GAS-EXCHANGE OF OIL PALM IN RELATION TO LIGHT, VAPOR-PRESSURE DEFICIT, TEMPERATURE AND LEAF AGE

1. Gas exchange in leaves of oil palm (Elaeis guineensis) was measured in the field over a range of photon fluxes, leaf temperatures and leaf-air vapour pressure differences (VPD). 2. There were no differences in leaflet photosynthesis within one frond, but a reduction depending on frond age. For fronds of the same age, between-tree variation was low. 3. The light saturated rate of net CO2 assimilation was 20 mumol m-2 s-1 when photosynthetically active radiation (PAR) exceeded 1100 mumol M-2 s-1. Apparent quantum yield averaged 0.051 mol mol-1. When leaf temperature increased from 30-degrees-C to 38-degrees-C, net CO2 uptake showed no variation as transpiration rate and stomatal conductance increased slightly. A small increase in VPD induced rapid stomatal closure and consequently a reduction in the transpiration rate. A VPD greater than 1.8 kPa was required before photosynthesis was reduced via stomatal closure in the case of plants growing in a soil with adequate water supply.