Limitation of leaf carbon gain by stomatal and photochemical processes in the top canopy of Macaranga conifera, a tropical pioneer tree

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Abstract
Diurnal changes in gas exchange and chlorophyll fluorescence were measured in the top canopy leaves of the tropical rainforest tree species, Macaranga conifera (Zoll.) Muell. Arg. during a drought year. Maximum values of net photosynthetic rate (Pn, 10 μmol m−2 s−1) and stomatal conductance (gs, 0.2 mol m−2 s−1) were found in east-facing leaves in early morning. After 1000 h, both Pn and gs decreased. Minimum daytime values of Pn, gs, and photosystem II (PSII) quantum yield (ΔF/Fm′) were found in horizontally fixed leaves. At a given electron transport rate through PSII (ETR), Pn was higher in early morning than at midday, suggesting a high rate of photorespiration at midday. We tested the hypothesis that the effect of low leaf temperature (Tleaf) on Pn is significant in the early morning, whereas the effect of low gs on Pn predominates at midday. In the early morning, when Tleaf was increased from 32 to 38 °C by artificial heating, Pn at a given ETR decreased 29%, suggesting that the low Tleaf was associated with a high Pn. When Tleaf at midday was decreased from 37 to 32 °C by artificial cooling, Pn increased 22%, but Pn at a given ETR was higher in early morning than at midday, even at the same low Tleaf (32 °C). This suggests that the rate of photorespiration was higher at midday than in early morning because low gs at midday caused a reduction in leaf intercellular CO2 concentration. We conclude that low Pn at midday was the result of both a reduction in the photochemical process and an increase in stomatal limitation.

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