PHOTOSYNTHETIC ACCLIMATION TO LIGHT IN JUVENILES OF 2 CLOUD-FOREST TREE SPECIES

The photosynthetic response of juveniles of Decussocarpus rospigliosii , an emergent primary forest species and shade tolerant in its juvenil e stages and Alchornea triplinervia, a gap-colonizing species of tropi cal cloud forest in Venezuela was studied. Daily courses of microenvir onmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment an d transplanted to different light regimes (shade and sun) in the field . The photosynthetic response and some anatomical characteristics of p lants from different treatments were analyzed in the laboratory. Photo synthetic rates were low for both species, and were negative during so me diurnal periods, related to the low photosynthetically active radia tion levels obtained at both gap (6% of total radiation) and understor y (2%). A. triplinervia shows higher rates (1.5-3.0 mu molm(-2)s(-1)) than D. rospigliosii (0.7-1.1 mu molm(-2)s(-1)). Both species showed i ncreased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light co nditions. In contrast, D. rospigliosii responded to increased light le vels. Both species showed low light compensation points when grown und er total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conduc tances, transpiration rates and lesser stomatal control. Some leaf ana tomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weig ht and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environme nts, therefore maintaining a favorable carbon balance during the day.