Is stomatal conductance of Central Amazonian saplings influenced by circadian rhythms under natural conditions?

Abstract

Stomata control CO2 uptake and transpirational water loss, and their functioning is closely regulated by external factors, such as light, CO2 concentration, air vapor pressure deficit, and temperature. In addition, endogenous factors (e.g. biological clock) also affect stomatal movement. In comparison with ambient factors, internal factors have received less attention in Amazonian tree species. The aim of this study was to determine how stomatal functioning and photosynthesis respond to continuous light exposure during a circadian cycle. We collected data from July to September 2010 in four saplings of Amphirrhox surinamensis. Stomatal conductance (g s50) and photosynthesis at a constant photosynthetically active radiation of 50 μmol m-2 s-1 (P N50) were measured continuously during a circadian period (24 h, day and night) under a [CO2] of 380 μmol mol-1, temperature of 25 ± 2 °C, air relative humidity of 73 ± 3 %. The highest values of g s50 and P N50 were observed before 02:00 pm. Values of these parameters decreased toward the end of the afternoon and night until reaching steady state around midnight. Stomatal reopening (inferred by the increase in g s values) began after midnight (01:00 am-02:00 am) when lower temperatures were recorded. Cooler temperatures observed after midnight and higher vapor pressure deficit values recorded early in the morning should promote closing rather than stomatal opening. Stomatal reopening in the early predawn hours suggests the involvement of an endogenous timer (circadian clock) in stomatal functioning. © 2014 Brazilian Society of Plant Physiology.