Seasonal changes in photochemical efficiency in leaves of Halimium halimifolium, a mediterranean semideciduous shrub

The relationship between soil water availability, physiological responses [ leaf chlorophyll (Chl) fluorescence, leaf water potential (Psi), and stomat al conductance (g(s))] and plant stress was studied in Halimium halimifoliu m (L.) Willk, Cistaceae, in three sites with contrasted water regimes (Mont e Blanco - MB, Monte Intermedio MI, and Monte Negro - MN) of Donana Nationa l Park (SW of Spain) along the day, in September (summer period) and Decemb er (winter period) 1994. In winter, differences among the areas were not si gnificant, with Psi, g(s), and photochemical efficiency values of -1.5 MPa, 0.200 cm s(-1), and 0.70, respectively. In summer, however, high declines of g(s) (0.014 cm s(-1) in MI and 0.021 cm s(-1) in MB), photochemical effi ciency (0.65 F-v/F-p in MB and MI sites) and Psi (-3.76 in MI and -3.04 MPa in MB) were recorded. Winter-summer differences were minimum in well-water ed sites (MN) and maximum in MI. The Psi and photosystem 2 (PS2) fluorescen ce were correlated and showed similar seasonal patterns in all three areas.