Functional implications of opposite phyllotaxis in 12 Mediterranean woody species

Leaf phyllotaxis influences light capture efficiency of the crown. Opposite leaves exhibit a greater overlapping than leaves spirally arrange along a shoot, decreasing the potential light capture efficiency of the crown. The abundance of woody species with opposite phyllotaxis is remarkably high in the Mediterranean flora: 76% of the most common genera exhibit this phyllotaxis in the Mediterranean vs. 17% in Central Europe. Since plants exhibit a suite of traits that conjunctly influence light capture efficiency, we wanted to explore the effect of phyllotaxis together with other environmentally influenced architectural features. Morphological measurements obtained from natural populations of 12 woody species from Mediterranean-type ecosystems differing in leaf phyllotaxis were used to reconstruct unbranched, vertical shoots with the three-dimensional model Y-plant. Results indicated that species with spiral vs. opposite phyllotaxis did not intrinsically differ in their light capture efficiencies. However, plants with spiral phyllotaxis were more efficient in intercepting light when the internode length was short. Species with opposite phyllotaxis exhibited a broad range of variation in internode length and leaf elevation angle, while species with spiral phyllotaxis formed a more homogeneous group. These results indicated that species with opposite phyllotaxis can compensate their less efficient leaf arrangement by adjusting other structural features. The elongation of the internodes, for instance, significantly increased light capture efficiency, especially in the case of species with large leaves. Thus, species with opposite phyllotaxis, and particularly those with large leaves, may experience a real phylogenetic constraint on light capture efficiency but only when costs in terms of biomass allocation to supporting tissues becomes excessive. This could be involved in the relative shade intolerance of many woody Mediterranean species exhibiting opposite phyllotaxis, such as those in the families Labiatae and Cistaceae.