but also FRL via phytochromes. Shade avoidance responses are triggered by the low R:FR ratio through phytochrome signaling, and result in a longer hypocotyl phenotype in Arabidopsis.¹⁵ In each species, pod elongation rate was similar under FRL and WL (Figs. 2B, 3). In Arabidopsis, RL inhibits hypocotyl elongation, and phyB plays a prominent role in the regulation of the hypocotyl elongation.¹³ On the other hand, in soybean, RL increases hypocotyl length.¹⁴ In this study, monochrome RL slowed the pod elongation rate in comparison with WL, whereas BL did not (Fig. 2A). RL significantly promoted cowpea pod elongation rate during the light period (Fig. 3).

Figure 2.

Soybean pod elongation rate for 5 d during the dark and light periods under (A) white light (WL), blue light (BL), red light (RL) and (B) WL and far-red light (FRL) treatments. P, period (dark × light); L, light treatments. The same letters within each treatment indicate no significant differences (Tukey–Kramer method; p < 0.05, n = 3). ANOVA: **, p < 0.01; n.s., not significant.

Figure 3.

Cowpea pod elongation rate for 5 d during the dark and light periods under white light (WL), blue light (BL), red light (RL), and far-red light (FRL) treatments. P, period (dark × light); L, light treatments. The same letters within each treatment indicate no significant differences (Tukey–Kramer method; p < 0.05, n = 3). ANOVA: **, p < 0.01; n.s., not significant.

We found that the response of pod elongation to RL differed between soybean and cowpea, although monochrome BL had little effect on pod elongation in either species. In Arabidopsis, RL prolonged phyB activity and inhibited phyB-mediated hypocotyl elongation.¹⁶A mutation of phytochrome-interacting factor 3 (PIF3) results in short hypocotyls under red light, indicating that RL has negative effects on light-mediated hypocotyl growth.^17,18 It seems likely that phytochrome-mediated signaling differs in soybean and cowpea pods. Under field conditions, the exposure of pods to RL is lower in soybean than in cowpea (Fig. 1, Fig. S1). Additionally, the photon flux density of RL is high during daytime (Fig. S1). These results suggested that the differences in RL responsiveness or sensitivity between soybean and cowpea might attribute the difference in the pod set sites. Further study is needed to clarify the molecular mechanisms of pod growth regulation by RL signaling.

Materials and methods

Seeds of soybean (Glycine max L. Merr.) ‘Fukuyutaka’ and cowpea (Vigna unguiculata (L.) Walp.) ‘IT98K-205–8’ were imbibed overnight and sown in 0.7 L pots with compound fertilizer (N:P:K = 3:10:10) and magnesium lime (1.25 g each). Plants were grown in a phytotron under a natural photoperiod at 25°C from 8 Nov. to 15 Dec. in 2016. On the day before soybean pod length reached 1 cm, plants were transferred to an incubator under a 10-h light/14-h dark photoperiod at 25°C. When a soybean pod reached 1 cm, the pods was tagged for WL, BL, RL, or FRL light treatments, which were performed with an LED light source (ISLM W/RB/FR, CCS). Light intensity was adjusted to 200 µmol m⁻² s⁻¹. In soybean, WL and FR intensities were 15 µmol m⁻² s⁻¹ at the pod position, as measured with a light quantum sensor (LI-190, LI-COR) connected to a light meter (LI-250, LI-COR). Light spectra were obtained from a light analyzer (LA-105). After the pod length reached 1 cm, one pod was measured from each pot; 3 pots per treatment. Pod length was measured twice a day (at the beginning and end of the light period) with a vernier micrometer for 5 d after treatment. Pod elongation rate was calculated by using all the growth during light or dark period divided by the duration in period over 5 d (light period: 50 h, dark period: 70 h).

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by JSPS KAKENHI Grant Number 16H04867.