The effect of elevated CO2 on photochemistry and antioxidative defence capacity in wheat depends on environmental growing conditions: a FACE study

The present study examines photosynthesis, photochemistry and low weight molecular antioxidants (ascorbic acid and glutathione) of two Triticum aestivum L. cultivars (H45 and Yitpi) in response to growth under two CO2 concentrations (elevated CO2, e[CO2] vs. ambient CO2, a[CO2]), two sowing times (time of sowing 1, TOS1, less stressful growing conditions vs. time of sowing 2, TOS2, more stressful growing conditions) and two water treatments (rain-fed vs. irrigated). The objective was to evaluate (1) if growth under e[CO2] will alleviate climate stresses such as higher temperature and/or limited water supply thereby reducing the need for photoprotection and concentrations of low weight molecular antioxidants and (2) cultivar-specific responses to combined climate change factors which may be useful to identify intra-specific variation in stress tolerance for future breeding. We compared gas exchange, chlorophyll fluorescence and antioxidative defence compounds (ascorbic acid, glutathione) of flag leaves of Australian Grains Free Air Carbon dioxide Enrichment (AGFACE) grown wheat. When plants were grown under the less stressful growing conditions of TOS1, e[CO2] increased light saturated net assimilation rates (Asat) and quantum yield of PSII electron transport (ΦPSII) but decreased thermal energy dissipation (indicated by increased efficiency of open PSII centres, Fv'/Fm'), while antioxidant concentrations did not change. Under the more stressful growing conditions of TOS2, e[CO2] also increased Asat (like at TOS1), however, photochemical processes were not affected while antioxidant concentrations (especially ascorbic acid) were decreased. Cultivar specific responses also varied between sowing dates: Only at TOS2 and additional irrigation, antioxidant concentrations were lower in e[CO2] grown H45 as compared to Yitpi indicating decreased photo-oxidative pressure in H45. These results suggest a photo-protective role of e[CO2] as well as some intra-specific variability between investigated cultivars in their stress responsiveness, all strongly modified by environmental growing conditions