Stomatal conductance in Amazonian tree saplings in response to variations in the physical environment

Abstract

In juvenile trees growing at the rainforest understory, light is the most limiting factor for growth. It has been assumed that stomata quickly respond to light irrespective of the physical conditions prevailing before leaf illumination. Nevertheless, so far this issue has not been addressed for saplings of Amazonian tree species. The aim of this study was to determine how stomatal conductance (g s) and photosynthetic parameters of Amazonian saplings respond to diurnal variation in the physical environment and to rainfall seasonality. Light-saturated net photosynthetic rate (P Nmax) and g s at light saturation (g smax) were measured in the dry (August) and rainy (January) season of 2008 in saplings of 10 Amazonian tree species (Minquartia guianensis, Myrcia paivae, Protium apiculatum, Guatteria olivacea, Unonopsis duckei, Rinorea guianensis, Dicypellium manausense, Eschweilera bracteosa, Gustavia elliptica, and Tapura amazonica). At the forest understory, variables of the physical environment were measured. Rainfall seasonality did not affect P Nmax and g smax, nor was the effect of species on P Nmax and g smax significant (p>0.05). The g s and P Nmax increased as the forest understory became brighter and warmer; as a result, P Nmax and g smax were higher at midday than early in the morning or in the afternoon. However, contrary to expectations, neither changes in air vapor pressure deficit nor air CO2 concentration at the forest understory affected stomatal opening. More investigation is needed to elucidate the role of environmental factors in modulating stomatal movements in juvenile trees growing beneath the dense canopy of tropical rainforests.