PATTERNS IN THE PHOTOSYNTHETIC METABOLISM OF MEDITERRANEAN MACROPHYTES

Here we examine the relationship between the photosynthetic performanc e of a wide range of Mediterranean marine macrophyte species (5 angios perms and 18 macroalgae of widely different size and growth form), and several descriptors of the photosynthetic tissue such as thickness, c hlorophyll a (chl a) and nutrient (C, N, P) content. The photosyntheti c performance of the plants is described by their photosynthesis-irrad iance curves, either as continuous functions, or as the individual par ameters defining the curves. Our results demonstrate a strong relation ship between the photosynthetic efficiency (alpha) measured at low lig ht and the photosynthetic rate at saturating light (P-max), and show a lpha to be closely related to differences in tissue thickness and chl a concentration among the marine macrophytes examined. The interaction between thickness and chl a concentration in the regulation of alpha is parallel to their interaction in the regulation of light absorption properties. Tissue thickness and pigment concentration not only expla ined variation in individual photosynthetic parameters (P-max and alph a), but were also able to summarise differences in the response of pho tosynthesis to irradiance. The strong relationship between tissue thic kness and photosynthetic performance lend support to the important ada ptive role attributed 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 previous demonstrations o f the functional importance of thickness for phytoplankton and for lan d plants, we suggest the existence of a general relationship between t issue thickness and the photosynthetic performance of photosynthetic o rganisms.