Ozone tolerance in snap bean is associated with elevated ascorbic acid in the leaf apoplast

Ascorbic acid (AA) in the leaf apoplast has the potential to limit ozone injury by participating in reactions that detoxify ozone and reactive oxygen intermediates and thus prevent plasma membrane damage. Genotypes of snap bean ( Phaseolus vulgaris L) were compared in controlled environments and in open-top field chambers to assess the relationship between extracellular AA content and ozone tolerance. Vacuum infiltration methods were employed to separate leaf AA into extracellular and intracellular fractions. For plants grown in controlled environments at low ozone concentration (4 nmol mol⁻¹ ozone), leaf apoplast AA was significantly higher in tolerant genotypes (300–400 nmol g⁻¹ FW) compared with sensitive genotypes (approximately 50 nmol g⁻¹ FW), evidence that ozone tolerance is associated with elevated extracellular AA. For the open top chamber study, plants were grown in pots under charcoal-filtered air (CF) conditions and then either maintained under CF conditions (29 nmol mol⁻¹ ozone) or exposed to elevated ozone (67 nmol mol⁻¹ ozone). Following an 8-day treatment period, leaf apoplast AA was in the range of 100–190 nmol g⁻¹ FW for all genotypes, but no relationship was observed between apoplast AA content and ozone tolerance. The contrasting results in the two studies demonstrated a potential limitation in the interpretation of extracellular AA data. Apoplast AA levels presumably reflect the steady-state condition between supply from the cytoplasm and utilization within the cell wall. The capacity to detoxify ozone in the extracellular space may be underestimated under elevated ozone conditions where the dynamics of AA supply and utilization are not adequately represented by a steady-state measurement.