| dc.description.abstract | Atmospheric CO₂ concentration (Ca) has been continuously rising due to direct and indirect anthropogenic activities since the industrial revolution. Thanks to this Ca rise, photosynthetic rate and net primary productivity (NPP) of plants is increasing. Subsequent ecophysiological changes results in altered biomass allocation and in changes in leaves area (Aleaf) in relation to biomass that in turn can regulate plants growth. Furthermore, changes in the leaf nitrogen (Nleaf) and leaf phosphorus (Pleaf) content can affect the maximal RuBisCO limited rate of photosynthesis (Vcmax) and maximum electron transport (Jmax), respectively. However, few studies have examined the role of nutrient limitation, especially phosphorus (P), in downregulating plant growth to rising Ca, despite its potential influence on the global carbon cycle. It is likely that fully coupled climate-carbon cycle model projections misrepresent future plant carbon sequestration since they do not integrate low soil phosphorus concentration (Ps) even though soil P is particularly scarce in many ecosystems and expected to decrease in the future. The present study investigated the combined effect of rising Ca and low Ps on the total plant biomass (Bt) and on its allocation to the roots or above ground section, especially to the canopy, on Aleaf in relation to Bt and leaves biomass (Bleaf) and on Nleaf and Pleaf and on their relationships with Vcmax and Jmax. Three species, Holcus lanatus, Solanum dulcamara and Panicum mileaceum were grown in three phytotrons with 150, 450 and 800 ppm respectively and treated with two nutrient solutions in which P was variated in relation to N (1N:1P, 1N:45P). Results suggest that low Ps could hamper NPP at the end of the century more strongly than at the present. Moreover, above-below biomass ratio (Ba:Bb), leaf mass ratio (LMR), specific leaf area (SLA) and leaf area ratio (LAR) responses showed high variability between the three species, suggesting that plants can adopt very different strategies under the independent and interactive effect of rising Ca and low Ps thanks to their different species physiological characteristics and ontogeny. In addition, the low statistical significance of SLA and LAR responses could suggest that, if well lighted, plants no longer need to invest in Aleaf regardless of variations in Ca and in Ps. Finally, Nleaf and Pleaf decreased similarly with increasing Ca, independently from the P treatment, and further decreased under low Ps. Results of the interactive effect of rising Ca and Ps indicate that plants under low Ps could suffer a milder reduction in Pleaf at present Ca compared with glacial Ca. Moreover, Nleaf and Pleaf showed a positive but not very strong linear relationship with Vcmax and Jmax, respectively. This confirms that Nleaf and Pleaf concentrations may be able to partially regulate the allocation of N to the ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and of P to the electron transport chain (ETC), respectively. However, low Ps weakened these positive relationships. | |
