Water use efficiency, transpiration and net CO₂ exchange of four alfalfa genotypes submitted to progressive drought and subsequent recovery

The predicted worldwide increase in arid areas and water stress episodes will strongly affect crop production. Plants have developed a wide diversity of physiological mechanisms for drought tolerance. A decline in photosynthesis and thus yield production is a common response to drought, as are increases in the water use efficiency of photosynthesis (WUEₚₕ) and productivity (WUEₚ). The aim of our study was to determine the physiological effects (especially WUEₚₕ and WUEₚ) of progressive drought and subsequent recovery in three cultivars adapted to a Mediterranean climate [Tafilalet (TA), Tierra de Campos (TC), and Moapa (MO)], and another representative from an oceanic climate (Europe (EU)). The accuracy of the relationships between WUEₚₕ or WUEₚ and carbon isotope discrimination (Δ¹³C) in shoots was also investigated as a function of water stress intensity. Mild drought (7 days of water withholding) decreased the net CO₂ exchange (A), leaf conductance to water (g) and transpiration in EU leading to an increased WUEₚₕ. Δ¹³C was correlated with WUEₚ but not with WUEₚₕ, probably due to a late decrease in g. After moderate drought (14 days), A and g decreased in all cultivars, increasing WUEₚₕ. In this period WUEₚ also increased. Both WUE parameters were correlated with Δ¹³C, which may indicate that the g value at the end of moderate water stress was representative of the growing period. After 21 days, TA was the most productive cultivar but under severe drought conditions there was no difference in DM accumulation among cultivars. After the recovery period, leaf area was increased but not total DM, showing that leaves were the most responsive organs to rewatering. Severe water stress did not decrease WUEₚₕ or WUEₚ, and Δ¹³C did not increase after recovery. This absence of a response to severe drought may indicate significant effects on the photosynthetic apparatus after 21 days of withholding water. As for mild drought, WUEₚ but not WUEₚₕ was correlated with Δ¹³C, supporting the view that WUEₚ is a more integrative parameter than WUEₚₕ.