Drought stress at jointing–booting directly affects plant growth and productivity in rice. Limited by natural factors, the jointing and booting stages of short-growth-period rice varieties are highly overlapped in high-latitude areas, which are more sensitive to water deficit. However, little is known about the dry matter translocation in rice and the strategies of starch synthesis and filling of superior and inferior grains under different drought stress was unclear. In this study, the rice plants were subjected to three degrees of drought stress (−10 kPa, −25 kPa, −40 kPa) for 15 days during the jointing–booting stage; we investigated dry matter accumulation and translocation, grain filling and enzyme activities to starch synthesis of superior and inferior grains in rice with overlapping growth stages from 2016 to 2017. The results showed that drought stress significantly reduced dry matter accumulation in the stems and leaves. Mild and moderate drought increased dry matter translocation efficiency. However, severe drought stress largely limited the dry matter accumulation and translocation. A large amount of dry matter remains in vegetative organs under severe drought stress. The high content in NSC in stem and sheath plays a key role in resisting drought stress. The drought stress at jointing–booting directly caused a change in the grain filling strategy. Under moderate and severe drought, the grain-filling active period of the superior grains was shortened to complete the necessary reproductive growth. The grain-filling active period of the inferior grains was significantly prolonged to avoid a decrease in grain yield. The significant decrease in the grain-filling rate of the superior and inferior grains caused a reduction in the thousand-grain weight. In particular, the influence of the grain-filling rate of inferior grains on the thousand-grain weight was more significant. Drought stress changed the starch synthesis strategies of the superior and inferior grains. Soluble starch synthase and starch branching enzyme activities of inferior grains increased significantly under drought stress. GBSS activity was not sensitive to drought stress. Therefore, amylose content was decreased and amylopectin synthesis was enhanced under drought stress, especially in inferior grains.

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.