Leaf ascorbic acid level--is it really important for ozone tolerance in rice?

Leaf ascorbic acid (ASA) level is thought to be an important trait conferring stress tolerance in plants, but definite evidence regarding its effectiveness in the breeding of stress tolerant crops is lacking. Therefore, the stress response of a rice TOS17 insertion mutant (ND6172) for a GDP-D-mannose-3',5'-epimerase gene, which is involved in ASA biosynthesis, was tested. Two fumigation experiments were conducted, in which rice plants (Oryza sativa L.) were exposed to (i) high ozone for ten days at the tillering stage (100 ppb, 7 h day⁻¹); and (ii) to four different ozone concentrations ranging from charcoal filtered air to 2.5 times the ambient concentration for the entire growth season. The mutant ND6172 had around 20-30% lower ASA level than the wild-type (Nipponbare), and exhibited a moderately higher level of visible leaf symptoms due to ozone exposure. Differences in ASA level between ND6172 and Nipponbare led to differential responses of the glutathione level, and the activities of glutathione reductase, ascorbate peroxidase, and dehydroascorbate reductase. With season-long ozone fumigation, yields and yield components were not negatively affected at ambient ozone level in both genotypes, but showed stronger decreases in ND6172 at higher ozone levels, especially at 2.5 times the ambient level. Similarly, the mature straw of ND6172 exhibited a higher degree of lignification at the 2.5 times ambient ozone level. In conclusion, a difference in leaf ASA level of around 20-30% is relevant for ozone tolerance in rice at levels exceeding the current ambient ozone concentrations.