Stomatal opening in response to the simultaneous increase in vapor pressure deficit and temperature over a 24-h period under constant light in a tropical rainforest of the central Amazon

Abstract

Stomatal functioning is modulated by several factors and, in most circumstances, stomatal aperture declines with increasing leaf-to-air vapor pressure difference (VPDL) and temperature, but stomatal functioning under the humid conditions of the tropical rainforest understory has been scarcely investigated. The aim of this study was to determine how stomatal conductance of saplings responds to the changing understory temperatures of a tropical rainforest. We measured gas-exchange in four saplings of Guarea carinata (1–2 m tall). On each plant, stomatal conductance (g s), VPDL and leaf temperature were measured continuously at 3-min intervals during 24 h. We used a constant irradiance of 50 µmol m−2 s−1 and nearly constant relative humidity (75.8 ± 1.9%). Within the leaf chamber, leaf temperature over plants varied from 25.8 to 30.7 °C and VPDL from 0.62 to 0.93 kPa. Contrary to expectations, g s increased with the increase in leaf temperature and VPDL, and throughout the 24-h cycle, g s was maximum (0.123 mol m−2 s−1) at midday when temperatures were higher and minimum (0.016 mol m−2 s−1) after midnight, when the lower temperatures were recorded. There was a positive relationship between g s and both leaf temperature (p < 0.0001) and VPDL (p < 0.0001). If global temperature continues to increase as predicted by models, the direct increase in g s of saplings associated with an increase in temperature can be nullified if global warming negatively affects the water balance of plants (e.g. Guarea carinata) in this part of the Amazon.