PHOTOSYNTHETIC ACCLIMATION OF SUBMERSED ANGIOSPERMS TO CO2 AND HCO3-

Photosynthetic acclimation after growth under a factorial combination of three concentrations of CO2 (1, 16 and 910 mu M) and two concentrat ions of HCO3- (0.2 and 1.5 mM) was measured for Callitriche cophocarpa Sendt., Elodea canadensis L.C. Rich. and Ranunculus peltatus Schrank. Callitriche cophocarpa was restricted to CO2 as a carbon source while the other two species also used HCO3-. None of the species showed C-4 -like photosynthesis as evidenced by low activities of phosphoenolpyru vate carboxylase. Carbon exchange characteristics and biochemical capa cities were down-regulated in response to increasing inorganic carbon during growth. In all three species, P-max initial slope of net photos ynthesis versus [CO2], rubisco activity, protein content and chlorophy ll content decreased, and CO2 compensation concentration increased wit h increased inorganic carbon, In addition, for the two HCO3- users, th e rate of HCO3--dependent photosynthesis at zero [CO2] and 1.5 mM HCO3 - decreased with inorganic carbon. The response to increased [GO,] was greater than that to increased [HCO3-]. Morphological acclimation to inorganic carbon was evident in all species. The root/shoot ratio incr eased with increasing [CO2] but was unaffected by [HCO3-]. The specifi c leaf area declined with carbon availability in Callitriche and Ranun culus, whereas no change was observed in Elodea. There was a significa nt positive correlation between various carbon exchange characteristic s and between these and the chlorophyll content and rubisco activity, suggesting that carbon exchange, light capture and carbon fixation are regulated in parallel in response to carbon availability. The general down-regulation response shown by these aquatic plants to elevated in organic carbon resembles the response of some terrestrial C-3 species to elevated CO2.