The effects of increasing CO2 on crop photosynthesis and productivity: a review of field studies

Abstract. Only a small proportion of elevated CO₂ studies on crops have taken place in the field. They generally confirm results obtained in controlled environments: CO₂ increases photosynthesis, dry matter production and yield, substantially in C₃ species, but less in C₄, it decreases stomatal conductance and transpiration in C₃ and C₄ species and greatly improves water-use efficiency in all plants. The increased productivity of crops with CO₂ enrichment is also related to the greater leaf area produced. Stimulation of yield is due more to an increase in the number of yield-forming structures than in their size. There is little evidence of a consistent effect of CO₂ on partitioning of dry matter between organs or on their chemical composition, except for tubers. Work has concentrated on a few crops (largely soybean) and more is needed on crops for which there are few data (e.g. rice). Field studies on the effects of elevated CO₂ in combination with temperature, water and nutrition are essential; they should be related to the development and improvement of mechanistic crop models, and designed to test their predictions.