Effect of Variations in Leaf Size on Morphology and Photosynthetic Rate of Twigs

C02-exchange rates were measured in the dark and at high irradiance on the current season's twigs (the leaf-bearing portion of the stem) of 10 desert species with twigs capable of positive net assimilation of atmospheric CO2, and for 28 species (some from warm desert communities and others from moderate elevation riparian communities bordering the deserts) whose twigs never exhibited positive net assimilation rates. Dark respiration rate of twigs was found to be strongly correlated with leaf size. Some of the causes of this correlation were revealed by harvest data on the current season's shoots of the same 28 species. As leaf size increased, total mass shoot-1 increased and twig diameter increased. This resulted in a decrease in twig surface volume-1 and an increase in twig respiration rate on a surface area basis. Regardless of the magnitude of the respiration rate in the dark, however, the rate of CO2 efflux from twigs which did not achieve positive net photosynthesis was reduced to near zero in the light. This was attributed to photosynthetic recycling of the respired CO2. By recycling endogenous CO2 derived from mitochondrial respiration, the twigs of largeleaved species could achieve rates of twig gross photosynthesis comparable to those of desert species specialized for positive net uptake by the twigs. These observations suggest that scaling relationships between leaf and twig dimensions may have a strong influence on the kind of photosynthetic activity developed in twig tissues. Positive net assimilation of atmospheric CO2 by twigs may generally be restricted to species with small leaves, small twig diameters and low rates of endogenous CO2 production. Since leaf size varies systematically along aridity gradients, some of the observed distribution patterns of species with positive net assimilation in twigs (they are largely restricted to arid and semi-arid environments) may be caused indirectly through allometric relationships between leaves and twigs. Key-words: Allometries, CO2, desert recycling, diameter, leaf, photosynthesis, twig respiration