Clonal differences in the response of dark and light reactions of photosynthesis to elevated atmospheric CO2 in poplar

Two hybrid poplar (Populus) clones (i.e., fast growing clone Beaupre and slow growing clone Robusta) were grown for two years from cuttings at close spacings in open top chambers (OTCs) under ambient (AC) and elevated [EC = AC + 350 mu mol(CO2) mol(-1)] CO2 treatments, For clone Beaupre no down-regulation of photosynthesis was observed. Two years of growing under EC resulted in an increase in quantum yield of photosystem 2 (PS2), steady state irradiance saturated rate of net photosynthesis (P-Nmax), chlorophyll (Chl) content, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC) activity for this clone. We suppose that under nonlimiting conditions of nitrogen and phosphorus content the response to EC was by building up light-harvesting complexes of PS2 and increasing photochemical efficiency of PS2, Due to a high rate of the primary reactions of photosynthesis and a high RuBPCO activity the end product of the response to EC was an increase in P-Nmax and a larger saccharides content, The Robusta clone showed a depression in the primary reactions of photosynthesis under EC, We found a decrease in quantum yield of PS2, Chl and phosphorus contents, and in RuBPCO activity. However, an increase in P-Nmax, saccharides content and Chi a/b ratio was observed. We speculate (1) that the phosphorus deficiency in combination with an increase in CO2 concentrations may lead to a potential damage of the assimilation apparatus of the primary reactions of photosynthesis and to a decrease in photochemical efficiency of PS2; (2) that the primary target of ''down-regulation'' takes place at PS2 for irradiances above 150 mu mol m(-2) s(-1).