LEAF EXTRACELLULAR ASCORBTE IN RELATION TO 03 TOLERANCE IN TWO SOYBEAN CULTIVARS

Authors: CHENG, FANG-YI - NORTH CAROLINA STATE UNIV; Burkey, Kent; ROBINSON, J. MICHAEL - RETIRED CSGCL,USDA ARS; Booker, Fitzgerald

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2007
Publication Date: 12/1/2007
Citation: Cheng, F., Burkey, K.O., Robinson, J., Booker, F.L. 2007. Leaf extracellular ascorbte in relation to 03 tolerance in two soybean cultivars. Environmental Pollution. 150:355-362

Interpretive Summary: Tropospheric ozone is a major air pollutant that has adverse effects on the growth and yield of agricultural crops. The use of ozone tolerant plants is one approach to alleviate this problem. Development of ozone tolerant plants requires knowledge of the critical points in plant metabolism that can be manipulated to provide greater protection against ozone stress without sacrificing yield or other desirable characteristics. Plants synthesize antioxidant compounds such as ascorbic acid (vitamin C) that have the potential to neutralize the effects of ozone. In this study, soybean genotypes were compared to determine whether a relationship exists between ozone tolerance and the presence of ascorbic acid in the external fluid surrounding leaf cells, a location in the cellular structure called the apoplast where antioxidants have the potential to neutralize ozone before cellular injury can occur. In soybean, ascorbic acid levels in the leaf apoplast were relatively low and did not appear to contribute to observed genetic differences in ozone tolerance. Evidence was obtained that the leaf apoplast contains antioxidant compounds other than ascorbic acid that may contribute to ozone tolerance in soybean. Identification of these compounds and their reactions with ozone is a critical topic for future research into the development of ozone tolerant crops.

Technical Abstract: Two soybean [Glycine max (L.) Merr.] cultivars that exhibit differences in ozone (O3) sensitivity were used to investigate the potential role of leaf extracellular antioxidants in ozone injury responses. Plants were grown in a greenhouse under charcoal-filtered air (CF) conditions to assess genetic and leaf age effects under low stress conditions, or were subjected to CF or O3 treatments in greenhouse exposure chambers to assess O3 effects. In both cultivars, the extracellular ascorbate pool consisted of 80 to 98 percent dehydroascorbic acid (DHA), the oxidized form of ascorbic acid (AA) that does not function as an antioxidant. The predominance of DHA in the apoplast was associated with relatively high ascorbate oxidase and ascorbate peroxidase activities in extracellular extracts from both young and old leaves. For all combinations of genotype and ozone treatment, extracellular AA levels were relatively low (1 to 32 nmol g-1FW) and represented only 3 to 30 percent of the total antioxidant capacity found. However, total extracellular antioxidant capacity was twofold greater in Essex compared with Forrest, which was consistent with lower foliar injury from O3 in Essex. Concentrations of AA and DHA and ascorbate redox status in whole-leaf tissue samples were generally not indicative of differential O3 sensitivity between the cultivars. The results suggest that extracellular antioxidant metabolites in addition to ascorbate contribute to detoxification of O3 in soybean leaves and possibly affect plant sensitivity to O3 injury.