The response of plants to carbon dioxide (CO 2 ) enrichment is an important aspect to be thoroughly investigated in order to accurately predict the impact on resource use as well ecosystem level responses. But the extent of response of individual plant species is dependent on the underlying biotic and abiotic stresses. In the current project, the response of Cenchrus ciliaris, a C4 grass, was investigated under CO 2 enrichment conditions under nutrient stress. Eco-physiological growth parameters were assessed within two plastic chambers with one chamber kept at ambient CO 2 conditions (500ppm) and the second enriched with CO 2 (1000ppm). Three treatments were studied: ambient (ACO2), enriched (ECO2) and alternating (alternating two weeks in each chamber; ALCO2). High atmospheric CO 2 concentrations did increase shoot and inflorescence production under nutrient stress. The blade area of ECO2 plants was significantly larger than that of ALCO2 plants on the 31st of March 2010 at P=0.06 and on the 7th of April 2010 (P=0.074). Stomatal density of C.ciliaris, however, did decrease for ECO2 and ALCO2. Under ACO2, average sheath biomass was signcantly higher under nutrient stressed condition than under non-stressed condition (65.81% vs. 52.96%; respectively). Growth allocations results revealed a rush toward reproductive production for ECO2 under nutrient stress conditions.

Salinity stress may affect the response of grass species to atmospheric CO2 enrichment. But the extent and timing of growth responses in hyperarid environments may be different depending on the specific environmental conditions and the type of grass species (C3 vs. C4). The aim of the present study was to assess the extent of Cenchrus ciliaris L. responses to CO2 enrichment and to evaluate the role of salt stress in affecting such response. Three treatments were tested ; namely ambient, alternating and enriched CO 2 concentrations (ACO2 , ALCO2 and ECO2 ; respectively). Shoot length of C. ciliaris was positively affected by atmospheric CO2 enrichment. The number of inflorescence increased with the increase of CO2 level under both salt stress and control conditions. Growth allocation to green blades increased ten times under salinity stress than under control conditions. Allocation to inflorescence parts was almost two times higher under control than under salinity stress. Moreover, root allocation underwent variations between control and salinity stress. In short, CO2 enrichment did influence growth responses of C. ciliaris, especially under salinity tress. Growth allocation was also influenced toward a more reproductive growth pattern. Moreover, plants under salt stress may have responded to the CO2 alternation as an added stress.