CARBOHYDRATE AND CARBON METABOLITE ACCUMULATION RESPONSES IN LEAVES OF OZONE TOLERANT AND OZONE SUSCEPTIBLE SPINACH PLANTS AFTER ACUTE OZONE EXPOSURE

The objective of this study was to determine whether exposure of plant s to ozone (O-3) increased the foliar levels of glucose, glucose sourc es, e.g., sucrose and starch, and glucose-6-phosphate (G6P), because i n leaf cells, glucose is the precursor of the antioxidant, L-ascorbate , and glucose-6-phosphate is a source of NADPH needed to support antio xidant capacity. A further objective was to establish whether the resp onse of increased levels of glucose, sucrose, starch and G6P in leaves could be correlated with a greater degree of plant tolerance to O-3. Four commercially available Spinacia oleracea varieties were screened for tolerance or susceptibility to detrimental effects of O-3 employin g one 6.5 hour acute exposure to 25O nL O-3 L(-1) air during the light . One day after the termination of ozonation (29 d post emergence), le aves of the plants were monitored both for damage and for gas exchange characteristics. Cultivar Winter Bloomsdale (cv Winter) leaves were l east damaged on a quantitative grading scale. The leaves of cv Nordic, the most susceptible, were approximately 2.5 times more damaged. Phot osynthesis (Pn) rates in the ozonated mature leaves of cv Winter were 48.9% less, and in cv Nordic, 66.2% less than in comparable leaves of their non-ozonated controls. Stomatal conductance of leaves of ozonate d plants was found not to be a factor in the lower Pn rates in the ozo nated plants. At some time points in the Light, leaves of ozonated cv Winter plants had significantly higher levels of glucose, sucrose, sta rch, G6P, G1P, pyruvate and malate than did leaves of ozonated cv Nord ic plants. It was concluded that leaves of cv Winter displayed a highe r tolerance to ozone mediated stress than those of cv Nordic, in part because they had higher levels of glucose and G6P that could be mobili zed during diminished photosynthesis to generate antioxidants (e.g., a scorbate) and reductants (e.g., NADPH). Elevated levels of both pyruva te and malate in the leaves of ozonated cv Winter suggested an increas ed availability of respiratory substrates to support higher respirator y capacity needed for repair, growth, and maintenance.