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 (O3 ) 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 O3 flux. Measurements were carried out periodically between full expansion and complete senescence of flag leaves. Fluxes to the canopy of CO2 (CERc .) (corrected for soilborne CO2 ), water vapour (Ec ) and O3 were determined by using open-top chambers as differential systems. Water use efficiency (WUEc .) was calculated from CERc , and Ec . Leaf CO2 (CERn ) and H2 O (En ) exchange rates, stomatal conductance (g,(H2 O)), and WUEn , were analyzed with a portable gas exchange analyzer. Effects of O3 flux on structural components of photosynthesis were examined by determining variable fluorescence (defined by the Fv /Fo ratio) in leaves after 60 minutes of dark-adaptation or during the night. The decline in CERc and CERn associated with senescence was accelerated by O3 . Average CERn between flag leaf unfolding and late milk stage declined linearly with increasing O3 flux. The corresponding decline in average CERc was less pronounced. The quantitative effect of O3 flux on CERc corresponded well with the effect on grain yield. In young leaves, gs (H2 O) was reduced in response to O3 but WUEn was unaffected. With progressing leaf age, WUEn declined. Thus, in the young leaves, O3 affected the stomata directly and, consequently, limitation of photosynthesis was primarily due to reduced CO2 diffusion. In contrast, in senescent leaves, the effect of O3 was mainly due to reduced carboxylation. Compared with WUEn WUEc responded differently to increasing O3 flux. During O2 fluxes at above-ambient levels, WUEC tended to increase rather than to decrease. It is suggested that under O3 stress, factors controlling WUE at the canopy level differ from those operating at the level of single flag leaves. The decline in Fv /Fo measured after anthesis was stimulated by O2 , but no effect of O3 was detected when Fv /Fo was measured during the night. This leads to the conclusion that the effect of O3 on photosynthetic structures is reversible and the reduction in photosynthesis in response to O3 flux is due to metabolic changes rather than to direct damage to structural components.