Abstract
A field experiment was conducted in the paddy field in which cool water (about 11ºC) was continuously irrigated, to determine the most low-temperature-sensitive stage of submerged direct-seeded paddy rice seedlings. Cool water treatments were given from just after seeding (JAS), full emergence stage (FES), and second leaf appearing stage (SLA), for 7 and 14 days. Seeds were sown in plastic nursery boxes used for seedling raising. Each treatment was carried out by moving the nursery boxes from the normal field to the cooling field at the starting stage. Average water temperature in each cooling treatment was below 12ºC. Percentage of seedling establishment was lowest in FES, followed by SLA. Leaf age just before and after the cooling was also examined to find the factors that decrease establishment percentage in each treatment. The decrease in seedling establishment in JAS mainly resulted from the decrease in emergence percentage, but that in FES and SLA mainly resulted from the failure of first leaf elongation, and also from the failure to survive after the first leaf elongation in SLA. These results indicate that the stage of seedlings most sensitive to low temperature is just before the first leaf elongation, in other words, the elongation stage of coleoptile.
