Quenching analysis in poplar clones exposed to ozone.

Effects of ozone (O(3)) on photo-inhibition of photosynthesis were investigated in two poplar clones (Populus deltoides Bartr. ex Marsh. x maximowiczii A. Henry clone Eridano, O(3)-sensitive, and P. x euramericana (Dode) Guinier clone I-214, O(3)-resistant) by using pulse amplitude modulation chlorophyll fluorescence. After a 15-day exposure to O(3) at 60 nl l(-1) for 5 h day(-1), the effective photosynthetic quantum yield of photosystem II (PSII), as indicated by the ratio of variable to maximum chlorophyll fluorescence, significantly declined in both young and mature leaves of the two clones. Compared with control samples, mature leaves of both clones showed differences in fluorescence quenching and quantum yield of PSII when exposed to ozone fumigation. We conclude that photo-inhibition is caused by transformation of active reaction centers to photochemically inactive centers that dissipate excitation energy into heat, thus causing non-photochemical fluorescence quenching. The changes observed in chlorophyll a fluorescence of O(3)-treated young leaves in both clones are consistent with O(3) having a negligible effect on photosynthetic performance, but inducing a premature decrease in chlorophyll a concentration. A possible role of O(3) pollution on chlorophyll fluorescence yield is discussed.