Seasonal variations in leaf nitrogen, phosphorus and potassium concentrations were studied in a mature carob (Ceratonia siliqua L. cv ‘‘Mulata’’) orchard subjected to a
4-year irrigation and fertilization experiment. Three irrigation regimes (0, 50 and 100%), based on the evaporation values
obtained from a class A pan, were tested in combination with 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 nitrate. Leaf nitrogen concentration increased throughout the experiment, independently of treatments. There were no significant differences in leaf N concentration between trees in the high-N and low-N treatments. Irrigation regimes had no effect on leaf mineral concentration but influenced the amount of leaves shed and slightly modified the pattern of leaf shedding that occurred during the summer drought period.
Nutritional balances between N and P and N and K were both closely and significantly correlated. Potassium was translocated
from leaves to fruits during 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 drought
periods may represent an adaptive mechanism. Nitrogen retranslocation was higher for trees in the high-N treatments than for trees in the low-N treatments, whereas phosphorus retranslocation was independent of the irrigation and fertilization treatments
