We measured the diurnal changes in net photosynthetic rate (PN) and stomatal conductance (gs) 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 m 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 gs, and gs 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 (T L). 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. and A. C. Tay ... [et al.].
The effects of leaf to air vapour pressure differences (ΔW) on net photosynthetic rate (PN) and stomatal conductance (gs) were examined in the leaves of two tropical rain forest trees, Eugenia grandis and Pongamia pinnata, and two temperate evergreen trees, Viburnum awabuki and Daphniphyllum macropodum. A single leaf was set inside a small chamber and ΔW was varied from 7 to 24 mmol mol-1 at 25 and 500 μmol m-2 s-1 of photon flux density. PN and gs of the two tropical rain forest trees decreased with increasing ΔW, while the two temperate evergreen trees were not highly responsive to ΔW. P. pinnata was more sensitive to ΔW in its stomatal response, and had a higher stomatal density and higher stomatal index than did the two temperate trees and another tropical tree. Significant reductions i n gs and intercellular CO2 concentrations in the two tropical trees at high ΔW suggest that the decline of PN was due to the decrease in gs. The responses of PN and gs indicated that the tropical trees were more sensitive to ΔW than were the temperate ones. and S.-Y. Park, A. Furukawa.