PLASTICITY AND ACCLIMATION TO LIGHT IN TROPICAL MORACEAE OF DIFFERENT SUCCESSIONAL POSITIONS

We evaluated both the photosynthetic plasticity and acclimation to light of seedlings of five co-occurring tropical tree species in the Moraceae, Cecropia obtusifolia, Ficus insipida, Poulsenia armata, Brosimum alicastrum, and Pseudolmedia oxyphyllaria. Distinct differences in the species' abilities to respond to increasing irradiance correlated with their known habitat breadths and successional status. The early successionals Cecropia and Ficus exhibited the highest photosynthetic rates and conductance values in high light. There was a several-fold difference in assimilation across light regimes, consistent with a high physiological plasticity. When individuals grown at low light were transferred to higher irradiances, seedlings of both Cecropia and Ficus produced leaves which photosynthesized at rates as high or higher than those of plants continuously grown in high light, indicating a high photosynthetic acclimation potential. In contrast, the late successionals were characterized by both a more restricted physiological plasticity and acclimation potential. Higher light levels resulted in only moderate increases in assimilation among the late successionals, and only Brosimum acclimated fully to increased irradiances. Neither Poulsenia nor Pseudolmedia increased appreciably their photosynthetic rates when transferred to high light. This suggests that acclimation potential cannot always be inferred from plasticity responses, and calls for a reevaluation of arguments developed solely from plasticity studies. Finally, differences between the early and late successional species in the allocation of nitrogen into RuBP carboxylase and thylakoid nitrogen pools or non-photosynthetic compounds are suggested by the distinct relationships between maximum photosynthetic capacity and nitrogen content.