RESPONSES TO OZONE OF VARIETIES OF COMMON BEAN (PHASEOLUS-VULGARIS L)

Varying modes of ozone sensitivity were detected in 12 varieties of Phaseolus vulgaris. Seedlings were exposed to 0.4-0.5 mul l-1 ozone for 75-135 min in growth chambers, until the most sensitive variety manifested visible damage at the leaf margins. Sensitive varieties showed ozone-induced chlorophyll loss and/or inhibition of photosynthesis. A set of physiological characters thought to mediate ozone response was also assayed in control and fumigated plants: stomatal conductance, the regulator of ozone entry into the leaf; glutathione (as non-protein sulphydryl) and ascorbate, the antioxidants; and polyphenol oxidase activity, a measure of the wound response. Some ozone-tolerant varieties appeared to lower chlorophyll loss solely by blocking ozone entry through decreased stomatal conductances, whereas other ozone-tolerant varieties appeared to mitigate ozone damage through the action of the antioxidants, ascorbate, and non-protein sulphydryl. Similarly, ozone-sensitive varieties showed possible alternative mechanisms of sensitivity. Damage appeared to derive from either comparatively high stomatal conductances, or comparatively low levels of antioxidants in spite of low stomatal conductances. Only one variety showed elevated photosynthesis rates in response to ozone, indicating a potential to repair ozone damage. Elevated ascorbate concentrations appeared to protect some varieties from post-fumigation depressions in photosynthesis. The results were inconclusive regarding the role of polyphenol oxidase as the wound response.