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

Abstract

Abstract. Only a small proportion of elevated CO 2 studies on crops have taken place in the field. They generally confirm results obtained in controlled environments: CO 2 increases photosynthesis, dry matter production and yield, substantially in C 3 species, but less in C 4, it decreases stomatal conductance and transpiration in C 3 and C 4 species and greatly improves water‐use efficiency in all plants. The increased productivity of crops with CO 2 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 CO 2 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 CO 2 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.