THE USE OF CHLOROPHYLL FLUORESCENCE AND LEAF GAS-EXCHANGE AS METHODS FOR STUDYING THE DIFFERENT RESPONSES TO OZONE OF 2 BEAN CULTIVARS

After exposure to 80 ppb of ozone for a single 4 h period, a significa nt reduction in photosynthetic activity was rapidly induced in two cul tivars of Phaseolus vulgaris, cv. Pinto, O-3-sensitive and cv. Groffy, O-3-resistant. Pinto displayed a strong and irreversible reduction in A(max)anied by stomatal closure and an increase in intercellular CO, concentration. The fluorescence parameters indicated that the electron transport around PSII had been altered. The increase in non-photochem ical quenching was related to an energy dissipation through non-radiat ive mechanisms. Photosynthesis was also reduced in the cv. Groffy, but the effect was reversible with time. Groffy showed alteration in the photochemical functioning of the thylakoids and specific disturbances of the water-splitting enzyme system of PSII, but these alterations di sappeared only 20 h after the end of fumigation. Recovery of photosynt hetic ability was more delayed with time and needed 7 d to show values similar to the controls. Physiological tools, such as gas exchange an d chlorophyll fluorescence, play a major role in the early detection o f environmental stress such as O-3 pollution, and in distinguishing th e responses between cultivars with different O-3-sensitivity.