Free-air CO2 enrichment in a wheat-weed assembly -- effects on water relations
Erbs, M. | Franzaring, J. | Hogy, P. | Fangmeier, A.
In a three-year free-air CO2 enrichment study (Mini-FACE), spring wheat associated with typical arable weeds were grown under present and elevated atmospheric carbon dioxide concentrations [CO2] (ambient air+150 μmol mol-1). Analyses of plant stable carbon isotope ratios and in vivo measurements of leaf gas exchange were used to describe the CO2 effects on water relations. For most species examined elevated [CO2] significantly increased the intrinsic water-use efficiency (A/g(s)) as derived from carbon isotope analyses. In some of the species, seasonal averages of the ratio between leaf internal to atmospheric CO2 (c(i)/c(a)) were found to be significantly reduced by elevated [CO2]. Periodic leaf gas exchange measurements confirmed the increased water-use efficiency, but significant CO2 effects became evident only over the entire season by carbon isotope analysis. In both types of analysis conducted, spring wheat was found to react significantly different from all other species examined. The relation between A/g(s) and biomass production was significantly influenced by elevated [CO2] in all three years of the study. At the end of the drier growing seasons 2003 and 2004, the soil water content tended to be increased in the CO2 enriched plots indicating a water saving effect. These observations demonstrate the impact of elevated [CO2] on plant water relations with a likely positive feedback leading to higher soil water availability. Due to the differences in the CO2 responses of spring wheat compared to the weeds we suggest that rising [CO2] may cause shifts in the species composition of crop-weed communities.
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