Photosynthetic performance and sucrose metabolism in superior and inferior rice grains with overlapping growth stages under water stress

Drought stress at jointing and booting stages of plant development directly affects plant growth and productivity in rice. Jointing and booting stages may overlap in high-latitude areas where water deficits occur. However, little is known about the effects of photosynthesis on grain sucrose metabolism and the differences of sucrose metabolism strategies between superior and inferior grains under different drought stress was unclear. In this study, rice plants were subjected to drought stress for 15 days at jointing-booting. Drought stress affected normal leaf growth, and decreased the leaf area index linearly. Short-term mild drought stress had positive effects on photosynthesis, but long-term drought stress reduced the transpiration rate, stomatal conductance, and intercellular CO₂ concentration. Stomatal conductance increased with drought stress duration but increased intercellular CO₂ concentration did not prevent decrease in net photosynthetic rate. Vacuolar invertase activity was important for panicle development (where its activity differed between superior and inferior grains), but not for rice grain filling. Vacuolar invertase activity of drought-sensitive rice varieties superior grains increased by 111.24%~118.46% under drought stress. Drought stress reduced sucrose-phosphate synthase activities in superior and inferior grains. SuSase activity of inferior grains affected sucrose content significantly.