SEASONAL WATER POTENTIAL CHANGES AND PROLINE ACCUMULATION IN MEDITERRANEAN SHRUBLAND SPECIES

We studied the water relations of 6 shrub and 3 tree species typical o f the mediterranean climate region of central Spain to identify differ ential responses to water stress between and within species, and to de termine if free proline concentration in leaves could be used as a wat er stress indicator. Predawn and midday water potentials (Psi w) on a seasonal basis, relative water content (RWC), leaf mass per area, foli ar nitrogen and free proline concentrations were measured. The lowest water potentials were observed at the end of the summer, with recovery to higher water potentials in the fall and winter seasons. Species di ffered regarding the annual Psi w fluctuation. Thymus zygis, Halimium viscosum, Genista hirsuta and Juniperus oxycedrus exhibited the most n egative midday and predawn Psi w (both less than -6 MPa) with a large magnitude of response to changing conditions in soil moisture of the u pper horizon of the soil. Lavandula pedunculata and Cistus ladanifer s howed a moderate response. Quercus rotundifolia, Quercus faginea and R etama sphaerocarpa showed a modest response. The Psi w of different si ze individuals of Quercus rotundifolia and Cistus ladanifer were compa red. The annual Psi w fluctuation was greater in small individuals as compared to large individuals. In every species, there was an increase in proline concentration of bulk leaf tissues when predawn Psi w drop ped below -5 MPa. Small plants of Cistus ladanifer reached lower water potentials and also higher concentration of proline than bigger plant s. Proline could possibly be used as a drought stress indicator in eve ry species except and. rotundifolia. It is suggested that in addition to water stress avoidance due to deep root systems, some mechanisms of water stress tolerance may operate among shrub and tree species of ce ntral Spain.