Patterns in the photosynthetic metabolism of Mediterranean macrophytes

Here we examine the relationship between the photosynthetic performance of a wide range of Mediterranean marine macrophyte species (5 angiosperms and 18 macroalgae of w d e l y different size and growth form), and several descriptors of the photosynthetic tissue such as thickness, chlorophyll a (chl a) and nutrient (C, N, P) content.The photosynthetic performance of the plants is descnbed by their photosynthesis-irrad~ance curves, elther as continuous functions, or as the individual parameters defining the curves.Our results demonstrate a strong relationship between the photosynthetic efficiency (a) measured at low llght and the photosynthetic rate at saturating light (P,,,), and show a to b e closely related to d~fferences in tissue th~ckness and chl a concentration among the marine macrophytes examlned.The interaction between thickness and chl a concentration in the regulation of a is parallel to their Interaction In the regulation of light absorption properties.Tissue thickness and pigment concentration not only explained variation In individual photosynthetic parameters (P,,,,, and a ) , but were also able to summanse differences in the response of photosynthesis to irradiance.The strong relationship between tissue thickness and photosynthetic performance lend support to the important adaptive role attnbuted to the thickness of marine macrophyte tissues in the past.This importance of thickness extends beyond the regulation of plant productivity to the resistance to grazing, mechanical damage and the longevity of the tissue.Considering prevlous den~onstrations of the functional importance of thickness for phytoplankton and for land plants, we suggest the existence of a general relationship between tissue thickness and the photosynthetic performance of photosynthetic organisms.