Abstract
This study evaluated leaf supply of zinc (Zn) and boron (B) using either soluble or sparingly soluble fertilizers in young sweet orange trees. Three experiments were set up in a greenhouse to compare two sources and four doses (control, low, adequate and high) of fertilizers as follows: (i) Experiment I (B): boric acid and calcium borate; (ii) Experiment II (Zn): Zn sulfate and Zn oxide; and (iii) Experiment III (B + Zn): boric acid + Zn Sulfate and Zn Borate. The sparingly soluble sources were effective in increasing the Zn and B leaf concentration. Dry matter of the aerial part increased 18 % with B applications in adequate concentration independent of the B fertilizer sources. In contrast, trees did not grow well with applications of adequate concentration of Zn as Zn Sulfate or high Zn concentration as Zn borate. Superoxide dismutase activity in leaves increased with applications of low concentration of Zn as Zn oxide and decreased with high concentration of Zn from either source. Polyphenol oxidase activity increased with application of adequate concentration of B as boric acid and high concentration of B as calcium borate. Furthermore, the upper concentrations of Zn were toxic in orange trees when the source was Zn sulfate. Increases in plant growth without damage to leaf tissue and positive responses of key enzymes of orange trees in a range of nutrient concentration applications demonstrated the practical use of sparingly soluble fertilizers to supplying B and Zn foliarily to plants.
Highlights
The majority of orange groves cultivated in acid to neutral pH soils require frequent supply of B and Zn, otherwise fruit production is limited (Quaggio et al, 2010; Wang et al, 2015; Mattos Jr. et al, 2017)
Efficiency of traditional foliar fertilizers depends on water solubility, while sparingly soluble sources rely upon particle size, in which the smallest particles (0.2 20 μm) facilitate the release and further absorption of the nutrient by plant leaves (Du et al, 2015), maintaining a constant nutrient supply for long periods reducing the risk of leaf tissue injury immediately after application (Li et al, 2012)
Leaf concentrations of B and Zn increased with doses of soluble and sparingly soluble fertilizers sources sprayed on plants in experiments I (B), II (Zn) and III (B and Zn)
Summary
The majority of orange groves cultivated in acid to neutral pH soils require frequent supply of B and Zn, otherwise fruit production is limited (Quaggio et al, 2010; Wang et al, 2015; Mattos Jr. et al, 2017). Efficiency of traditional foliar fertilizers depends on water solubility, while sparingly soluble sources rely upon particle size, in which the smallest particles (0.2 20 μm) facilitate the release and further absorption of the nutrient by plant leaves (Du et al, 2015), maintaining a constant nutrient supply for long periods reducing the risk of leaf tissue injury immediately after application (Li et al, 2012). In addition to the quantification of nutrient concentration in leaves, the analysis of superoxide dismutase activity (SOD) was proposed to study the interaction between metals and plants (Shenker et al, 2004) as Zn is a constituent of Cu / Zn-SOD (Hansch and Mendel, 2009; Hippler et al, 2015a) and may be indicative of the risk of phytotoxicity in plants supplied with excess micronutrients (Hippler et al, 2015a; Hippler et al, 2018). The activity of polyphenol oxidase (PPO) has been assessed as an indicator of the nutritional status of B since rises in the concentration of phenols in B-deficient tissues may result from restrictions in the biosynthesis of phenolic alcohols (Marschner, 2011)
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