Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2001
Publication Date: 9/24/2001
Citation: Burkey, K.O., Eason, G. 2001. Ozone tolerance in snap bean is associated with elevated ascorbic acid in the leaf apoplast. Physiologia Plantarum. 114(3):387-394. 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, snap bean genotypes were compared to determine whether a relationship exists between ozone tolerance and leaf ascorbic acid. Although tolerant genotypes had somewhat higher leaf ascorbic acid content, cellular location of ascorbic acid was found to be a more important factor. Ozone-tolerant genotypes had higher levels of ascorbic acid in the external fluid surrounding leaf cells, and thus appear to have a greater capacity to neutralize ozone before cellular injury can occur. The results suggest that one important aspect of tolerance mechanism is the capacity to transport ascorbic acid from the site of synthesis in the cytoplasm into the extracellular space of leaf cells. If extracellular ascorbate is a key factor in ozone tolerance, then understanding how to control the amount of ascorbic acid in the cell wall is a first step toward developing plants with greater tolerance to ozone and other oxidative stresses.
Technical Abstract: Ascorbic acid in the leaf apoplast has the potential to limit ozone injury by participating in reactions that detoxify ozone and reactive oxygen intermediates to prevent plasma membrane damage. Genotypes of snap bean were compared in the field and in controlled environments to assess the relationship between extracellular ascorbate content and ozone tolerance. Vacuum infiltration methods were employed to separate leaf ascorbic acid into apoplast and cytoplasm fractions. For the field study, plants were grown in pots in open top chambers under charcoal filtered air (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 ascorbic acid was in the range of 100 to 190 nmol g-1 fresh weight for all genotypes, but no relationship was observed between ozone tolerance and extracellular ascorbic acid content. Under controlled environment conditions (4 nmol mol-1 ozone), apoplast ascorbate was significantly higher in tolerant genotypes (300 to 400 nmol g-1 fresh weight) compared with sensitive genotypes ( 50 nmol g-1 fresh weight), evidence that ozone tolerance is associated with elevated extracellular ascorbic acid. The contrasting genotype results from the two studies demonstrated a potential limitation in the interpretation of extracellular ascorbate data. The capacity to detoxify ozone in the extracellular space may be underestimated under stress conditions where the dynamics of ascorbate supply and utilization are not adequately represented by a steady-state measurement.