Midday depression of photosynthesis in Enkleia malaccensis, a woody climber in a tropical rainforest

We measured the diurnal changes in net photosynthetic rate (PN) and stomatal conductance (g(s)) of the leaves of a liana, Enkleia malaccensis Griff. (Thymelaeaceae), at the canopy level in the lowland tropical rainforest at Pasoh, Peninsular Malaysia. The measurements were made from a canopy walkway system, 30 in from the ground for 3 d in March 2003. PN increased with increasing photosynthetically active radiation (PAR) before noon, though PN was not enhanced by the strong radiation hit in the afternoon. Plotting g(s), at saturating PAR (> 0.5 mmol m(-2) s(-1)) against the vapour pressure deficit (VPD) failed to reveal a significant correlation between VPD and g(s), and g(s), became very low at VPD > 2.5 kPa. The relationship between PN and gs was fitted on the same regression line irrespective of measuring day, indicating that this relationship was not influenced by either VPD or leaf temperature (TL). Therefore, in the liana E. malaccensis, an increase in VPD leads to partial stomatal closure and, subsequently, reductions In PN and the midday depression of PN of this plant.