Sensitivity to ozone is highly variable in cultivars of different wheat species, leading to differences in leaf injury and yield. Not much is known about the physiological background of these differences. The objective of this study was to compare the effects of ozone on photosynthetic parameters in Triticum aestivum L. (spring wheat cv. Nandu, winter wheat cv. Perlo) and Triticum durum Desf. (cv. Extradur). Plants cultivated in pots were exposed to 80 nmol mol(-1) ozone, or were used as control plants in a greenhouse. Stages of growth and senescence of single leaves were recorded. Light-saturated net photosynthesis, leaf conductance for water vapour, and chlorophyll fluorescence were measured. Stomatal limitation was calculated from CO2 response curves, and the free radical content of whole leaves measured by EPR spectroscopy. Senescence of single leaves was enhanced by the ozone-treatment in all three cultivars, in the order Nandu > Perlo > Extradur. Development of whole plants was slightly delayed in Perlo and Nandu, but was accelerated significantly in Extradur. The rate of net photosynthesis under light saturation (A(sat)) decreased significantly in older, ozone-fumigated leaves of Perlo and Nandu but not of Extradur. Leaf conductance (g(1)) showed a similar behaviour, but stomatal limitation (l) was similar between ozone-treated and control plants. Thus, an ozone-induced closure of stomata was not the reason for the observed difference in A(sat). Perlo and Nandu showed a significant, only partly reversible decrease in F-v/F-m in ozone-fumigated leaves, whereas in Extradur the decrease was fully reversible only in older leaves. Whole leaves of Extradur, in contrast to Perlo and Nandu, showed no increase in EPR free radical signals. The higher ozone tolerance of Extradur was thus not caused by decreased ozone uptake via the stomata, but by a better ability of photosynthetically active mesophyll cells to cope with photooxidative stress.
