Interactions between fruit load and macroelement concentrations in fertigated olive (Olea europaea L.) trees under arid saline conditions

While the intensification of olive cultivation has brought about increased yields and allowed the expansion of olive production to arid and saline environments, alternate bearing - a significant problem of the olive oil industry - remains unresolved. Developing fruit is a major sink for nutrients, and therefore extreme fruit loads are likely to disturb tree-scale mineral balance. Mineral availability, a prerequisite of plant performance, is largely determined by an intrinsic ability to store and remobilize nutrients timely, according to seasonal supply and demand. Unfavorable environmental conditions (aridity and salinity) might disrupt nutrient uptake and interrupt tree growth and reproduction. The objectives of the present study were: (1) to characterize the effect of fruit load on annual macroelement balance in olive trees under intensive management practices in a saline and arid environment and (2) to identify acute risks of mineral deficiencies occurring along the annual production cycle under the same conditions. Nitrogen, P, K, Na, and Ca concentrations were determined every 2 months during 2006 in the leaves, branches, bark, and roots of corresponding heavily (On) and lightly (Off) yielding 'Barnea' trees grown under arid and saline conditions. Nitrogen concentrations in trees were generally unaffected by the fruit load. During fruit development, P and K concentrations were significantly higher in leaves of Off-trees. Potassium concentrations also showed a typical alternate pattern in bark and roots. Sodium concentrations were particularly higher in roots, while Ca accumulated in leaves. An alternating pattern was observed for Na in branches and for Ca in leaves, pointing to a possible mechanism employing Ca to avoid Na-toxic hazards in leaves of fruit-loaded trees. Large quantities of N, P, and K were found in the fruit at harvest. The role of the fruit as an ultimate sink of N, P, K, and Ca, versus the function of leaves or branches as instantaneous sources of these nutrients was demonstrated in a separate fruit removal experiment in 'Coratina'. Macroelement balance at the whole tree level calculated after harvest indicated that the functional reserves of N and P, but not of K, were sufficient to furnish a successive On-year yield. It is concluded that no risks of acute mineral deficiency occur during the biennial cycle. Consideration of alternate bearing cycles is necessary for optimization of N, P, and K application in olive in order to achieve more efficient production, improved produce quality, and minimized environmental consequences.