Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contra sted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and P istacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf wat er potential dropped below a threshold value of -3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag be tween decrease of leaf water potential and proline accumulation could be ob served. In Halimium halimifolium, proline accumulation appeared to be assoc iated with severe stress conditions as most plants with high proline conten ts suffered irreversible wilting, especially in the greenhouse. P. lentiscu s showed a different pattern, accumulating proline at two different times o f the year, as a response to cold or to drought. The results of our study i ndicated that the role of proline in this species, rather than an osmotic a gent, seems to be more related to a protective action in cases of severe st ress conditions.