Effect of Calcium Acetate and Calcium Chloride on Grain Morphology and Antioxidant Regulation in rice Under Ozone Stress

In this study, 5 mM calcium acetate and calcium chloride were individually sprayed twice on rice leaves to study their function in antioxidant response under ozone stress (125 ± 25 ppb). Results showed that elevated O3 during rice cultivation impaired photosystem II (PSII) and carbon dioxide assimilation, as indicated by a decrease in effective quantum yield of PSII and electron transport rate and an increase in intracellular carbon dioxide. This occurrence led to growth inhibition resulting in rice yield and grain size (width and thickness) reduction, accompanied with the increase of malondialdehyde (MDA) content in plant cells under O3 stress. Rice plant pretreated with calcium acetate could endure long-term O3 exposure through the enhancement of NAD kinase activity and NADPH content, leading to an increase in ascorbate peroxidase (APX) and glutathione reductase (GR) activities and higher levels of reduced ascorbic acid (ASA) and reduced glutathione (GSH) under long-term O3 exposure. Consequently, the application of calcium acetate led to increase PSII efficiency and reduce cell damages (MDA content), resulting in an increase in rice yield and grain size under O3 stress. Under long-term O3 exposure, pretreatment of rice with calcium chloride resulted in a decrease in APX activity and an increase in MDA level, resulting in a rice yield similar to that of the non-pretreatment. It suggested that calcium acetate pretreatment of rice was more effective than pretreatment with calcium chloride in terms of reducing stress and maintaining antioxidant enzyme activities, and redox compounds in cells under long-term O3 exposure.