INCREASED ACCUMULATION OF CARBOHYDRATES AND DECREASED PHOTOSYNTHETIC GENE TRANSCRIPT LEVELS IN WHEAT GROWN AT AN ELEVATED CO2 CONCENTRATIONIN THE FIELD

Repression of photosynthetic genes by increased soluble carbohydrate c oncentrations may explain acclimation of photosynthesis to elevated CO 2 concentration. This hypothesis was examined in a field crop of sprin g wheat (Triticum aestivum L.) grown at both ambient (approximately 36 0 mu mol mol(-1)) and elevated (550 mu mol mol(-1)) atmospheric CO2 co ncentrations using free-air CO2 enrichment at Maricopa, Arizona. The c orrespondence of steady-state levels of mRNA transcripts (coding for t he 83-kD photosystem I apoprotein, sedoheptulose-1,7-bisphosphatase, p hosphoribulokinase, phosphoglycerokinase, and the large and small subu nits of ribulose-1,5-bisphosphate carboxylase/oxygenase) with leaf car bohydrate concentrations (glucose-6-phosphate, glucose, fructose, sucr ose, fructans, and starch) was examined at different stages of crop an d leaf development and through the diurnal cycle. Overall only a weak correspondence between increased soluble carbohydrate concentrations a nd decreased levels for nuclear gene transcripts was found. The differ ence in soluble carbohydrate concentration between leaves grown at ele vated and current ambient CO2 concentrations diminished with crop deve lopment, whereas the difference in transcript levels increased. In the flag leaf, soluble carbohydrate concentrations declined markedly with the onset of grain filling; yet transcript levels also declined. The results suggest that, whereas the hypothesis may hold well in model la boratory systems, many other factors modified its significance in this field wheat crop.