Mineral and carbohydrates changes in leaves and roots of olive trees receiving biostimulants and foliar fertilizers

Abstract

Fertilization practices in Tunisian olive growing are widely used in order to improve the yields and the mineral status of the trees grown under rain-fed conditions (low received rainfall). The aim of this study is to assess the effect of foliar fertilization on mineral composition and primary metabolites (soluble sugars) in leaves and roots of cultivar Chemlali. A nitrogen based fertilizer and two biostimulants were sprayed separately in different seasons and the treatments were the following: F1 (nitrogen based fertilizer), F2 (biostimulant rich in boron), F3 (combination of F1 and F2), F4 (biostimulant with a calcium base) and F5 (Combination of F1, F2 and F4). Untreated trees (no foliar fertilization) were used as control (C). Leaf and root samples of treatments F1, F2 and F3 were collected at different phenological stages while samples of F4 and F5 were collected at fruit growth. Mineral content and carbohydrate composition were carried out in all the samples. Results showed that foliar fertilization generally improved the mineral profile of leaves for both macro and micronutrients. Macronutrient concentrations were more important in leaves than in roots while the micronutrients were the highest in roots. Concerning carbohydrates status, the most abundantly soluble sugar in leaves and roots was mannitol followed by glucose. Differences in carbohydrates concentration were observed in leaves through the season and the treatments. The combined treatment F3 induced a positive effect on the pool of soluble sugars in leaves at flowering. Then, both mannitol and glucose decreased in July compared to the levels in May, a fact which can be explained by the high carbohydrate demand for fruit growth during this period. Roots accumulated lower contents of carbohydrates. The ratios macronutrients/total carbohydrates and micronutrients/total carbohydrates were higher in the roots than in the leaves. All these results showed the impact of foliar fertilization on the mineral and carbohydrate status and highlighted a relationship between them for different organs of the olive tree cultivated under rain-fed conditions.