RESPONSES OF CO2 ASSIMILATION, TRANSPIRATION AND WATER-USE EFFICIENCYTO LONG-TERM ELEVATED CO2 IN PERENNIAL C-3 XERIC LOESS STEPPE SPECIES

CO2 assimilation (A), transpiration (E), water use efficiency (WUE), l eaf-nitrogen and carbohydrate responses to 11 months elevated (700 mu mol mol(-1)) CO2 exposure in four perennial C-3 species (Festuca rupic ola, Dactylis glomerata, Filipendula vulgaris, Salvia nemorosa) from a xeric temperate loess steppe are reported. The responses in the speci es varied greatly owing to their differing acclimation. The acclimatio n of photosynthesis was somewhat downward in F. rupicola, fully downwa rd in D. glomerata, and upward in S. nemorosa and F. vulgaris. The red uction in the initial slope of the A/c(i) response curve in E rupicola and D. glomerata suggested a decrease in Rubisco capacity. Net CO2 as similation at 700 mu mol mol(-1) CO2 c(a) in the high CO2 F. rupicola was higher than in those grown at present (350 mu mol mol(-1)) CO2; th ere was no difference in D. glomerata. The initial slope of the A/c(i) curve indicated an increased Rubisco capacity in high CO2 F. vulgaris and S. nemorosa. Their net CO2 assimilation was higher in the plants grown in the high CO2 treatment at c(i)'s over 200 mu mol mol(-1) than that in the plants grown at present CO2. The A/c(i) response curves, which were saturated in all species grown at present CO2, did not reac h saturation in the plants grown at elevated CO2, reflecting that the Pi limitation of CO2 assimilation was alleviated in the plants grown a t high CO2. Transpiration decreased with an increase in c(i) in both t he present and elevated CO2 F. rupicola and D. glomerata. In F. vulgar is, an increase in ci caused a reduction in transpiration in the plant s grown at high CO2 only. Transpiration rate in both the present and e levated CO2 S. nemorosa was not affected by any change in c(i). It is suggested then that long-term exposure to high CO2 causes a similar ac climation of stomatal regulation and transpiration to that of photosyn thesis. High CO2 caused a significant decrease in protein-nitrogen con tent only in D. glomerata. Starch increased in F. rupicola and D. glom erata and soluble sugar content was higher in all species grown at hig h CO2 than at ambient. Instantaneous WUE significantly increased in ai l species grown at elevated CO2.