Growth and physiological changes in saplings of Minquartia guianensis and Swietenia macrophylla during acclimation to full sunlight

Low light availability under a forest canopy often limits plant growth; however, sudden increase in light intensity may induce photoinhibition of photosynthesis. The aim of this study was to evaluate the ecophysiological changes that occur in potted plants of Minquartia guianensis and Swietenia macrophylla during the acclimation process to full sunlight. We used six full-sun independent acclimation periods (30, 60, 90, 120, 150, and 180 days) and a control kept in the shade. Shading was obtained by placing plants under the canopy of a small forest. The F-v/F-m ratio, net photosynthetic rate (P (N)), the maximum carboxylation velocity of Rubisco (V (cmax)), maximum electron transport rate (J (max)), specific leaf area (SLA), and growth were assessed at the end of each of the six acclimation periods. Plant exposure to full sunlight caused a sudden decrease in the F-v/F-m ratio (photoinhibition) particularly in Minquartia. Photooxidation (necrotic patches) of the leaf tissue was observed in upper leaves of Minquartia. The higher P (N) values were observed in Swietenia under full sun, about 12 mu mol(CO2) m(-2) s(-1). V (cmax25) values were higher after 90 days of acclimation, about 14 mu mol(CO2) m(-2) s(-1) for Minquartia, and 35 mu mol(CO2) m(-2) s(-1) for Swietenia. At the end of a 180-d acclimation period J (max25) was 35 mu mol(electron) m(-2) s(-1) for Minquartia and 60 mu mol(electron) m(-2) s(-1) for Swietenia. SLA was higher in Swietenia than in Minquartia. In Minquartia, monthly rate of leaf production per plant (MRLP) was positive (0.22 leaf month(-1)) after four months in the open. Whereas, in Swietenia MRLP was positive (0.56 leaf month(-1)) after an acclimation period of two months. After six months in the open, height growth rates were 3.5 and 28 mm month(-1) for Minquartia and Swietenia, respectively. The greater acclimation capacity of Swietenia was associated to an enhanced photosynthetic plasticity under full sun. In Minquartia, transition to full-sun conditions and lack of physiological adjustment resulted in severe photoinhibition and loss of leaves.