Annals of Botany

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Live fast – die young

How long does a leaf last? Well, it all depends… but what it all depends on is a matter for discussion. In the words of Gregoire Vincent (Montpellier, France, pp. 245–255), available data pinpoint ‘the strong trade-off that exists between leaf photosynthetic capacity and leaf life span’. To test this idea further, the author has carried out experiments on four tropical tree species with differing light requirements: Lansium domesticum (fully shade adapted), Durio zibethinus (establishes in the shade but grows into the top canopy), Hevea brasiliensis (light-demanding but seedlings/saplings are partly shade tolerant) and Alstonia scholaris (light-demanding). Seedlings were grown under 12 %, 45 % or 100 % of full sunlight for 18 months; leaf emergence, leaf life span and several physiological parameters were monitored. Some plants died during the experiment: seedlings of Lansium and Durio were unable to tolerate full sunlight. For the remaining plants, the effects of the light regime on leaf life span were very clear: in all four species, the leaves lived longest in the deepest shade, with Hevea showing the least plasticity and Astonia the most. The physiological effects of increasing shade included increases in specific leaf area (SLA) and in leaf N per unit mass, and decreases in photosynthetic capacity and in dark respiration. There was a strong correlation between the degree of plasticity in leaf life span and the degree of plasticity in SLA and in photosynthetic capacity. Overall, the data are consistent with the view that increased leaf life span under shaded conditions is a direct or indirect consequence of reduced photosynthetic capacity or rate of photosynthesis. Exactly how a decrease in photosynthesis leads to an increase in leaf longevity is not clear, but these data are certainly in line with observations that moving plants from high light to low light delays leaf senescence.