The effect of air and water temperature on the number of days to heading in japonica rice cultivars.

Abstract

The effect of combination of air and water temperatures (29, 23 and 17±2°C respectively) on the number of days to heading, the rate of leaf development and the total number of plant leaves were investigated by using three japonica rice cultivars (Oryza sativa L.) which differ in the length of their basic vegetative phases (BVP) and photoperiod-sensitive phases (PSP). The name of cultivars were Norin 20 (short BVP and short PSP), Norin 1 (long BVP and short SPS), and Zuiho (short BVP and long PSP). The results are summarized as follows : 1) In the weakly photoperiod-sensitive cultivar Norin 2, compared with higher temperature (23/23 or 29/29°C : air/water temperature) the number of days to heading under low temperature due to a retardation of (17/17°C) slightly increased. This was the rate of leaf development. The total numbers of leaves in each treatment remained fairly constant despite varying air and water temperatures. 2) In the strongly photoperiod-sensitive cultivar Zuiho, the number of days to heading under low temperature remarkably increased not only because of a retardation of the rate of leaf development but also because of an increase in the total number of leaves. This leaf increase was determined more by a low air ternperature than by a low water temperature. 3) In the weakly photoperiod-sensitive cultivar Norin 1, the number of days to heading under low temperature did not increase as much as Zuiho. By retarding the rate of leaf development under low temperature the number of days to heading decreased in relation to the decrease in the total number of leaves. This leaf number decrease was determined more by low water temperature than by low air temperature. The general conclusion that can be drawn from these results is that the variation among the cultivars with respect to the effect of temperature on the number of days to heading can not be attributed to the rate of leaf development. Instead it can be attributed to total number of leaves which is determined by the interaction between the air and water temperatures.