Screening of bean cultivars for their response to ozone as evaluated by visible symptoms and leaf chlorophyll fluorescence

Fourteen Italian cultivars of Phaseolus vulgaris were exposed to a single pulse of ozone (O(3), 150 nl l(-1)) or to filtered air (<3 nl l(-1)) for 3.5 h. O(3) sensitivity was assessed by recording the extent of visible symptoms, effects on chlorophyll (Chl) content and changes in Chl a fluorescence parameters. This paper reports the results of an initial screening of 14 bean cultivars that was used to select a small number of cultivars for further work. Seven cultivars showed visible symptoms of injury in the range of 2-60 h after the end of the O(3) fumigation. O(3) significantly depressed total Chl content in most cultivars and a significant correlation was found between Chl content and visible symptoms. Most cultivars showed a significant change in the F(v)/F(m) ratio, even when there were no visual symptoms. There was no relationship between the extent of visual symptoms and quenching coefficients, indicating that these parameters were of no use in the determination of sensitivity to O(3) stress.     

Effect of chronic ozone fumigation on the photosynthetic process of poplar clones showing different sensitivity.

Rooted cuttings from two poplar clones (Populus x euramericana, I-214, and Populus deltoides x maximowiczii, Eridano) were exposed for 15 days to diurnal square-wave treatment with ozone (60 nL L-1 for 5 h day-1). Completely fully expanded leaves exposed to ozone showed a reduction in net CO2 assimilation rate as compared to the control leaves during whole exposure period in both the clones. The reduction was related to a strong stomatal closure in clone I-214, but also to an altered mesophyll activity ascribed to limitation of the dark reactions of photosynthetic process. The results obtained in leaves of I-214 subjected to long-term fumigation seem to support the view that the decrease in quantum yield of electron transport may be a mechanism to down-regulate photosynthetic electron transport so that production of ATP and NADPH would be in equilibrium with the decreased demand in the Calvin cycle. In Eridano the CO2 assimilation was reduced because of the exposure and any alteration in stomatal conductance was observed. Thus, chlorophyll fluorescence parameters showed that an inhibition of photosystem II had occurred (reduction in Fv/Fm ratio), while no alterations in quenching parameters were observed upon illumination. The results seem to indicate that an alternative sink for reducing equivalent, other than carbon metabolism is present.     

Ozone differentially affects physiological and biochemical responses of two clover species; Trifolium repens and Trifolium pratense

The effect of acute ozone exposure (150 ppb for 3 h) on two clover species, white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) was investigated through the analysis of 10 different physiological and biochemical parameters. Twenty-four hours after fumigation, visible symptoms of injury on leaves were observed only in red clover, but from the biochemical point of view, both species revealed significant ozone-induced modifications. A decrease in the photosynthetic efficiency as well as an increase in the de-epoxidation index and a decrease in the redox state of ascorbate were detected only in T. pratense leaves; no significant change in pigment content was found in either of the two species. On the other hand, both white and red clover showed, although to different extents, significant decreases in the activities of the antioxidant enzymes peroxidase and ascorbate peroxidase. Multivariate statistical analysis revealed not only that ozone affects both species, but also that they differentially respond to the pollutant, confirming the higher sensitivity of Trifolium pratense to ozone exposure.