Abstract
The deep water irrigation is one of the most effective countermeasures against the sterility induced by a cool temperature at the booting stage. For the purpose of estimating the water depth necessary for protecting susceptible spikelets, the vertical distribution of spikeltts at the critical stage was surveyed in the whole panicles of a rice plant in paddy fields, uslng 4 varieties in Hokkaido during the years 1972 to 1975. The following relations were observed as to the height of susceptible spikelets above ground. Variety: Norin 20, Yukara > Eiko, Shiokari, Yunami. Year: 1973 > 1972. Fertilizer level : high > medium > low. Seedling age: 5th≒4th-leaf stage. Tiller: main stem > primary > secondary. The results of the present study showed that the water depth covering about 80% of the cool-susceptible spikelets is l7-20 cm and higher than the conventional 12-15 cm depth recommended by Sakai (1949).
