Zinc Nutrition of Fruit Trees by Foliar Sprays

Effective methods of supplying Zn to fruit trees are needed to combat widespread deficiency of this element all over the world. Soil applications are not very effective because the roots of fruit crops occupy deep soil layers and zinc does not easily move in the soil. Although foliar sprays are more effective, foliar-absorbed Zn is not easily translocated in plants, which necessitates repeated spray applications and diminishes the ability of foliar sprays to alleviate Zn deficiency in all plant parts. Conditions under which fruit trees are most likely to respond to corrective Zn treatments in terms of growth, yield, and fruit quality are not completely understood. In citrus and apples, the occurrence of severe deficiency symptoms appears to be a prerequisite for tree responses. Zinc foliar sprays applied before anthesis may be most beneficial in terms of fruit yield in citrus and grapes. More research is needed to better define the critical periods for Zn supply to assure optimal fruit set, fruit growth, and high external and internal fruit quality. INTRODUCTION Foliar sprays with Zn are widely practiced by fruit growers because: (1) the metal is an essential element for normal plant development; (2) large areas of agricultural soils are deficient in this element; and (3) soil applications are generally ineffective in correcting Zn deficiency in fruit crops (Swietlik 1999). Although the subject has been studied for a long time, we still need to develop a better understanding of the factors limiting the effectiveness of Zn foliar sprays such as: (1) the rate of absorption of Zn by the aboveground plant parts; (2) translocation of the absorbed Zn into specific organ(s) to elicit the desired physiological effect(s); (3) the critical timing for Zn foliar sprays; and (4) the critical Zn nutritional level below which the corrective treatments would be expected to improve growth, yield, or fruit quality. ABSORPTION AND TRANSLOCATION OF FOLIAR-ABSORBED ZN Mineral nutrients enter into leaves in three steps involving: (1) penetration through the cuticle and epidermal walls; (2) adsorption on the surface of the plasmalemma, and (3) passage through the plasmalemma into the cytoplasm (Swietlik and Faust, 1984). The cuticle is covered by epicuticular waxes, which constitute the most hydrophobic component of the leaf surface. Discontinuities and cracks in these waxes, however, open a pathway for penetration of leaf-applied nutrients (Swietlik and Faust, 1984). The underlying cutin is much more hydrophilic because its building blocks, polyesterified hydroxy fatty acids, contain polar groups. Schonherr and Bukovac (1970) demonstrated the existence of polar pathways in the cuticle and Schonherr (1976) proposed the existence of transculticular canals lined with carboxyl groups serving as polar pathways for penetration. Due to its pectin and polar nature, the epidermal cell wall is much less of a barrier to nutrient diffusion than is the cuticle (Schonherr and Bukovac, 1970). Our knowledge of absorption pathways for Zn following cuticular penetration is still very limited. Some of the possible pathways are depicted in Fig. 1. After passing through the cuticle, Zn may diffuse through the free space (apoplast) of cell walls to vascular tissue where, after loading into phloem, it may be transported out of the leaf. Alternatively, after lateral and inward diffusion, Zn may get adsorbed on negatively charged sites and remain in the apoplast of leaf mesophyll tissue. Another pathway of foliar Zn absorption may involve transport across the plasmalemma and the cytosol of Proc. IS on Foliar Nutrition Eds. M.Tagliavini et al. Acta Hort. 594, ISHS 2002 124 leaf mesophyll (symplast) to the vascular tissue. In studies conducted by Zhang and Brown (1999b) and Ferrandon and Chamel (1988), approximately 89-95 % of Zn recovered after foliar application to pistachio and pea (Pisum sativum) was found in the treated leaf after 10 and 1 day, respectively (Table 1). This shows very poor translocation of foliar-applied Zn and indicates that only a very small proportion of Zn recovered in plants was actually transported across the cell membranes into the symplast. Furthermore, there was a linear relationship between the concentration of foliar-applied Zn and the amount of Zn recovered in plants (Zhang and Brown, 1999a). This kind of kinetics strongly indicates the involvement of physical processes in Zn uptake such as diffusion and binding to the cuticle and cell walls, rather than enzymatically-dependent Zn transport across the cell membranes. Indeed, Zhang and Brown (1999a) showed metabolic inhibitors and light had no effect on leaf Zn absorption by pistachio and walnut leaves whereas temperature had only a weak effect (Q10= 1.21.4). Quite a different picture emerges from the studies on Zn absorption by sugarcane leaf discs (Bowen, 1969). The relationship between Zn absorption and the concentration of applied Zn followed Michaelis-Menten kinetics as the rate of Zn absorption reached saturation at elevated Zn concentrations. The rate of absorption was reduced by a number of metabolic inhibitors and it also showed stronger dependence on temperature (Q10= 1.8) than in Zhang and Brown’s (1999a) study. Light, however, still had no effect on Zn absorption. The discrepancy between Zhang and Brown’s (1999a) and Bowen’s (1969) studies could be due to different plant species and concentrations of foliar-applied Zn. Most likely, however, various experimental procedures were responsible for the observed differences. Bowen (1969) employed leaf discs whose edges with exposed mesophyll cells had a direct contact with the Zn treatment solution. Zhang and Brown (1999a) used intact leaves therefore the cuticle and the underlying cell walls could adsorb Zn on negatively charged sites as the metal diffused into the leaf interior. The difficulty of separating adsorption from absorption is one of the reasons there is controversy in the literature as to whether Zn absorption by plants is an active or passive process (Swietlik, 1999). The lack of a rigorous definition for an active uptake process is a further contributing factor. In this paper, active uptake is defined as transport against an electrical potential gradient (Kochian, 1991). Thus, a mere dependence of uptake on metabolic processes does not prove the existence of active uptake across cell membranes. In fact, a cell membrane potential of –120mV to –180 mV generated by a proton pump is believed to be large enough to drive passive uptake of Zn (Kochian, 1991). This process is illustrated in Fig. 2. One must keep in mind, that the proton pump, whose functioning depends on metabolic processes, generates the voltage potential. That is why, Zn uptake across cell membranes, although passive is dependent on metabolic processes (Bowen, 1969). Hacisalihoglu et al. (2001) reported that Zn transport across root-cell membranes followed Michaelis-Menten kinetics because it had a saturable component, suggesting the involvement of a protein-mediated transport system. Three Zn transport genes have been identified in Arabidopsis thaliana and one in the Zn hyper-accumulator species, Thlaspi caerulescens (Kochian, 2000). Since the rate of absorption and mobility of foliar-applied Zn is low (Zhang and Brown, 1999a b), one may speculate that leaf cell membranes may be lacking an efficient Zn transporter(s). Poor mobility of foliar-absorbed Zn reported by Zhang and Brown (1999b) and Ferrandon and Chamel (1988) was also found in other studies (Swietlik, 1996). It is also reflected by the inability of foliar sprays with Zn to eliminate Zn deficiency in roots as measured by growth and Zn tissue concentrations (Swietlik and Zhang, 1994) (Table 2). The foliar treatments reported in Table 2, however, alleviated the deficiency in the aboveground plant parts.