An Analysis of Some Effects of Humidity on Photosynthesis by a Tomato Canopy under Winter Light Conditions and a Range of Carbon Dioxide Concentrations

Abstract

The rates of net photosynthesis by closed canopies of tomato plants were measured at three CO₂ concentrations and three humidities over a range of natural light flux densities. The data have been analysed using a model of canopy photosynthesis which allows for 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 photochemical efficiency, α. The leaves were found to have a photochemical efficiency in the absence of photorespiration, α_m, of 12·6 × 10⁻⁹ kg (CO₂) J⁻¹. At a CO₂ concentration of 0·73 × 10⁻³ kg m⁻³ (400 vpm) the leaf photochemical efficiency, α, and canopy light utilization efficiency, α_c, were 18 per cent greater at a vapour pressure deficit of 0·5 kPa than at 1·0 kPa. At a CO₂ concentration of 2·2 × 10⁻³ kg m⁻³ (1200 vpm) they were only 5 per cent greater.