Effect of shading signal on growth and photosynthetic characteristics of soybean seedlings

The reduction in the ratio of red light (R) to far-red light (FR) is an important signal of shade stress in nature. The objective of this study was to investigate the response mechanism of soybean seedlings to low R/FR signals. To this end, a pot culture experiment was conducted to study the effect of normal and low R/FR light irradiation on the morphology, photosynthetic characteristics and chlorophyll fluorescence of two soybean varieties (Glycine max L.) seedlings — ‘Nandou12 ‘and ‘Nan032-4’. The results indicated that compared with seedlings under normal light, low-R/FR light increased soybean plant height, leaf area, petiole length and internode length, but reduced stem diameter. Plant height, petiole length and internode length of ‘Nandou12’ increased by 7.86%, 81.48% and 26.55%, and that of ‘Nan032-4’ increased by 3.95%, 76.67% and 20.00%, respectively, compared with the corresponding normal light treatment. Also compared with seedlings under normal light, leaf photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and photochemical quenching coefficient (qN) of seedlings under low R/FR treatment increased. However, intercellular CO2 concentration (Ci), initial fluorescence intensity (Fo), maximum fluorescence (Fm), maximum photochemical efficiency (Fv/Fm), actual PSⅡ photochemical efficiency (ΦPSⅡ) and light chemical quenching coefficient (qP) decreased under low R/FR treatment. Meanwhile, low-R/FR increased dry matter accumulation. Under low R/FR treatment, Pn and dry matter accumulation of ‘Nandou12’ increased by 37.21% and 12.35% and those of ‘Nan032-4’ increased by 39.04% and 17.36%, respectively. Compared with seedlings under normal light treatment, Ci of ‘Nandou12’ and ‘Nan032-4’ decreased by 9.29% and 11.72%, respectively. However, different soybean varieties had different degrees of response to low-R/FR. Under low-R/FR light condition, the plant height, internode length and petiole length of ‘Nandou12’ were lower than those of ‘Nan032-4’, however, the leaf area, stem diameter, light energy conversion efficiency, photosynthetic rate (Pn), stomatal conductance (Gs) and dry matter accumulation of ‘Nandou12’ seedlings were higher than those of ‘Nan032-4’. This indicated that ‘Nandou12’ variety had stronger shade tolerance than ‘Nan032-4’ variety. Furthermore, this study confirmed that soybean was sufficiently sensitive to low-R/FR shading signal, but different varieties had different sensitivities. Therefore, in intercropping systems, it was recommended to use shade-tolerant soybean varieties to reduce lodging and increase soybean yield.