Quenching analysis in poplar clones exposed to ozone

Effects of ozone (O-3) On photo-inhibition of photosynthesis were investiga ted in two poplar clones (Populus deltoides Bartr, ex Marsh. x maximowiczii A. Henry clone Eridano, O-3-sensitive, and P. x euramericana (Dode) Guinie r clone I-214, O-3-resistant) by using pulse amplitude modulation chlorophy ll 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), a s indicated by the ratio of variable to maximum chlorophyll fluorescence, s ignificantly declined in both young and mature leaves of the two clones. Co mpared with control samples, mature leaves of both clones showed difference s 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 dissipa te excitation energy into heat, thus causing non-photochemical fluorescence quenching. The changes observed in chlorophyll a fluorescence of O-3-treat ed 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 chlorophyl l fluorescence yield is discussed.