Physiological responses to water stress following a conditioning period inberseem clover

Berseem clover (Trifolium alexandrinum L.) is an important crop in semi-ari d regions; its herbage and seed yields are often reduced by water stress. O ur objectives were (i) to determine the effect of water stress, applied aft er a conditioning period, on water relations, proline accumulation and plan t dry weight, and (ii) to investigate if some physiological responses diffe red in varieties of berseem. Five cultivars (Axi, Bigbee, Lilibeo, Sacromon te and Saniros) were grown in a controlled environment, and subjected to fo ur irrigation treatments (T1, T2, T3 and T4 referring to plants irrigated t o field capacity every 1, 2, 3 or 4 d, respectively) during a conditioning period (12 d). T1 treatment indicated the well-watered control, whereas T2, T3 and T4 treatments represented the conditioned plants. Leaf water potent ial (Psi), osmotic potential (Psi pi), relative water content (RWC), gravim etric soil water content (GSWC) and leaf proline concentration were recorde d during the conditioning period and a subsequent water deficit period (3 d ) applied at early flowering growth stage. The conditioned plants subjected to subsequent water deficit maintained higher values of Psi, Psi pi, RWC a nd GSWC, and lower values of leaf proline concentration. Reductions in para meter values were inversely related to the water stress severity that plant s had previously experienced. At the end of the experiment, T1 showed 42%, 58% and 31% lower values for Psi, Psi pi and RWC, respectively, than those of T4. Conditioned plants were also shorter and accumulated less leaf, stem and total dry weight. The conditioning treatments did not affect the relat ion between Psi and Psi pi since conditioned plants show similar values of Psi pi as the control at the same Psi value. Thus, drought acclimation in b erseem clover contributed to water stress tolerance by the maintenance of t issue hydration. The berseem cultivars examined showed differences in plant growth parameters, but they were very similar for physiological responses to water deficit. The main genetic difference was recorded for turgor maint enance capacity.