LEAF NUTRIENT VARIATION IN MATURE CAROB (CERATONIA-SILIQUA) TREES IN RESPONSE TO IRRIGATION AND FERTILIZATION

Seasonal variations in leaf nitrogen, phosphorus and potassium concent rations were studied in a mature carob (Ceratonia siliqua L. cv ''Mula ta'') orchard subjected to a 4-year irrigation and fertilization exper iment. Three irrigation regimes (0, 50 and 100%), based on the evapora tion values obtained from a class A pan, were tested in combination wi th two nitrogen (N) supply regimes in which 21 kg ha(-1) year(-1) (low -N) and 63 kg ha(-1) year(-1) (high-N) were supplied as ammonium nitra te. Leaf nitrogen concentration increased throughout the experiment, i ndependently of treatments. There were no significant differences in l eaf N concentration between trees in the high-N and low-N treatments. Irrigation regimes had no effect on leaf mineral concentration but inf luenced the amount of leaves shed and slightly modified the pattern of leaf shedding that occurred during the summer drought period. Nutriti onal balances between N and P and N and K were both closely and signif icantly correlated. Potassium was translocated from leaves to fruits d uring spring, independently of treatments. Severe water stress periods occurring during spring or autumn induced shedding of leaves leading to nutrient mobilization. Nutrient retranslocation during these drough t periods may represent an adaptive mechanism. Nitrogen retranslocatio n was higher for trees in the high-N treatments than for trees in the low-N treatments, whereas phosphorus retranslocation was independent o f the irrigation and fertilization treatments.