Maximum carboxylation velocity of rubisco and maximum rate of electron transport in saplings in response to variations in environmental factors in central Amazonia

Abstract

Light availability is one of the main factors that limits photosynthetic rates in saplings in the forest understory. The aims of this study were to determine how photosynthetic parameters (maximum carboxylation velocity of Rubisco – Vc-max and maximum rate of electron transport – Jmax) respond to irradiance, temperature and diurnal variations in the physical environment. Gas exchange data were collected between 08h00 and 16h00, and from September to November (2010) under ambient conditions, temperature of 27± 2 ºC and relative humidity of 70 ± 3%. CO2 saturated photosynthesis (Apot) was measured under light saturation and [CO2] of 2000 μmol mol-1. Data were subjected to analysis of variance and then the Tukey test was used for mean separations. Vc-max and Jmax correlated with photosynthetic rates at [CO2] of 380 μmol mol-1 (A380) and Apot. The increase in intercellular CO2 concentration (Ci) led to increase in photosynthetic rates, up to the point of photosynthesis limitation by the electron transport rate. Below the collimation point (300-569 μmol mol-1) photosynthetic rates were limited by Vc-max. Increases in irradiance and temperature in the forest understory led to increasing the values of A380, Apot, Vc-max and Jmax. However, the effect of time of day on Vc-max and Jmax was not significant (p > 0.05). Absence of an effect of time of day on Vc-max and Jmax is important for the leaf to efficiently use the sun flecks that occasionally puncture the forest floor. © 2017, Universidade Federal de Santa Maria. All rights reserved.