Photosynthesis in Two Neotropical Palm Species

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Abstract
The photosynthetic responses of the palms Socratea durissima (Oerst.) H. Wendl. and Scheelea zonensis Bailey, two species with contrasting growth forms, were studied. Light-saturated rates in the laboratory were 3.38 and 5.74 mu mol m(-2)s(-1) for Socratea and Scheelea respectively, grown at 25% full sunlight; the respective rates were 1.16 and 1.61 mu mol m(-2)s(-1) when grown at 1-5% of full sunlight. In the field, leaf nitrogen declines more rapidly with age in Socratea than in Scheelea. Data on stomatal density indicate that maximum conductance contributes to between-species and between-treatment differences in light-saturated photosynthesis in the laboratory, while declining leaf nitrogen with leaf age may reduce photosynthesis more in Socratea than in Scheelea. The shade-adapted photosynthetic responses of these two species allows them to grow as seedlings in the shade of the tropical forest understorey. Scheelea shows greater differences in photosynthetic responses when grown under high vs low light than Socratea does; this difference between the two species suggests that Scheelea is more suited to exploiting treefall gaps than Socratea is. The characterization, based on physiological responses, of Scheelea as more of a gap species than Socratea is consistent with interpretations based on plant architecture and with field observations of the population biology of these two species.