GROWTH-RESPONSE OF GRASSES TO ELEVATED CO2 - A PHYSIOLOGICAL PLURISPECIFIC ANALYSIS

The effect of CO2 enrichment on the growth and the economy of carbon a nd nitrogen of 11 Mediterranean grass species was investigated in orde r to determine the underlying causes of the large variation observed b etween species in their responses to elevated CO2. Plants were grown f or 26-43 d (depending on species growth rate) under productive conditi ons at ambient (350 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) concentrations of CO2. Plant parameters were determined at a common bi omass of 0.15 g to determine the CO2 effect independent of ontogenic e ffects. The effect of CO2 on RGR ranged from -6.7 to 22.5%, with a mea n stimulation of 10.3%. Averaged over the 11 species, the growth enhan cement resulted from an increase in net assimilation rate per unit lea f d. wt. (NAR(w)) of 10.6%. This was the result of a large increase (1 8.7%) in NAR per unit leaf area (NAR(a)) associated with a 8.1% decrea se in the specific leaf area (SLA). This decrease in SLA was due to a large increase of the non-structural carbohydrates. The increase in sh oot activity was balanced by a 7.6% increase in the specific absorptio n rate of nitrogen (SAR). AS a result, plant nitrogen content was not modified. Leaf nitrogen productivity was significantly increased (14.9 %). Shoot vs. root allocation of biomass and nitrogen was not modified . An analysis across the 11 species of the relationships between the s timulation of RGR and the alteration in RGR components showed a signif icant correlation only with increases in NAR(w), SAR and nitrogen prod uctivity. The co-ordinated increase in these three parameters constitu tes a single response syndrome, whose intensity is responsible for mos t of the species variability.