Postanthesis Water Deficits Enhance Grain Filling in Two-Line Hybrid Rice

Two-line hybrid rice (Oryza sativa L.) is a hybrid from a thermosensitive genetic male sterile line and its corresponding restoring line. This hybrid rice shows strong heterosis but a slow grain-filling rate, as a result of unfavorably delayed senescence. This study investigated the possibility that a moderate water deficit imposed during grain filling may enhance plant senescence, and therefore facilitate the remobilization of reserve C to the grains and improve grain filling. Two two-line indica hybrids were grown in both the pot and field. Three levels of soil water potential, well watered (WW), moderate water deficit (MD), and severe water deficit (SD), were imposed from 9 d after anthesis to maturity in both pot and field experiments. Results showed that Chlorophyll content and photosynthetic rate of the flag leaves declined more quickly as plants approached maturity for MD and SD plants than for WW plants, indicating the water deficits enhanced senescence. The remobilized C reserve and reallocation of pre-fixed ¹⁴C from stems to grains increased by 1.5- to 2-fold and 55 to 67%, respectively, for water deficit plants, when compared with WW plants. The water deficit treatments shortened the grain-filling by 5 to 17 d and increased the grain-filling rate by 0.09 to 0.27 mg d^-1 grain^-1. The grain yield of MD plants in both experiments was increased by 8.2 to 10% but that of SD plants in the pot was reduced. We conclude that if water deficit is properly controlled during the grain filling, whole-plant senescence is enhanced. The enhanced senescence can facilitate remobilization of C reserves, accelerate grain filling, and increase grain yield in the two-line hybrid rice.