Combining 4 classes of air-temperature (36, 31, 21 and 16°C) with 4 classes of water-temperature (36, 31, 21 and 16°C) to give 16 different factorial conditions, and subjecting rice plants to these different conditions for a 15-day period at different stages of growth under natural sunlight, the authors examined the combined effects of air- and water-temperature on the grain yield and its components. The variety used is Norin No. 25, a mid-term variety. The results may be summarized as follows : 1. In the early growth period (Stage I and II, cf. Table 1), only water-temper-ature has a remarkable effect and air-temperature has no effect on yield, i. e. a water-temperature of 31°C always has the most favourable effect and that of 21°C has the second most favourable effect, while that of 16°C has the most unfavourable effect on yield under any air-temperatures (cf. Table. 2). 2. In the middle growth period (Stage III and IV), both air-temperature and water-temperature have serious effects on yield, i. e. under any water-temperatures an air-temperature of 31°C increases yield most, while that of 16°C decreases most, and under most air-temperatures a water-temperature of 31°C increases yield most, but under some other air-temperatures a water-temperature of 21°C increases most (cf. Table. 2). 3. In the late growth period (Stage V and VI), only air-temerature has a remarkable effect, while water-temerature has no effect on yield, an air-temperature of 21°C having the most favourable effect on yield. At the Stage V a remarkable detrimental effect of a high temperature of 36°C as well as that of a low temperature of 16°C is found on yield, but at the Stage VI a low temperature as low as 16°C has a beneficial effect on yield and high temperatures such as 36°C and 31°C have remarkable detrimental effects on yield (cf. Table. 2). 4. It has also been confirmed from the results of the present experiment that the influences of air-temperature and water-temperature which increase the yield positively can act only up to the spikelet initiation stage, in particular they act most at the initial stage of panicle differentiation, and after that stage only the influences which can prevent a reduction in yield can act. It has therefore been made clear that the importance of water-temperature in increasing yield in the early growth period as well as the seriousness of the detrimental effect of high air-temperature on yield in the late growth period cannot be over-emphasized. 5. The above mentioned facts are reasonably supported by the responses of yield-components to air- and water-temperature in different stages of growth (cf. Table. 2). 6. The main reason why the grain yield and its compents are strongly influenced by only water-temperature in the early growth period (up to the necknode initiation stage), by both air-temperature and water-temperature in the middle growth period (from the neck-node initiation stage up to the final stage of reduction division) and by only air-temperature in the late growth period can likely be ascribed to the fact that all the growing points of rice plants are always below the surface of water during the early growth period, and some of them are located above the water-surface and some of them still below the water-surface during the middle growth period, while all of them are definitely located above the water-surface in the late growth period.
Crop Science Society of Japan