Rice Chalky Ring Formation Caused by Temporal Reduction in Starch Biosynthesis during Osmotic Adjustment under Foehn-Induced Dry Wind

Abstract

Foehn-like extreme hot and dry wind conditions (34°C, >2.5 kPa vapor pressure deficit, and 7 m s−1) strongly affect grain quality in rice (Oryza sativa L.). This is a current concern because of the increasing frequency and intensity of combined heat and water-deficit stress under climate change. Foehn-induced dry wind conditions during the grain-filling stage increase ring-shaped chalkiness as a result of spatiotemporal reduction in starch accumulation in the endosperm, but kernel growth is sometimes maintained by osmotic adjustment. Here, we assess the effects of dry wind on chalky ring formation in environmentally controlled growth chambers. Our results showed that hot and dry wind conditions that lasted for >24 h dramatically increased chalky ring formation. Hot and dry wind conditions temporarily reduced panicle water potential to –0.65 MPa; however, kernel growth was maintained by osmotic adjustment at control levels with increased transport of assimilate to the growing kernels. Dynamic tracer analysis with a nano-electrospray-ionization Orbitrap mass spectrometer and quantitative polymerase chain reaction analysis revealed that starch degradation was negligible in the short-term treatment. Overall expression of starch synthesis-related genes was found to be down-regulated at moderately low water potential. Because the events observed at low water potential preceded the packing of starch granules in cells, we concluded that reduced rates of starch biosynthesis play a central role in the events of cellular metabolism that are altered at osmotic adjustment, which leads to chalky ring formation under short-term hot and dry wind conditions.

Multiple strategies for heat adaptation in rice endosperms revealed by on-site cell-specific analysis

“…During the osmotic adjustment, turgor pressure can be maintained by accumulating osmotically active solutes, such as sugars and amino acids, into the cells (Morgan, 1977; Meyer and Boyer, 1981). As reported previously (Wada et al, 2011; Wada et al, 2014), osmotic adjustment also occurred in the rice endosperms cells growing under dry wind conditions prior to chalky ring formation. On-site cell metabolomics and in situ turgor assay conducted in outer endosperms indicated clear treatment differences in the heat adaptive responses (Figure 3B-E).…”

Chalky part differs in chemical composition from translucent part of japonica rice grains as revealed by a notched‐belly mutant with white‐belly

“…Within the endosperm, formation of chalky tissues is considered as related to insufficient vascular supply of metabolites such as sucrose, perturbation of growing of starch granules in amyloplast or interruption in starch biosynthesis . Similarly, SEM images of this study show the incomplete development of starch granules in the chalky parts, and analysis of amylopectin structure demonstrates that impaired synthesis of long chains occurs in the chalky parts.…”

Rice grain quality degradation and economic loss due to global warming in Japan

“…There are some limitations and challenges in the model. The occurrence of CRG is known to depend on not only air temperature, but also radiation [21,41], wind speed [42], CO 2 concentration [43], and management, including nitrogen application [44] and planting density [45]. Although our simple model was accurate enough for estimating the occurrence of CRG under current conditions, it is an important challenge to develop and use a model that considers the other factors.…”