The temperature-sensitive mutant flo11-2 reveals that rice grain chalkiness is more strongly influenced by flowering order than by carbohydrate supply.

Context and objective Rice ( Oryza sativa L.) grain chalkiness is a serious problem under high temperature (HT) and/or low solar radiation. The degree of chalkiness varies among grains within a panicle. However, the spatial and temporal patterns of chalkiness within a panicle and the underlying mechanisms causing this variation remain unclear. Methods In this study, the temperature-sensitive mutant flo11–2 and wild-type cultivars were used to elucidate the distribution of chalkiness ratio and grain area within a panicle under four short-daylength treatments and three nitrogen levels over three years. A two-day HT treatment was applied to examine spatial and temporal changes in susceptibility to chalkiness during grain filling. Results and conclusions Grain area, a possible indicator of available carbohydrates per spikelet, was greater at the upper and middle positions of each primary rachis branch (PB), regardless of the PB position within a panicle. In contrast, the distribution of chalkiness ratio differed markedly from that of grain area and changed progressively from the upper grains of the top PB to the lower grains of the bottom PB under short-daylength treatments that covered a wider range of grain-filling periods. Nitrogen fertilizer treatments conducted over three years further showed that the upper grain position (G1) had a significantly larger grain area than the lower grain position (G7), whereas the upper branch position (B1) exhibited a significantly higher chalkiness ratio than the lower branch position (B10). Because grain sensitivity to HT varied, being higher at 13–14 days after flowering (DAF) than at 21–22 DAF, the distribution of chalkiness ratio could be influenced by how this sensitive period coincided with HT. Significance Rice grain chalkiness is more strongly influenced by flowering order than by carbohydrate supply, which was demonstrated for the first time through the use of the temperature-sensitive mutant flo11–2 .