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 stomatal conductance (g(s))] and plant stress was studied in Halimium halimifolium (L.) Willk, Cistaceae, in three sites with contrasted water regimes (Monte Blanco - MB, Monte Intermedio MI, and Monte Negro - MN) of Donana National Park (SW of Spain) along the day, in September (summer period) and December (winter period) 1994. In winter, differences among the areas were not significant, 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 efficiency (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-watered sites (MN) and maximum in MI. The Psi and photosystem 2 (PS2) fluorescence were correlated and showed similar seasonal patterns in all three areas.