The influence of temperature of irrigation water upon the growth of paddy rice in the warmer district. : VI. Effects of day and night water-temperatures on the vegetative growth : In relation to air temperature change accompanied with shifting of cultivation period.

The present study was designed to evaluate the effects of different water temperatures under different air temperatures upon the vegetative growth of rice plant. Pot cultured rice plants were grouped in sixteen plots. Each plot was subjected to different combination of day and night water temperatures for twenty days in four water bathes regulated at 26°C, 30°C, 34°C, and 37°C, respectively. These tests were conducted at nine different periods from April to November. (1) Plant height increased lineary as the air temperature ascended, but those gradients were weaker under the conditions of high water temperature, especially, more than 34°C at night. Optimal water temperature for plant height lowered with increasing air temperature. (2) Favourable effects of daily wide range of water temperature on the number of tillers were clear, especially, at the period with optimal air temperature (ca. 20°C-26°C) for increasing of tillers. (3) High air temperature had effectiveness to weiden the optimal water temperature range in which the number of main stem's leaves increases. (4) Both effects of air and water temperatures on the weights of top and root, showed similar trend of the effect on the plant height. In general, it is assumed that opimal water temperature for rice growth lowered inversly as the air temperature ries.