Leaf gas exchange of oil palm (elaeis guineensis jacq.) grown in ditch and raised bed plots

The Central Plain of Thailand is a river flood plain on which farmers have constructed raised planting beds by building dikes and excavating drainage ditches. Oil palm (Elaeis guineensis Jacq.) is grown on such reshaped land in the Rangsit area, which is located farther north than the recommended latitude for commercial oil palm plantation. This study examines the limiting factors on leaf gas exchange rates of oil palm in this area. The soil matric potential showed that the root activity was limited to 45 cm with the deeper soil saturated by water penetrating from the surrounding ditches. The plants experienced high air vapor pressure deficit (vpdair) during the dry season and low vpdair during the raining season, in the latter case coupled with a saturated root zone for up to 3 months. Leaf net photosynthesis (A) and transpiration (E) commenced at sunrise and rose quickly to their highest levels as did stomatal conductance (gs) by 10 AM. Around noon, total water potential dropped to a low of-1,800 kPa and turgor potential decreased from 1,700 kPa to almost zero. Evidently root water uptake rate did not match the transpiration demand. The increasing radiation raised leaf-to-air vpd (vpdleaf) to levels inducing stomatal closure which caused drastic decreases in A, E and gs. Leaf water potentials recovered subsequently. Substantial rates of leaf gas exchange lasted only for 3 hours in the morning. We conclude that vpd is the controlling parameter on gas exchange through the opening mechanism of stomates. Nevertheless, as the oil palm biomass was still small, the leaf assimilation rate was sufficient to produce high yields of up to 35 ton ha-1 y-1.