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 a nd slow growing clone Robusta) were grown for two years from cuttings at close spacings in open top chambers (OTCs) under ambient (AC) and e levated [EC = AC + 350 mu mol(CO2) mol(-1)] CO2 treatments, For clone Beaupre no down-regulation of photosynthesis was observed. Two years o f growing under EC resulted in an increase in quantum yield of photosy stem 2 (PS2), steady state irradiance saturated rate of net photosynth esis (P-Nmax), chlorophyll (Chl) content, and ribulose-1,5-bisphosphat e carboxylase/oxygenase (RuBPC) activity for this clone. We suppose th at under nonlimiting conditions of nitrogen and phosphorus content the response to EC was by building up light-harvesting complexes of PS2 a nd increasing photochemical efficiency of PS2, Due to a high rate of t he 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 larg er saccharides content, The Robusta clone showed a depression in the p rimary reactions of photosynthesis under EC, We found a decrease in qu antum yield of PS2, Chl and phosphorus contents, and in RuBPCO activit y. However, an increase in P-Nmax, saccharides content and Chi a/b rat io was observed. We speculate (1) that the phosphorus deficiency in co mbination with an increase in CO2 concentrations may lead to a potenti al damage of the assimilation apparatus of the primary reactions of ph otosynthesis 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).