Growth response of grasses to elevated CO2: A physiological plurispecific analysis

The effect of CO2 enrichment on the growth and the economy of carbon and nitrogen of 11 Mediterranean grass species was investigated in order to determine the underlying causes of the large variation observed between species in their responses to elevated CO2. Plants were grown for 26-43 d (depending on species growth rate) under productive conditions at ambient (350 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) concentrations of CO2. Plant parameters were determined at a common biomass of 0.15 g to determine the CO2 effect independent of ontogenic effects. The effect of CO2 on RGR ranged from -6.7 to 22.5%, with a mean stimulation of 10.3%. Averaged over the 11 species, the growth enhancement resulted from an increase in net assimilation rate per unit leaf d. wt. (NAR(w)) of 10.6%. This was the result of a large increase (18.7%) in NAR per unit leaf area (NAR(a)) associated with a 8.1% decrease in the specific leaf area (SLA). This decrease in SLA was due to a large increase of the non-structural carbohydrates. The increase in shoot activity was balanced by a 7.6% increase in the specific absorption 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 stimulation of RGR and the alteration in RGR components showed a significant correlation only with increases in NAR(w), SAR and nitrogen productivity. The co-ordinated increase in these three parameters constitutes a single response syndrome, whose intensity is responsible for most of the species variability.