Influence of low/high temperature on water status in developing and maturing rice grains

Temperature limits productivity and kernel quality of rice grains. Water status of rice (Oryza sativa L. cv. Hinohikari) grains grown at 20, 25 and 30°C was evaluated by NMR relaxation times (T₁ and T₂) in relation to kernel quality during the period of grain development and maturation. During milky and dough stages, 7 to 15 days after flowering (DAF), T₁ values of long and short fractions in rice grains grown at 20°C slightly increased but those grown at 30°C decreased markedly. In rice grains grown at 25 and 30°C, T₂ values of long and short fractions were about 100 and 10 ms, respectively, until 22 DAF but rapidly decreased to about 1 ms and 20 μs, respectively, at 29 DAF (yellow-ripe stage). This means that these grains had free water until 22 DAF , but only loosely bound water and bound water thereafter. On the other hand, in the grains grown at 20°C, T₂ values of long and short fractions were about 100 and 10 ms, respectively, until 29 DAF, indicating that free water was maintained for seven days longer than at higher temperatures. The grains grown at 30°C had white-back kernels in 85% of them and no perfect kernels, while the grains grown at 25 and 20°C had perfect kernels in 68 and 27% of them, respectively. In contrast, the grains grown at 20°C had notched-belly kernels in 28% of them though the grains grown at 25 and 30°C had only a few and no notched-belly kernels, respectively. The present study revealed that the changes in T₁ of rice grains closely related with the quantity of water until mid-mature stage, while T₂ was more sensitive diagnostic indicator for accumulation of dry matter and quality of kernels, which were influenced by low/high temperature stresses.