MOBILIZATION OF THE RESERVE N IN CITRUS

The mobilization of N from reserve organs (leaves, roots, branches and trunk) to developing new organs was studied at different moments of t he growth cycle. Three-year-old Valencia Late orange trees (Citrus sin ensis (L.) Osbeck) were grown individually outdoors in 150 L container s filled with siliceous sand. Trees were irrigated with a nutrient sol ution labelled with potassium nitrate with an enrichment of 4 atom N-1 5 % excess during a complete growth cycle. At the following year, plan ts were irrigated with unlabelled nutrient solution, and harvested thr oughout the growth cycle (flowering, fruit set, second flush, third fl ush, and dormancy). Total N and N-15 analyses were carried out in the different organs of the plants. The highest amounts of N were found in leaves and roots (33-42% and 30- 38%, respectively). Distribution of N-15 was similar to that obtained for total N (42 and 39% of total N-1 5 in leaves and roots, respectively) as is expected after a long perio d of labelling. Old leaves were the main reserve organs, contributing a 40-50% of the total N exported. Roots and aerial woody tissues expor ted between 30-35% and 15-25% of total reserve N, respectively. N expo rted by old leaves was approximately 57% of the N accumulated during t he preceding year, while roots translocated 40% and trunk plus branche s 35%. More than 70% of N accumulated in new organs during spring came from N stored in old organs.