EFFECT OF VARIATIONS IN LEAF SIZE ON MORPHOLOGY AND PHOTOSYNTHETIC RATE OF TWIGS

CO2-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 commuinities bordering the deserts) whose twigs never exhibited positive net assimilation rates. Dark respiration rate of twigs was strongly correlated with leaf size. 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, attributed to photosynthetic recycling of the respired CO2. -from Authors