Increased Ring-Shaped Chalkiness and Osmotic Adjustment when Growing Rice Grains under Foehn-Induced Dry Wind Condition

Foehn-induced dry wind during grain filling increased ring-shaped chalky kernels in rice (Oryza sativa L.) plants. The objective of this study was to determine physiological mechanisms of the occurrence of ring-shaped chalky kernels. Rice plants were subjected to water deficit in a paddy field after shade by applying dry high-speed wind. Additionally, a growth chamber experiment was conducted with plants in pots to measure the water status under the dry wind condition for 24 h by combining in situ turgor (Ψ p ) assay in developing endosperms with the water potential measurements. The dry (high vapor pressure deficit [VPD]) wind treatment produced the largest number of ring-shaped chalky kernels due to poor starch accumulation, compared with shade or low-VPD wind treatment. The inner endosperm cells, where a high frequency of chalkiness was observed, spatially maintained Ψ p by osmotic adjustment before the chalky formation with no decline of grain weight. Dry wind reduced photosynthesis due to a partial stomatal closure after water deficit developed. However, these responses, including those related to the plant water status, returned to a level similar to those of the control plants in a day after the dry wind was stopped. We conclude that (i) Ψ p maintenance by osmotic adjustment contributes to grain development under water deficit under foehn conditions and (ii) osmotic adjustment has a role in temporally inhibiting starch accumulation in endosperms, resulting in ring-shaped chalky kernels under foehn-induced water deficit conditions.