Structural and functional variability within the canopy and its relevance for carbon gain and stress avoidance

The functional variability in leaf angle distribution within the canopy was analysed with respect to regulation of light interception and photoprotect ion. Leaf orientation strongly determined the maximum photochemical efficie ncy of PSII (F-v/F-m) during summer: horizontal leaves were highly photoinh ibited whereas vertical leaf orientation protected the leaves from severe p hotoinhibition. The importance of leaf orientation within the canopy was an alysed in two Mediterranean macchia species with distinct strategies for dr ought and photoinhibition avoidance during summer. The semi-deciduous speci es (Cistus monspeliensis) exhibited strong seasonal but minimal spatial var iability in leaf orientation. Reversible structural regulation of light int erception by vertical leaf orientation during summer protected the leaves f rom severe photoinhibition. The evergreen sclerophyll (Quercus coccifera) e xhibited high spatial variability in leaf angle distribution throughout the year and was less susceptible to photoinhibition. The importance of both s trategies for plant primary production was analysed with a three-dimensiona l canopy photoinhibition model (CANO-PI). Simulations indicated that high v ariability in leaf angle orientation in Q. coccifera resulted in whole-plan t carbon gain during the summer, which was 94 % of the maximum rate achieve d by theoretical homogeneous leaf orientations. The high spatial variabilit y in leaf angle orientation may be an effective compromise between efficien t light harvesting and avoidance of excessive radiation in evergreen plants and may optimize annual primary production. Whole plant photosynthesis was strongly reduced by water stress and photoinhibition in C. monspeliensis; however, the simulations indicated that growth-related structural regulatio n of light interception served as an important protection against photoinhi bitory reduction in whole-plant carbon gain. (C) 2001 Editions scientifique s et medicales Elsevier SAS.