Grain Yield, Biomass Accumulation, and Leaf Photosynthetic Characteristics of Rice under Combined Salinity-Drought Stress

The simultaneous stresses of salinity and drought often co-occur during rice-growing periods in saline lands, due to inadequate water resources and poor irrigation facilities. Hence, combined salinity-drought stress poses a major threat to saline rice production. This study performed with two salinity levels (NS, non-salinity; HS, high salinity) and three drought treatments (CC, control condition; DJ, drought imposed at jointing; DH, drought imposed at heading) was conducted to investigate the combined influences of salinity and drought on leaf photosynthetic characteristics, biomass accumulation and yield formation in rice. Salinity, drought and their combination shortened the growth duration from heading to maturity and total growth duration. Grain yield was reduced under the salinity and drought stress, and the reduction was more extensive under the combined salinity-drought stress. The combined stress at heading induced a greater yield loss of rice than the corresponding stress at jointing. The combined salinity-drought stress caused greater decreases in shoot biomass accumulation from heading to maturity, and shoot biomass and nonstructural carbohydrate (NSC) content in the stem at heading and maturity, while increasing the harvest index and NSC remobilization reserve. Salinity and drought reduced the leaf area index and SPAD value of flag leaves and weakened the leaf photosynthetic characteristics as indicated by the lower photosynthesis rate, transpiration rate and stomatal conductance, and the reductions were more pronounced under the combined stress. Salinity, drought, and especially their combined stress decreased the activities of ascorbate peroxidase, catalase and superoxide dismutase, while increasing the contents of malondialdehyde, hydrogen peroxide, and superoxide radical. Our results indicated a greater yield loss of rice when subjected to combined salinity-drought stress. The individual and especially combined stress of salinity and drought reduced the antioxidant enzyme activities, inhibited leaf photosynthetic functions, accelerated leaf senescence, and thereby lowered assimilate accumulation and grain yield.