BUSH BEAN (PHASEOLUS-VULGARIS L) LEAF INJURY, PHOTOSYNTHESIS AND STOMATAL FUNCTIONS UNDER ELEVATED OZONE LEVELS

Three bush bean cultivars (Lit, Groffy and Stella) were grown under fo ur levels of ozone exposure (ambient air+50 ppb O-3 ambient air+25 ppb O-3, ambient air and charcoal filtered air) in open- top chambers. Nu mber and leaf injury statistics showed significant reduction in the nu mber of healthy leaves as the level of O-3 increased. The area based l eaf injury percentages of the cvs. Lit, Groffy and Stella were 69.8, 5 7.9 and 71.1% at the highest O-3 level, 24.1, 19.6 and 30.3%, at the 2 nd highest O-3 level, and 4.5, 0.7 and 5.6% at the ambient air, respec tively. The plants grown in the filtered air revealed no injury sympto ms. The stomatal conductances were found to decrease gradually in each cultivar as the O-3 level increased. At the highest O-3 level, Chloro phyll fluorescence measurements on the 2nd leaf from the top on 24th d ay of exposure resulted in significantly the highest Fv/Fm values, the lowest F-0 and the highest F-m values whereas the 4th leaf showed the smallest Fm and correspondingly the smallest F-v/F-m values. This is an indication of photosystem II damage after accumulation of a high oz one dose in the 4th leaf. The photosynthetic rate of the 2nd leaf meas ured on 30th day of exposure was comparatively higher at the highest e xposure but the data taken from the same leaf on 40th day of exposure showed significantly tower photosynthetic rate than the plants in ambi ent air. Both chlorophyll fluorescence and photosynthetic measurements indicated that ozone stressed leaves experience a stimulation of phot osynthesis (possibly due to increased assimilate demand) prior to irre versible damage. Bush bean leaves need to accumulate a critical ozone dose(an AOT40 of presumably > 18 ppm-h) for reduction of the photosynt hetic capacitys.