We tested the hypothesis that, in tropical pioneer tree species, vertical leaf angle contributes to high carbon gain because it minimizes damage caused by high irradiances. Diurnal changes in leaf gas exchange and chlorophyll fluorescence were measured in east-facing (EL), west-facing (WL) leaves, and in leaves artificially held horizontal (HL) in the uppermost canopy of Macaranga conifera (Zoll.) Muell. Arg. Maximum values of net photosynthetic rate (Pn) for EL and HL reached 12 μmol m−2 s−1, whereas maximum Pn for WL was only 6 μmol m−2 s−1. Midday depressions of Pn and stomatal conductance occurred at high photosynthetic photon flux densities (PPFD), especially for HL. Photosystem II quantum yield (ΔF/Fm′) of HL for a given PPFD at the leaf surface was lower in the afternoon than in the morning. Values of ΔF/Fm′ for HL measured at dusk were lower than those measured just before dawn, suggesting that HL suffered from high light and heat load. Variations in the morphology and physiology of the canopy leaves were associated with different light environments, and there was circumstantial evidence of a transitional point at a PPFD of about 20–30% of full sunlight. Maximum Pn and nitrogen (N) content were higher in upper canopy leaves than in lower canopy leaves, and the differences were mainly associated with differences in lamina thickness. We conclude that the vertical leaf angle and thick lamina of the top canopy leaves contributed to enhance total carbon gain of the whole plant.
Keywords: leaf azimuth; photosynthesis; pioneer tree; tropical forest
Journal Article. 0 words.
Subjects: Plant Sciences and Forestry
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