Du Cloux, H. C, André, M., Daguenet, A. and Massinuno, J. 1987. Wheat response to CO2 enrichment: Growth and CO2 exchanges at two plant densities.—J. exp. Bot. 38: 1421–1431. The vegetative growth of wheat (Triticum aestivum L., var. Capitole) was followed for almost 40 d after germination in controlled conditions. Four different treatments were carried out by combining two air concentrations of CO2, either normal (330 mm3 dm ⊟3) or doubled (660 mm3 dm ⊟3) with two plant densities, either 200 plants m ⊟2 or 40 plants m ⊟2. Throughout the experiment the CO2 gas exchanges of each canopy were measured 24 h d⊟1. These provided a continuous growth curve for each treatment, which were compared with dry weights. After a small stimulation at the start (first 13 d), no further effect of CO2 enrichment was observed on relative growth rate (RGR). However, RGR was stimulated throughout the experiment when plotted as a function of biomass. The final stimulation ol dry weight at 660 mm3 dm ⊟3 CO2 was a factor of 1·45 at high density and 1·50 at low density, contrary to other studies, no diminution of this CO2 effect on dry weight was observed over time. Nevertheless, at low density, a transient additional enhancement of biomass (up to 1·70) was obtained at a leaf area index (LAI) below 1. This effect was attributed to a different build up of the gain of carbon in the case of an isolated plant or a closed canopy. In the former, the stimulation of leaf area and the net assimilation rate are both involved; in the latter the enhancement becomes independent of the effect on leaf area because the canopy photosynthesis per unit ground area as a function of LAI reaches a plateau.