Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2002
Publication Date: 1/1/2003
Citation: Burkey, K.O., Eason, G., Fiscus, E.L. 2003. Factors that affect leaf extracellular ascorbic acid content and redox status. Physiologia Plantarum. 117(1):51-57. 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 genotypes is one approach to alleviate this problem. Development of ozone tolerant plants requires knowledge of 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 help protect sensitive tissues from the oxidative stress associated with exposure to elevated ozone. Previous studies from this laboratory have suggested that ozone tolerance in snap bean is correlated with elevated levels of ascorbic acid in the external fluid surrounding leaf cells. This study confirmed the earlier finding, and provided the first evidence that extracellular ascorbic acid content is regulated by both genetics and a second unknown mechanism operating on a short- term daily basis. The results suggest that the process of transporting ascorbic acid from the cytoplasm into the leaf extracellular space involves steps that could be manipulated to enhance the ozone tolerance of crop plants.
Technical Abstract: Leaf ascorbic acid content and redox status were compared in ozone- tolerant (Provider) and ozone-sensitive (S156) genotypes of snap bean (Phaseolus vulgaris L.). Plants were grown in pots for 24 days under charcoal filtered air (CF) conditions in open-top field chambers and then maintained as CF controls (29 nmol mol-1 ozone) or exposed to elevated ozone (71 nmol mol-1 ozone). Following a 10-day treatment, mature leaves o the same age were harvested early in the morning (06:00 to 08:00 hours) or in the afternoon (13:00 to 15:00 hours) for analysis of ascorbic acid (AA) and dehydroascorbic acid (DHA). Vacuum infiltration methods were used to separate leaf AA into apoplast and symplast fractions. The total ascorbate content [AA+DHA] of leaf tissue was 20 percent highter in Provider relative to S156, and Provider exhibited a greater capacity to maintain [AA+DHA] content under ozone stress. Apoplast [AA+DHA] content was 2-fold higher in tolerant Provider (360 nmol g-1 fresh weight maximum) relative to sensitiv S156 (160 nmol g-1 fresh weight maximum) regardless of sampling period or treatment, supporting the the hypothessis that extracellular AA is a factor in ozone tolerance. Apoplast [AA+DHA] levels were significantly higher in the afternoon than early morning for both genotypes, evidence of short-term regulation of extracellular ascorbate content. Total leaf ascorbate was primarily reduced with AA/[AA+DHA] ratios of 0.85 to 0.90. In contrast, apoplast AA/[AA+DHA] ratios were 0.01 to 0.06 and depended on genotype and ozone treatment. Provider exhibited a greater capacity to maintain high AA/[AA+DHA] ratios under ozone stress, suggesting that ozone tolerance is associated with apoplast ascorbate redox status.