Summary and Conclusions

Wheat productivity (biomass and grain yield) will increase by some 7-11% per 100 mmol mol-1 increase in [CO2] without other environmental changes under well-fertilized and watered conditions, but less where there are nutrient limitations. Under conditions of water limitation the benefit to production is potentially greater, increasing water-use efficiency, although water use will probably be relatively unaffected. All of the observed responses of wheat to elevated [CO2] can be explained by two direct effects: (i) increased photosynthesis and decreased photorespiration; and (ii) decreased stomatal conductance. Effects of elevated [CO2] on tillers, leaf area, ears, grain, photo-synthetic capacity and development are all explicable as consequences of these and it is unnecessary to invoke other direct effects. A constant 1°C increase in temperature over the whole season would decrease yields by 6-10%, due to shorter duration of crop growth. This effect might be overcome by selection of slower-developing varieties where water is not limiting, but such varieties would be undesirable in today's environment. The overall effect of GEC on wheat productivity cannot be predicted with any certainty, partly because local changes in radiation and evaporative demand and interactions with changing technology are unknown. Wheat models have been used to suggest that GEC will reduce productivity, but the ability of such models to predict environmental effects on wheat yield over widely varying climate, varieties, soils and agricultural practices is unproven. The response to GEC will also require technological adjustments in wheat production by breeding, by incorporating new genetic resources and by agronomic techniques. Such changes will be required worldwide if the GEC problem and the demands arising from large increases in population are to be met.

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