EFFECTS OF LIGHT AND CO2 ON NET PHOTOSYNTHETIC RATES OF STANDS OF AUBERGINE AND AMARANTHUS

Net photosynthetic rates per unit ground area for plant stands of Solarium metongena L. var. esculentum (aubergine) and Amaranthus caudatus L. var. edulis (grain amaranth) were measured over 10 min intervals in an airtight, glass, control led-environment cabinet for a range of light flux densities provided by the diurnal variation in daylight. Light response curves for photosynthesis of stands, grown at ambient C02concentration, were defined at 400, 800 and 1200 vpm CO2. Light compensation points for these stands were around 20-30 J m-2s-1and decreased slightly at higher CO2concentrations. For aubergine, a C3species, the short-term effects of C02enrichment were to increase the initial slope as well as the asymptote of the light response curve, reducing light saturation at moderate to high light flux densities; but for amaranthus, a C4 species, saturation was less apparent and C02enrichment scarcely increased photosynthesis except at light flux densities above 150 J m-2s-1. The canopies intercepted 93-98% of incident light. The efficiency of utilization of intercepted light in photosynthesis (µg C02J-1) increased from zero at the light compensation point to a maximum at an optimum tight flux density of about 100 J m-2s-1(the optimum rose a little with C0.2enrichment) and decreased slightly with further increase in light. Maximum utilization efficiencies at 400 vpm C02were 8-9 yg CO, J-1. Enrichment to 1200 vpm did not affect the peak utilization efficiency of the C4 amaranthus, but increased that of aubergine to 12.2 µg CO2J-1(equivalent to some 14% when using the heat of combustion of plant dry matter to convert to the dimensionless form). This is among the highest recorded efficiencies of light utilization for stands, and relates to the exceptionally favourable environment, with optimal control of C02concentration, humidity, temperature, water supply and mineral nutrition. © 1993 Annals of Botany Company.