Shortage of assimilate supply to grain increases the proportion of milky white rice kernels under high temperatures

Increase in the proportion of milky white rice kernels (MWK) has seriously affected rice quality in west Japan during the last half-decade. Rising of temperature has been considered to be the primary cause of the spread of MWK. It is suspected that a lack of assimilate supply to grains increases the proportion of MWK, because high temperatures during the grain-filling period could increase the grain growth rate without profoundly affecting assimilate production. Our objective was to determine whether MWK in the higher temperature conditions could be reduced if assimilate supply during the grain-filling period met the requirement for realizing the accelerated grain-growth rate. Rice cv. Koshihikari was grown at three locations in western Japan over three years. At one location, plots were covered with plastic-film during the grain-filling period to increase temperature. When the mean temperature ranged between 23 and 29°C during the grain-filling period, the rate of MWK varied between 1 and 16% and spikelet filling percentages (F%) at maturity ranged between 70 and 90%. When the plots were thinned to half density during the grain-filling period, all F% attained a ceiling of 90%. MWK almost decreased to below 6%. Hence, thinning can overcome the lower F% and higher MWK. These results suggested that a lack of assimilate supply to the grains resulted in MWK. Any technique that increases assimilation after the heading will be suitable for decreasing MWK.