Effects of elevated CO2 concentrations on three montane grass species. III. Source leaf metabolism and whole plant carbon partitioning

Agrostis capillaris L.5, Festuca vivipara L. and Poa alpine L. were grown in outdoor open-top chambers at either ambient (340 +/- 3 micromole mol-1) or elevated (680 +/- 4 micromole mol-1) concentrations of atmospheric carbon dioxide (CO2) for periods from 79-189 d. Photosynthetic capacity of source leaves of plants grown at both ambient and elevated CO2 concentrations was measured at saturating light and 5% CO2. Dark respiration of leaves was measured using a liquid phase oxygen electrode with the buffer solution in equilibrium with air (21% O2, 0.034% CO2). Photosynthetic capacity of P. alpine was reduced by growth at 680 micromole mol-1 CO2 by 105 d, and that of F. vivipara was reduced at 65 d and 189 d after CO2 enrichment began, suggesting down-regulation or acclimation. Dark respiration of successive leaf blades of all three species was unaltered by growth at 680 relative to 340 micromole mol-1 CO2. In F. vivipara, leaf respiration rate was markedly lower at 189 d than at either 0 d or 65 d, irrespective of growth CO2 concentration. There was a significantly lower total non-structural carbohydrate (TNC) concentration in the leaf blades and leaf sheaths of A. capillaris grown at 680 micromole mol-1 CO2. TNC of roots of A. capillaris was unaltered by CO2 treatment. TNC concentration was increased in both leaves and sheaths of P. alpine and F. vivipara after 105 d and 65 d growth, respectively. A 4-fold increase in the water-soluble fraction (fructan) in P. alpine and in all carbohydrate fractions in F. vivipara accounted for the increased TNC content. In F. vivipara the relationship between leaf photosynthetic capacity and leaf carbohydrate concentration was such that there was a strong positive correlation between photosynthetic capacity and total leaf N concentration (expressed on a per unit structural dry weight basis), and total nitrogen concentration of successive mature leaves reduced with time. Multiple regression of leaf photosynthetic capacity upon leaf nitrogen and carbohydrate concentrations further confirmed that leaf photosynthetic capacity was mainly determined by leaf N concentration. In P. alpine, leaf photosynthetic capacity was mainly determined by leaf CHO concentration. Thus there is evidence for down-regulation of photosynthetic capacity in P. alpine resulting from increased carbohydrate accumulation in source leaves. Leaf dark respiration and total N concentration were positively correlated in P. alpine and F. vivipara. Leaf dark respiration and soluble carbohydrate concentration of source leaves were positively correlated in A. capillaris. Changes in source leaf photosynthetic capacity and carbohydrate concentration of plants grown at ambient or elevated CO2 are discussed in relation to plant growth, nutrient relations and availability of sinks for carbon.