Carbon remobilization and grain filling in japonica/indica hybrid rice subjected to postanthesis water deficits

Poor grain filling, as in unfilled grains, and unused carbohydrate in straws are two major problems in japonica/indica (J/I) hybrid rice (Oryza sativa L.). This study investigated if an early senescence, induced by controlled postanthesis soil drying, could enhance the remobilization of reserved C to the grains, and therefore improve the grain filling. Two J/I hybrids and one indica/indica (I/I) hybrid were field-grown. Three levels of soil water potential (ψ_soil) - well watered (ψ_soil = 0), moderate water deficit (ψ_soil = -0.025 MPa), and severe water deficit (ψ_soil = -0.05 MPa) - were imposed 9 d after anthesis to maturity. As a result of water deficit treatment, chlorophyll concentrations and photosynthetic rates of flag leaves declined more quickly, indicating an enhanced senescence. The remobilized C reserve from the culm and sheath during grain filling was increased by 47 to 61% for the J/I hybrids and 12 to 26% for the I/I hybrid. Partitioning of ¹⁴C from the flag leaves to the grains was increased by 18 to 28%. Active grain-filling period was shortened by 2.7 to 8.7 d, and grainfilling rate increased by 0.14 to 0.36 mg d^-1 grain^-1. Water deficits reduced the grain yield of the I/I hybrid by 2.9 to 16.4% but increased that of the two J/I hybrids by 4.4 to 13.3%. We conclude that if water deficit is properly controlled for J/I, the gain from enhanced remobilization of stored C may outweigh the loss due to shortened photosynthetic duration, leading to a fast grain filling and high grain yield in cases where senescence is unfavorably delayed.