The response of spring wheat (Triticum aestivum L.) to ozone at higher elevations. III. Responses of leaf and canopy gas exchange, and chlorophyll fluorescence to ozone flux.

Spring wheat (Triticum aestivum L., cv. Albis) was grown in open-top chambers and exposed to four different levels of ozone (O ) from the three-leaf stage until maturity. The aim was to examine changes in leaf and canopy gas exchange, and in chlorophyll fluorescence, in response to O flux. Measurements were carried out periodically between full expansion and complete senescence of flag leaves. Fluxes to the canopy of CO (CER .) (corrected for soilborne CO ), water vapour (E ) and O were determined by using open-top chambers as differential systems. Water use efficiency (WUE .) was calculated from CER , and E . Leaf CO (CER ) and H O (E ) exchange rates, stomatal conductance (g,(H O)), and WUE , were analyzed with a portable gas exchange analyzer. Effects of O flux on structural components of photosynthesis were examined by determining variable fluorescence (defined by the F /F ratio) in leaves after 60 minutes of dark-adaptation or during the night. The decline in CER and CER associated with senescence was accelerated by O . Average CER between flag leaf unfolding and late milk stage declined linearly with increasing O flux. The corresponding decline in average CER was less pronounced. The quantitative effect of O flux on CER corresponded well with the effect on grain yield. In young leaves, g (H O) was reduced in response to O but WUE was unaffected. With progressing leaf age, WUE declined. Thus, in the young leaves, O affected the stomata directly and, consequently, limitation of photosynthesis was primarily due to reduced CO diffusion. In contrast, in senescent leaves, the effect of O was mainly due to reduced carboxylation. Compared with WUE WUE responded differently to increasing O flux. During O fluxes at above-ambient levels, WUE tended to increase rather than to decrease. It is suggested that under O stress, factors controlling WUE at the canopy level differ from those operating at the level of single flag leaves. The decline in F /F measured after anthesis was stimulated by O , but no effect of O was detected when F /F was measured during the night. This leads to the conclusion that the effect of O on photosynthetic structures is reversible and the reduction in photosynthesis in response to O flux is due to metabolic changes rather than to direct damage to structural components.