Ozone sensitivity in Triticum durum and T-aestivum with respect to leaf injury, photosynthetic activity and free radical content

Sensitivity to ozone is highly variable in cultivars of different wheat spe cies, leading to differences in leaf injury and yield. Not much is known ab out the physiological background of these differences. The objective of thi s 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-satur ated net photosynthesis, leaf conductance for water vapour, and chlorophyll fluorescence were measured. Stomatal limitation was calculated from CO2 re sponse 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 thre e cultivars, in the order Nandu > Perlo > Extradur. Development of whole pl ants was slightly delayed in Perlo and Nandu, but was accelerated significa ntly 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 behavi our, but stomatal limitation (l) was similar between ozone-treated and cont rol plants. Thus, an ozone-induced closure of stomata was not the reason fo r 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, where as in Extradur the decrease was fully reversible only in older leaves. Whol e 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 abili ty of photosynthetically active mesophyll cells to cope with photooxidative stress.