Evidence for preservation of vacuolar compartments during foehn-induced chalky ring formation of Oryza sativa L.

Vacuolar compartments being sustained among the amyloplasts inadequately accumulated in rice endosperm cells are the main cause of chalky ring formation under dry wind conditions. Foehn-induced dry wind during the grain-filling stage induces shoot water deficit in rice (Oryza sativa L.) plants, which form a ring-shaped chalkiness in their endosperm that degrades milling quality and rice appearance. Air spaces formed in several inner cells cause significant transparency loss due to irregular light reflection. Although starch synthesis was suggested to be retarded by osmotic adjustment at foehn-induced moderately low water potential, the source of these air spaces remains unknown. We hypothesised that the preservation of vacuoles accompanied by a temporary reduction in starch biosynthesis in the inner cells leads to the chalky ring formation. Panicle water status measurement, light and transmission electron microscopic (TEM) observations, and an absolute qPCR analysis were conducted. Most starch synthesis-related genes exhibited temporarily reduced expression in the inner zone in accordance with the decrease in panicle water status. TEM observations provided evidence that vacuolar compartments remained among the loosely packed starch granules in the inner endosperm cells, where a chalky ring appeared after kernel dehydration. Taken together, we propose that vacuolar compartments sustained among the amyloplasts inadequately accumulated in rice endosperm cells and caused air space formation that leads to ring-shaped chalkiness under dry wind conditions.