22. Effects of temperature on the occurrence of notched grain in rice

The occurrence of notched grain in rice is certainly a genetic character, but is markedly affected by environmental conditions. To investigate the effect of temperature on the occurrence of notched grain, F\2\ plants derived from a cross between a highly notching line L-45 and a normal Japanese cultivar Akiminori were grown at 15 different conditions (Table 1) including Aomori (located at northernmost part of Hohshu) and Naha (the southernmost part of Japan). The plant, and heading date was recorded for each F\2\ population as a whole. The range of heading date within each population was ca. 10 days. Records were also taken for average temperature during the caryopsis elongation period, represented by the average temperature during a 15 day-period starting from the heading date of the earliest plant among the population. It varied from 20-29 degrees C as shown in Table 1.

Table 1. Growing conditions, average temperature at caryopsis elongation period, notched grain expression and caryopsis length in L-45 X Akiminori F\2\ population

In each F\2\ population, percent notched grain showed a continuous distribution. The distribution pattern markedly differed according to growing conditions, namely, the percentage of plants with notched grain varied from 25 to 100%, and the maximum notched grain percentage within a population varied from 43 to 100%. The population mean of notched grain percentage was as high as 64% when grown in the phytotron with a temperature after heading of 20 degrees C, while it was lower than 10% when the temperature was above 24 degrees C.

The correlation coefficient between the population mean of notched grain percentage (in probit scale) or percentage of plants with notched grains (tranformed to angles) and average temperature during the caryopsis elongation period was as high as -0.915 and -.930, respectively. This suggests that the temperature during the caryopsis elongation period was the primary determinant factor for the environmental variation in notched grain occurrence. On the other hand, population mean for the caryopsis length showed a very small range of variation, and it was independent of the notched grain occurrence.