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Effects of light and CO2 on net photosynthetic rates of stands of aubergine and Amaranthus

Hand, D.W., Wilson, J.W., Acock, B.
Annals of botany 1993 v.71 no.3 pp. 209-216
Amaranthus caudatus, Solanum melongena, photosynthesis, canopy, net assimilation rate, temperature, carbon dioxide
Net photosynthetic rates per unit ground area for plant stands of Solanum melongena L. var. esculentum (aubergine) and Amaranthus caudatus L. var. edulis (grain amaranth) were measured over 10 min intervals in an airtight, glass, controlled-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 CO2 concentration, were defined at 400, 800 and 1200 vpm CO2. Light compensation points for these stands were around 20-30 J m-2s-1 and decreased slightly at higher CO2 concentrations. For aubergine, a C3 species, the short-term effects of CO2 enrichment 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 CO2 enrichment 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 (microgram CO2 J-1) increased from zero at the light compensation point to a maximum at an optimum light flux density of about 100 J m-2s-1 (the optimum rose a little with CO2 enrichment) and decreased slightly with further increase in light. Maximum utilization efficiencies at 400 vpm CO2 were 8-9 micrograms CO2 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 micrograms CO2 J-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 fir stands, and relates to the exceptionally favourable environment with optimal control of CO2 concentration, humidity, temperature, water supply and mineral nutrition.