BROAD-SCALE COMPARISON OF PHOTOSYNTHETIC RATES ACROSS PHOTOTROPHIC ORGANISMS

We tested the existence of general patterns in the photosynthetic meta bolism of oxygen-evolving organisms, based on a compilation of data fo r 315 species ranging from cyanobacteria to tree leaves. We used thick ness and chlorophyll a concentration of the photosynthetic structure ( cell, thallus, leaf) to scale differences in photosynthetic metabolism among plants, because of the demonstrated importance of these plant t raits in regulating light absorption properties and photosynthetic rat es of particular plant groups. We examined only the properties of the photosynthetic structure because this is the plant unit responsible fo r the photosynthetic process and thus is closely related to plant prod uctivity, whereas there is a lack of general quantitative descriptors of the whole organism useful for such broad-scale comparisons, and few studies report net photosynthetic rates of whole organisms, including respiration rates of all nonphotosynthetic structures. The results de monstrated that descriptors of plant metabolism such as maximum net ph otosynthesis, initial slope of the photosynthesis-irradiance (PI) curv e and dark respiration display strong positive interrelationships. The metabolic rates declined with increasing thickness of the photosynthe tic structures and more steeply for photosynthesis than respiration. P hotosynthetic rates also changed with increment of volume of the photo synthetic structure resembling patterns that have been previously desc ribed for animal metabolism related to body weight. The strong relatio nship of metabolic rate and chlorophyll a concentration to the thickne ss of photosynthetic tissue reflects broad-scale patterns and not the adaptive response of individual or closely-related species of similar tissue thickness to varying environmental conditions. Thickness of the photosynthetic structures, therefore, plays an important role in the environmental control of plant performance and, consequently, it might have been an important driver of plant evolution, setting thresholds to the metabolism and productivity of phototrophic organisms.