RELATIONSHIP OF PHOTOSYNTHETIC ACCLIMATION TO CHANGES OF RUBISCO ACTIVITY IN FIELD-GROWN WINTER-WHEAT AND BARLEY DURING GROWTH IN ELEVATED CARBON-DIOXIDE

The responses of photosynthesis, Rubisco activity, Rubisco protein, le af carbohydrates and total soluble protein to three carbon dioxide tre atments were studied in winter wheat [Triticum aestivum (L.)] and barl ey [Hordeum vulgare (L.)]. Barley and wheat plants were grown in small field plots during 1995 and 1996 in clear, acrylic chambers (1.2-2.4 m(2)) and were provided with continuous carbon dioxide fertilization a t concentrations of 350, 525 and 700 mu mol mol(-1). Photosynthetic ra tes of barley penultimate leaves and wheat flag leaves measured at gro wth carbon dioxide concentrations decreased with leaf age in all three CO2 treatments during 1995 and 1996. Photosynthetic acclimation to el evated CO2 was observed on seven of eight measurement dates for barley and ten of eleven measurement dates for wheat over both years. Initia l Rubisco activity, total soluble protein and Rubisco protein in barle y penultimate leaves and wheat flag leaves also decreased with leaf ag e. Total Rubisco activity was not used because of enzyme degradation. There was a significant CO2 treatment effect on initial Rubisco activi ty, total soluble protein and Rubisco protein for wheat in 1995 and 19 96 and for barley in 1995. Responses of barley penultimate leaf Rubisc o activity and leaf protein concentrations to elevated carbon dioxide were nonsignificant in 1996. A significant CO2 treatment effect also w as detected when means of Rubisco activity, soluble protein and Rubisc o protein for wheat flag leaves were combined over harvests and years. These three flag leaf parameters were not significantly different in the 350 and 525 mu mol mol(-1) CO2 treatments but were decreased durin g growth in 700 mu mol mol(-1) CO2 relative to the other two CO2 treat ments. Ratios of photosynthesis at 700 and 350 mu mol mol(-1) were com pared to ratios of Rubisco activity at 700 and 350 mu mol mol(-1) usin g wheat flag leaf data from 1995 and 1996. Regression analysis of thes e data were linear [y = 0.586 + 1.103x(r(2) = 0.432)] and were signifi cant at P less than or equal to 0.05. This result indicated that photo synthetic acclimation was positively correlated with changes of initia l Rubisco activity in wheat flag leaves in response to CO2 enrichment. Effects of elevated CO2 on wheat leaf proteins during 1995 and 1996 a nd on barley during 1995 were consistent with an acceleration of senes cence.