Seasonal variations of leaf water potential and growth in fertigated carob-trees (Ceratonia siliqua L.)

Variations of predawn and midday leaf water potential and relative growth rates were studied in mature carob trees (Ceratonia siliqua L. cv Mulata) submitted to a fertigation experiment. Three levels of irrigation were tested: 0%, 50% and 100%, based on daily standard evaporation values. For each irrigation level two nitrogen amounts were applied –21 and 63 kg N ha^-1 year^-1 as ammonium nitrate. The experiment was run between July 91 and August 1993. Measurements of leaf water potential and absolute branch length increments were made at monthly intervals, during the entire experimental period or during seasonal growth, respectively. Leaf water potential was related to soil volumetric water content, maximum and minimum air temperature and daily evaporation. Predawn leaf water potentials were always higher than –1.1 MPa. Midday leaf water potential values presented very large seasonal variations and very low values independent of treatments. The low leaf water potentials observed for the fertigated trees during summer, suggest that this parameter may be related not only to the evaporative demand but also to growth investment. The amount of fertigation was positively correlated with vegetative growth increment and fruit production. Practical implications for irrigation schedules of leaf water potential patterns together with drought adaptation mechanisms of carob tree are discussed.