Influência dos ritmos circadianos na condutância estomática e fotossíntese em espécies florestais sob condições de sub-bosque na Amazônia Central

Abstract

This study we investigated how stomatal functioning and photosynthesis respond to continuous light exposure on a circadian cycle in saplings of five forest species native to Central Amazonia. It was also studied how photosynthetic parameters (maximum carboxylation velocity of Rubisco - Vc-max and maximum rate of electron transport - Jmax) respond to light, temperature and diurnal variations in the physical environment, as well as to study the relationship between luminosity, the leaf traits (thickness, specific leaf area - AFE and nutrients) and photosynthesis. The study was conducted at the Tropical Forestry Experimental Station (ZF2 reserve), of the (Instituto Nacional de Pesquisas da Amazônia). We used five species at the juvenile stage, with up to 3 m tall. Stomatal conductance (gs) and photosynthesis (A) data during the circadian cycle were collected from June to September 2010, using a portable gas exchange using a leaf per plant and four plants per species. Light saturated photosynthesis (Amax) and photosynthetic capacity of the leaf (Apot) were measured under light saturation and CO2 concentration at 380 μmol mol-1 and 2000 μmol mol-1, respectively. To assess the effect of time of day in photosynthetic parameters (Vc-max and Jmax), A/RFA and A/(Ci) curves were generated between 08:00 and 16:00 h. Growth data (height and diameter) were collected from July 2010 to July 2012. The other variables were determined after measurements of gas exchange. The highest values of gs, A and Φ were observed before 14:00. These three parameters decreased until late afternoon and evening, reaching a steady state at around midnight. Stomatal reopening (inferred by the increase in the values of gs) began after midnight (01:00 and 02:00 h), a period in which were recorded the lowest temperature values. Vc-max and Jmax significantly correlated with Amax and Apot. There was no significant effect (P>0.05) of the time of day on Vc-max and Jmax. However, the increase in understory irradiance and leaf temperature led to an increase in Amax, Apot, Vc-max and Jmax. There was also a positive relationship between understory irradiance and leaf thickness, as well as between leaf thickness and AFE and Apot. There was no significant relationship between photosynthetic rates (Apot-área, Apot-mas) and the concentrations of N and P per unit area and mass, but Apot, Vc-max and Jmax were responsive to nutrient use efficiency ( N, P, K, Ca, Mg). The efficient use of N decreases with the increase of the N/P ratio, but no effect of the N/P ratio in P use efficiency. Furthermore, it was observed that N and P use efficiency decreased rapidly with increasing leaf content of N and P. Understory irradiance had a significant effect on N and P use efficiency. The average annual increment in diameter (IAD) was 1.08 mm year-1, whereas the annual increment in height (IAA) did not exceed the 120 mm year-1.The stomatal reopening at early pre-dawn suggests the involvement of an biological clock in stomatal functioning. The lack of an effect of the time of day on Vc-max and Jmax indicates that these photosynthetic parameters are associated to diffusive factors, and also to factors that influence Rubisco, such as the activation of the enzyme. It was also shown that small variations in forest understory irradiance significantly influence the physiology and anatomy of the leaf (leaf thickness). Finally it was concluded that photosynthesis in most studied species is limited by the content of P rather than for leaf nitrogen content.