Factors that affect leaf extracellular ascorbic acid content and redox status

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 of the same age were harvested early in the morning (06:00–08:00 h) or in the afternoon (13:00–15:00 h) 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 averaged 28% higher 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 FW maximum) relative to sensitive S156 (160 nmol g −1 FW maximum) regardless of sampling period or treatment, supporting the hypothesis 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 for short‐term regulation of extracellular ascorbate content. Total leaf ascorbate was primarily reduced with AA/[AA + DHA] ratios of 0.81–0.90. In contrast, apoplast AA/[AA + DHA] ratios were 0.01–0.60 and depended on genotype and ozone treatment. Provider exhibited a greater capacity to maintain extracellular AA/[AA + DHA] ratios under ozone stress, suggesting that ozone tolerance is associated with apoplast ascorbate redox status.