ACCLIMATION OF PHOTOSYNTHESIS TO ELEVATED CO2 UNDER LOW-NITROGEN NUTRITION IS AFFECTED BY THE CAPACITY FOR ASSIMILATE UTILIZATION - PERENNIAL RYEGRASS UNDER FREE-AIR CO2 ENRICHMENT

Acclimation of photosynthesis to elevated CO2 has previously been show n to be more pronounced when N supply is poor. Is this a direct effect of N or an indirect effect of N by limiting the development of sinks for photoassimilate? This question was tested by growing a perennial r yegrass (Lolium perenne) in the field under elevated (60 Pa) and curre nt (36 Pa) partial pressures of CO2 (pCO2) at low and high levels of N fertilization. Cutting of this herbage crop at 4- to 8-week intervals removed about 80% of the canopy, therefore decreasing the ratio of ph otosynthetic area to sinks for photoassimilate. Leaf photosynthesis, i n vivo carboxylation capacity, carbohydrate, N, ribulose-1,5-bisphosph ate carboxylase/oxygenase, sedoheptulose-l,7-bisphosphatase, and chlor oplastic fructose-1,6-bisphosphatase levels were determined for mature lamina during two consecutive summers. just before the cut, when the canopy was relatively large, growth at elevated pCO(2) and low N resul ted in significant decreases in carboxylation capacity and the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase protein. In high N there were no significant decreases in carboxylation capacity or prote ins, but chloroplastic fructose-1,6-bisphosphatase protein levels incr eased significantly. Elevated pCO(2) resulted in a marked and signific ant increase in leaf carbohydrate content at low N, but had no effect at high N. This acclimation at low N was absent after the harvest, whe n the canopy size was small. These results suggest that acclimation un der low N is caused by limitation of sink development rather than bein g a direct effect of N supply on photosynthesis.