Abstract: | The rates of net photosynthesis by closed canopies of tomatoplants were measured at three CO2 concentrations and three humiditiesover a range of natural light flux densities. The data havebeen analysed using a model of canopy photosynthesis which allowsfor variation in leaf area index and other leaf and canopy characteristics.The model also deals explicitly with the effects of CO2 concentration,leaf conductance, and photorespiration on the leaf photochemicalefficiency, . The leaves were found to have a photochemicalefficiency in the absence of photorespiration, m, of 12?6 ?109 kg (CO2) J1. At a CO2 concentration of 0?73 ? 103 kg m3 (400vpm) the leaf photochemical efficiency, , and canopy light utilizationefficiency, c, were 18 per cent greater at a vapour pressuredeficit of 0?5 kPa than at 1?0 kPa. At a CO2 concentration of2?2 ? 103 kg m3 (1200 vpm) they were only 5 percent greater. |