EFFECTS OF EXPOSURE TO OZONE AT DIFFERENT STAGES IN THE DEVELOPMENT OF PLANTAGO-MAJOR L ON CHLOROPHYLL FLUORESCENCE AND GAS-EXCHANGE

Similar ozone treatments were applied at different stages of growth to a population of Plantago major L., which is as sensitive to ozone as Bel-W3 tobacco. Plants were grown from seed for 8 wk in controlled-env ironment chambers and exposed to 70 mu l O-3 1(-1) 7h d-(1) for the wh ole period or for 2-wk episodes during weeks 1+2, 3+4, 5+6 or 7+8. Con trols had charcoal-filtered air. Effects on stomatal conductance, chlo rophyll fluorescence and net photosynthesis are described. The fluores cence character, t1/2, proved to be very sensitive to ozone, and it re sponded at all stages of plant development, but the ratio of variable to maximum fluorescence decreased only when plants received ozone duri ng the first 2 wk of growth. The reduction was caused by a drop in max imum fluorescence. Ozone had no effect on F-0. Whenever exposure occur red, O-3, significantly reduced net CO2 assimilation and increased sto matal conductance. Unlike t1/2, the effects were persistent, affecting leaves that were in bud or enshrouded by leaves when the exposure occ urred. It is suggested that these persistent effects may have been cau sed by changes in resource allocation, flowering and senescence, which resulted in altered hormonal balance.