The effect of drought on the water relations of annual forage species in a low elevation grassland

Water is a restrictive factor that affects the growth and the productivity of forage species in the Mediterranean region. The aim of this study was to investigate the water relations under water deficit conditions and the drought adaptation ecophysiological mechanisms of two annual forage species, Hordeum murinum (grass) and Medicago αrabica (legume). The experiment was conducted in the spring – early summer of 2010 in a low elevation grassland. The midday leaf water potential (Ψ), the stomatal conductance (gs) and the transpiration rate (E) were measured, while the Vapor Pressure Deficit (VPD) over the canopy and the leaf Relative Water Content (RWC) were estimated. Different responses under water deficit conditions were evident between the two species. During the growing season, high VPD (2,5-4,1kPa) values dominated and, thus, both species presented different transpiration demands. In the end of the growing season, M. arabica exhibited lower Ψ (-2,84 MPa) and higher RWC (60,64%) in relation to H. murinum (-2,30 MPa, 56.27%, respectively). The annual grass H. murinum completed earlier its biological cycle, having higher water losses than M. arabica which reached the critical threshold point later (middle of June) and adapted better to water deficit conditions. Our results suggest that, M. arabica probably displays ecophysiological mechanisms that enable it to continue to grow, even under intense water deficit conditions.