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

Varying modes of ozone sensitivity were detected in 12 varieties of Ph aseolus 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 manife sted visible damage at the leaf margins. Sensitive varieties showed oz one-induced chlorophyll loss and/or inhibition of photosynthesis. A se t of physiological characters thought to mediate ozone response was al so assayed in control and fumigated plants: stomatal conductance, the regulator of ozone entry into the leaf; glutathione (as non-protein su lphydryl) and ascorbate, the antioxidants; and polyphenol oxidase acti vity, a measure of the wound response. Some ozone-tolerant varieties a ppeared to lower chlorophyll loss solely by blocking ozone entry throu gh decreased stomatal conductances, whereas other ozone-tolerant varie ties appeared to mitigate ozone damage through the action of the antio xidants, ascorbate, and non-protein sulphydryl. Similarly, ozone-sensi tive varieties showed possible alternative mechanisms of sensitivity. Damage appeared to derive from either comparatively high stomatal cond uctances, 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 da mage. Elevated ascorbate concentrations appeared to protect some varie ties from post-fumigation depressions in photosynthesis. The results w ere inconclusive regarding the role of polyphenol oxidase as the wound response.