Abstract
In 1964, the authors carried out a series of experiments to examine the differences in severall items as mentioned hereunder among seventeen verieties of lowland rice raised in cool regions of Japan, each having its own peculiar characteristic. These varieties were treated by being left in cool irrigation water (16°C) for twenty-two days at their tillering stages in order to clarify the varietal differences in the delay of heading time, the number of leaves on a main culm, and the leaf emerging velocity caused by such a low temperature. The results are summarized as follows: 1. The varietal difference of delaying degree of young panicle differentiation stage as well as heading time was clearly observed by the treatment with cool irrigation water (Fig. 1). No significant correlation, however, could be recognized between the delaying degree and the intrinsic earliness in growth (Fig. 2). 2. In most varieties the total number of leaved on a main culm was decreased by low water temperature at the tillering stage and in all the varieties the reduction of the leaf emerging velocity was observed (Table 1). Among these findings it is worthy of notice that there exists a high positive correlation between the leaf emerging velocity of varieties under ordinary water temperature (22°C) and that of those under low water temperature with a few exceptional varieties, having cleary lower reducing degree of the leaf emerging velocity under low water temperature in comparison with that of others (Fig. 3). 3. The variations both in the total number of leaves on a main culm and in the leaf emerging velocity were found closely related to the delaying degree of heading (Fig. 4, 5), but as a rule the latter was greater in the influential ability upon the delaying degree than the former. 4. The delaying index of heading composed of the changing rates of the above mentioned two factors has much higher correlation with the delaying degree of heading in comparison with the case of the individual factor (Fig. 7).
