The ozone is the primary gaseous pollutant with significant adverse effects on vegetation. Those effects include visible leaf injury, growth and yield reductions, accelerated senescence and altering sensitivity to biotic and abiotic stresses. Reductions in photosynthesis rate, stomata conductance, and root/shoot ratio, increase in respiration, and change in crop quality has also been observed in many crops. With fast industrial development, tropospheric ozone in the Yangtze River Delta, in eastern China, has risen since the later of the last century. The elevated ozone has become a prominent environmental and economic issue in this region.
In order to alleviate the phytotoxin of elevated level O₃ to crops, the effect of exogenous ascorbate acid (ExAsA) on membrane protective system of rice (Oryza sativa L.) leaves was investigated. Rice was grown in open top chambers (OTCs) under field conditions and exposed to five levels of O₃ treatments: charcoal-filtered air (CF), non-charcoal-filtered air (NF), 50 nl/L, 100 nl／L， and 200nl/L O₃. In addition, half of the plot within the OTC was treated by spraying 0.1% ExAsA solution once per week. The results showed that with spraying ExAsA, chlorophyll a content of rice leaves distinctly increased; H_２Ｏ_２ content, membrane permeability (indicated by the relative electrical conductivity, REC), and lipid peroxidation (indicated by the content of Malondialdehyde, MDA) of rice leaves declined; activities of superoxide dismutase (SOD), ascrobate peroxidase (APX), and carotenoid (Car) content increased. From these results, it can speculated that ExAsA defers leaf senescence, reduces accumulation of active oxygen species (AOS) and lipid peroxidation (LP) of leaves, enhances the function of the membrane protective system, and increases the tolerance ability of rice leaves under elevated O₃ exposure.