STOMATAL DYNAMICS AND ITS IMPORTANCE TO CARBON GAIN IN 2 RAIN-FOREST PIPER-SPECIES .1. VPD EFFECTS ON THE TRANSIENT STOMATAL RESPONSE TO LIGHTFLECKS

The effects of leaf-air vapor pressure deficit (VPD) on the transient and steady-state stomatal responses to photon flux density (PFD) were evaluated in Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub, that are both native to tropical forests at Los Tuxtl as, Veracruz, Mexico. Under constant high-PFD conditions, the stomata of shade-acclimated plants of both species were sensitive to VPD, exhi biting a nearly uniform decrease in g(s) as VPD increased. Acclimation of P. auritum to high light increased the stomatal sensitivity to VPD that was sufflcient to cause a reduction in transpiration at high VPD 's. At low PFD, where g(s) was already reduced, there was little addit ional absolute change with VPD for any species or growth condition. Th e stomatal response to 8-min duration lightflecks was strongly modulat ed by VPD and varied between the species and growth light conditions. In P. aequale shade plants, increased VPD had no effect on the extent of stomatal opening but caused the rate of closure after the lightflec k to be faster. Thus, the overall response to a lightfleck changed fro m hysteretic (faster opening than closure) to symmetric (similar openi ng and closing rates). Either high or low VPD caused g(s) not to retur n to the steady-state value present before the lightfleck. At high VPD the value after was considerably less than the value before whereas a t low VPD the opposite occurred. Shade-acclimated plants of P. auritum showed only a small g(s) response to lightflecks, which was not affec ted by VPD. Under sunfleck regimes in the understory, the stomatal res ponse of P. aequale at low VPD may function to enhance carbon gain by increasing the induction state. At high VPD, the shift in the response enhances water use efficiency but at the cost of reduced assimilation .