CO2 photoassimilation and chlorophyll fluorescence in two clover species showing different response to O3

The current study evaluated photosynthetic processes in two clover species based on their performance under high O3 conditions (150 nl l–1 for 3 h). These species are Trifolium repens L. and Trifolium pratense L., which are well known for their different sensitivity to ozone. Ozone affected the two clover species very differently. In T. pratense, ozone induced visible symptoms of damage even though CO2 photoassimilation, stomatal conductance and the electron transport rate were not affected. A decrease in the optimal quantum yield was observed in T. pratense immediately after the end of the period of O3 stress but it reverted to a value similar to the control 24 h after removing the stress, indicating that non-irreversible photoinhibition had occurred. Data obtained for symptomatic T. pratense indicate that still-green living tissue of the leaf is able to carry out CO2 assimilation; the only parameter found to be affected by O3 was the efficiency of excitation capture. In T. repens, acute O3 fumigation induced inhibition of photosynthetic activity, enhanced stomatal closure and increased the reduction state of the PSII primary acceptor. A possible explanation for the inhibition of photosynthesis could reside in inhibition of the Calvin cycle over-reducing PSII, thus increasing (1 – qP) and increasing non-photochemical quenching.