Response of Faba Bean Subjected to Drought Stress Under Free Air CO2 Enrichment Facility (FACE) in a Mediterranean Dry Environment.

Atmospheric CO₂ concentration ([CO₂]) has been increased from pre-industrial era of 280 ppm to 406 ppm at present with concomitant rise in the severity and frequency of drought in many cropping regions. Lower stomatal conductance under elevated [CO₂] (e[CO₂]) improves plant water productivity especially provides advantages in the region of water limited environment. The performance of faba bean in terms of growth, yield and N₂ fixation under e[CO₂] is scarce in dry land Mediterranean environment. This study evaluated response of faba bean (Vicia faba L. cv. Fiesta) under supplemental irrigation and drought condition by exposing either ambient [CO₂](a[CO₂], ~400 ppm) and or e[CO₂] (~550 ppm) in the Soil Free CO₂ Enrichment Facility (SoilFACE) array located in the Australian Grain FACE (AGFACE), Horsham, Australia.

Elevated [CO₂] increased net CO₂ assimilation rate and biomass accumulation of faba bean under drought. Stomatal conductance was lower under e[CO₂] in drought which resulted in significantly greater water use efficiency. Higher number of nodules was achieved under e[CO₂] regardless of growing conditions. N₂ fixation increased significantly under e[CO₂] and faba bean continued to fix N₂ from atmosphere even during the period of water shortage. Elevated [CO₂] accumulated lower level of total free amino acids in nodules under drought compared to a[CO₂]. Stimulation of grain yield by e[CO₂] was greater under drought. Elevated [CO₂] decreased grain N concentration under supplemental irrigation but increased substantially under drought. Elevated [CO₂] maintain N₂ fixation and grain [N] concentration of faba bean by improving water use efficiency under drought which will provide advantage of growing faba bean in dry land semi- arid environment in a unforeseen climate change.

Keywords: Growth, yield, N₂ fixation, Vicia faba L., drought, supplemental irrigation, water use efficiency