Water deficit effects on osmotic adjustment and solute accumulation in leaves of annual clovers

Annual clovers are very popular forage crops in the Mediterranean areas. However, their herbage and seed yields are often reduced by high temperatures and water stress occurring during spring and summer. Field experiments using the four most widely grown forage legumes, berseem (Trifolium alexandrinum L.), crimson clover (T. incarnatum L.), Persian clover (T resupination L.) and squarrosum clover (T. squarrosum L.), were conducted in 1992 and 1993 at Foggia (Italy) to evaluate the osmotic adjustment capability and the relevant contribution of inorganic and organic solutes in response to water deficits. Soil water depletion reduced leaf water potential (Psi(w)) and leaf osmotic potential at full turgor (Psi(pi)(100)) in both years. In particular, berseem and squarrosum clovers showed lower values ( - 1.46 and - 1.51 MPa on average, for Psi(w) and Psi(pi)(100), respectively). Furthermore, water-stressed plants showed an increase in potassium, reducing sugars and proline concentrations and a decrease in non-reducing sugar contents. Generally, about 80% of the measured cellular osmotic potential was attributable to assayed osmotically active solutes; the inorganic ions represented the major contributors (about 59%). However, only proline levels appeared to change clearly in terms of relative contribution to Psi(pi)(100), showing an increase of about 2.2%, under water-stressed conditions. Statistically significant differences (P less than or equal to 0.05) in the osmotic adjustment trait were not found for either species or year. Because clover species showed the same value for solute accumulation capability (0.34 MPa), degree of tolerance to dehydration and turgor maintenance but differed in leaf water parameters, other physiological and/or morphological traits will have to be investigated to better explain the performance of each species under water-stress conditions. However, the tendency of crimson and Persian clovers to maintain high leaf water potentials could be an example of a stress-avoidance mechanism. (C) 2002 Elsevier Science B.V. All rights reserved.