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Leaf-age effect on photosynthetic rates as a key parameter in the theory of leaf longevity.
Kitajima, Kaoru*,1, 4, Kikuzawa, Kihachiro2, Ackerly, David3, Mulkey, Stephen1, Wright, Joseph4, 1 University of Florida, Gainesville, FL4 Smithsonian Tropical Research Institute, Balboa, Panama2 Kyoto University, Kyoto, Japan3 Stanford University, Stanford, CA
ABSTRACT- The cost-benefit theory of leaf longevity predicts that optimal leaf longevity is shorter either when the initial photosynthetic rate is high or the rate of its decline with leaf age is fast. The first prediction is well supported among a wide range of plants species at both regional and global scales. In contrast, the second prediction, the link between the regression slope of photosynthetic capacity against leaf age, has been rarely shown. Here, we present strong support for both predictions from eight neotropical canopy tree species ranging in mean leaf longevity from 29 to 400 days. Interestingly, the parameter decreases with increasing leaf longevity more than predicted by the theory. In other words, short-lived leaves die at the age their photosynthetic capacity reaches zero, while long-lived leaves senesce and abscise much earlier than the age when their photosynthetic capacity would reach zero. We propose several possible explanations for this emergent property. Further, we demonstrate that the analysis of age-effects on photosynthesis opens a novel way to estimate daily photosynthetic gain from instantaneous measurement of photosynthetic capacity.
Key words: photosynthesis, tropical trees, leaf longevity, cost-benefit theory