In tropical forests, light is considered the most limiting resource for plant growth and reproduction. Besides light, water deficit leads invariably to a decrease in photosynthesis. Thus, despite of the unquestionable role of light in CO2 assimilation (A), it is expected that water deficit affects and limits the light utilization by plants. In this study, we investigated how water deficit influenced the photosynthetic induction of the early successional tree Bauhinia forficata (Fabaceae) and the late successional Esenbeckia leiocarpa (Rutaceae) in the understorey and in the forest gap. Field measurements were carried out in June and August 2006 in plants of approximately two-years-old. In August, the plants were subjected to a period of 45 d without rain characterizing a water deficit situation. Leaf water potential was significantly lower in August, both in forest gap and understorey (-2.5 and -3.8 MPa, respectively), than in June (-0.6 and -1.6 MPa, respectively). In June, both species presented a rapid increase in A after a saturating light pulse under gap conditions. However, in the understorey the increase in A was slower in B. forficata than in E. leiocarpa. In August water deficit limited the increase in Amax in both species, indicating that potential utilization of increasing irradiance was reduced by water deficit conditions. The constrain in Amax was less pronounced in the understorey where plants at least reached the irradiance compensation point, whereas carbon gain by photosynthesis of the plants grown in the gap did not compensate the carbon loss by respiration.
drought; ecophysiology; gas exchange; light utilization; sunflecks; tropical forest succession
