Partial Mechanism Of Photosynthetic Suppression In Soybean Varieties Under Salt Stress

Salinity is one of the most important abiotic factors limiting crop productivity. Because crop productivity is closely related to photosynthesis, it is very important to study the effect of salinity on photosynthesis in order to illuminate the photosynthetic internal mechanism of crop salt tolerance. Glycine max Melrose a cultivated soybean and S111-9 a somatic hybrid descendant between Melrose and a wild species Glycine cyrtoloba ACC547 were used for studying the effects of salinity on the gas exchange parameters, chlorophyll fluorescence parameters, chlorophyll content and Rubisco activity in them. The photosynthesis was suppressed to a more degree in Melrose than in S111-9. Under salt stress, the photosynthetic rate (P_N) and stomatal conductance (g_s) decreased, while the intercellular CO₂ concentration (C_i) was not influenced significantly at 50 and 100 mM NaCl and even increased at 150 mM NaCl compared with the control of Melrose, indicating that the photosynthetic suppression in Melrose was mainly due to the non-stomatal limitation. However, the decreased P_N and g_s was accompanied by a decrease in C_i in S111-9, indicating that the photosynthetic suppression in S111-9 was mainly due to the stomatal limitation. Although the maximal efficiency of PS II photochemistry (F_v/F_m) decreased at 150 mM NaCl in both Melrose and S111-9, it was lower in Melrose than in S111-9 compared with the control. Saline treatment depressed the practical efficiency of PS II photochemistry (Φ_{PS II}) much more in Melrose than in S111-9. The photochemical quenching (qp) had the same tendency toΦ_{PS II}. By analyzing the CO₂ response curve, it was indicated that Rubisco activity was affected to a more extent in Melrose than in S111-9. Saline treatment depressed the initial activity and total activity of Rubisco in Melrose, which were unchanged in S111-9. It was very interesting that the activation state of Rubisco was not influenced in both Melrose and S111-9. The depression of Rubisco activity diminished the electron transport activity by negative feedback, thus qp fell, causing the depression ofΦ_{PS II} in Melrose under salt stress. In conclusion, the stronger resistance of photosynthetic apparatus to salinity in S111-9 than that in Melrose is due to the fact that the chlorophyll content, PS II photochemistry, Rubisco activity were less affected in S111-9 under salt stress.