Macronutrients play a major role in most plant physiological processes. However, the influence of macronutrient availability on flower properties and fruit set has not been sufficiently investigated, especially in perennials. We explored the effect of increasing internal nutritional levels of nitrogen (N), phosphorus (P) and potassium (K) on parameters relating to olive (Olea europaea L.) flowering and fruit set over three successive seasons. Bearing olive trees grown in inert growing media were exposed to 8 levels of N ranging from 0.1 to 15.1 mM, 7 levels of P ranging from 0.00 to 0.66 mM and 6 levels of K ranging from 0.23 to 5.04 mM. Plant nutritional status was assessed via mineral analysis of leaves. Only substantial low levels of N, P or K negatively affected flowering differentiation. Significant reduction in flowering level was found in response to leaf N lower than 1.35%, leaf P lower than 0.09% and leaf K lower than 0.26%. More pronounced effect of nutritional level was observed on fruit setting. Fruit set, and consequently, fruit number per tree, were consistently impaired by high N levels. Thus, maximum fruit production corresponded to intermediate N levels. Leaf P concentration was strongly and linearly related to rate of perfect flowers, fruit set and total fruit production. The positive response for P was measured throughout all the range of P. even when leaf concentrations in diagnostic leaves were well above the standard commonly considered adequate. Potassium nutrition had the least impact on flower properties and fruit production. However, prolonged and severe K deficiency reduced flowering intensity and productivity. During the three years of study extreme climatic conditions were evidenced, creating different inductive conditions. In seasons characterized by high flower load, staminate flowers were more abundant and number of flowers per inflorescence was lower. Although the nutritional effect on flowering parameter was comparable in the three studied years the effect of nutrition was magnified in season characterized by favorable induction conditions. (C) 2013 Elsevier B.V. All rights reserved.
