Effect of Copper Foliar Spray upon the Contents of Other Elements in Apple Leaves

In order to cope with fungal diseases during organic viticulture and fruit farming, use of foliar copper spraying up to 6 kg/ha. a has been generally permitted within the EU [1]. Fixation of copper-hydroxide or copper-oxychloride particles at leaf surfaces can be achieved by emulsions together with adherents like ethoxylated rapeseed oil or soy oil. Simulation of elution by rain drops showed that adherents decreased Cu-losses from leaves within the first 30 mm of rain got decreased from 75 - 90% down to 10-25%, and increased the covered surface area per particle. Washout to the soil left the particles rather unchanged [2,3]. Within the soil, the Cu-containing particles get irreversibly adsorbed at humics and pedogenic oxides within a few hours, leaving exchangeable fractions within a few percent of total [4]. 

Because Cu is much more toxic towards fungi than towards bacteria, Cu inputs decreased fungal biomass, whereas bacterial phospholipids and xylanase were not affected, but effects upon the mycorrhiza remained unclear. Thus, effects upon bacterially driven N -, P-and C - cycles seem less pronounced [5].

 Though numerous studies about Cu-speciaton and mobilities in soil, as well as toxicity symptoms in green plants are available, effects upon the metabolism of other elements after Cu-spraying are largely unknown.

 In green plants, about 98% of total Cu is bound to various organic molecules. Cu shows high affinities to thiol groups of peptides, and thus to proteins rich in cystein. But it may also form stable chelates with ca rboxylic and hydroxylic groups, often assisted by groups containing basic nitrogen. Cu-containing enzymes catalyse electron transfer reactions, like photosynthesis, respiration, perception of ethylene, metabolism of reactive oxygen, and remodeling of cell walls. Many Cu - proteins have a functional counterpart that uses Fe [6,7]. 

Whereas divalent Cu shows high affinity to histidine, monovalent Cu favors cystein or methionine. In case, metals are transported by a common transport protein, Cu can displace other essential metals in metallo-proteins because of its high stability of its thiol complexes. According to the Irving-Williams-series, the stability of bondings between metallo-proteins and metals increases within Ca2+< Mg2+< Fe2+< Co2+< Ni2+-Cu2+-Cu+ [7,8]. 

Small organic molecules like mugeinic acid or nicotianamine (N-N-(3-amino-3-carboxypropyl) -3-amino-3-carboxypropyl) azetidine-2-carboxylic acid) are utilized to transport essential metals like Cu, Fe, Mn, Ni or Zn within the green plant. In vitro, the complexation capability of nicotianamine increases within the sequence Mn-Fe-Co-Zn-Ni-Cu, and peaks at pH 6,5. In case of Cu, Fe or Zn deficiencies, nicotianamide get increasingly formed, but this necessitates, however, high supply of nitrogen [6]. At Cu-deficiency, Cu-treatment induces the formation of metallothioneins, which assist the reconstruction of plasma membranes and act as anti-oxidants [9]. Within the roots, both Cu and Fe get reduced by root cell ferric reductase. When given in excess, plan ts have reduced Fe uptake, and vice versa [6]. Excess Cu concentrations tend to decrease root growth because of preferential Cu - accumulation in that organ. 

In case Cu is taken from the soil, the most frequent symptom of Cu - intoxications is chlorosis as well as reduced uptake of Fe. In addition to chlorosis, excess Cu causes symptoms like necrosis, and reduced growth. Inside the cells, excess Cu can disrupt protein structures, reduce enzyme activities, interfere in the biosynthesis of photosynthetic pigmen ts and membranes, cause deficiencies of other essential elements, and induce oxidative effects [8]. Cu-induced Fe deficiency, replacement of Mg by Cu, or destruction of the oxygen transporting polypeptide decreases chlorophyll content. 

Therefore, the Cu - tissue levels are regulated within a narrow physiological range by homeostasis. Frequently, green plants have specific metal sensors, which start a cascade of signals to induce corresponding reactions. The green plant can protect itself against excess Cu by stimulation of excretion, increase of chelators, and separation into a vacuole [8]. 

Immissions of toxic amounts of Cu can be caused by industrial and residential wastes, pig manure and poultry dung, and also Cu foliar sprays. Most papers about metal tole rance deal with elevated soil or hydroponic Cu levels. Among green plants, tolerance versus excess Cu is highly variable. Cu tolerant plants are mainly excluders, reacting by reduced secretion of root exsudates, and immobilisation inside the root. Excesss oil Cu gets at first enriched within the roots, lowers root growth, promotes root damage and lowers transport processes inside the plant. Transport of excess Cu from roots to shoots gets prevented by adherence to cell walls, reduced flux across plasma membranes, increased outflow from the cytoplasma as well as intracellular chelation by organic acids, special phytochelatins, and metallo-thioneins [7]. 

The reverse transport of Cu from the leaves back to stalks and roots takes place at leaf aging. Increased N-supply delays aging and affects availability and mobility inside the plant by binding more Cu to amino acids and proteins. Cu gets hardly redistributed from old leaves to younger ones [6,7]. 

In case of Cu-deficiency, because of limited mobility from soil , foliar Cu-spray acts much faster and more effectively than additions to the soil. Cu levels applied as fungicides, however, are 10-100 times higher than usually needed for fertilization in case of deficiencies [7]. A hydrophobic cuticula protects leaves from external damage. Because the cuticula of young leaves is not so strong, effects of foliar spraying are higher in this case. The adherence of sprayed solutions depends on genotypical differences of leaf surface properties, like hairs and smoothness of surface. 

A known interlement effect facilitates the decision to spray each element separately, or to use a mixture and thus safe work. The uptake of foliar-sprayed Cu, Mn, and B into apple leaves was higher in May than in June and September, and elevated levels of Cu lasted longer than of Mn and B. In combination with Mn, the leaves adsorbedless Cu than without [10]. Cu addition to sandy soil also increased the uptake of Ba, Ca, Sr and Fe into the leaves of MacIntosh apple seedlings, but decreased Mn and Mo. Addition of peat to this sandy soil increased soil adsorption, but also decreased Cu uptake into the seedlings [11]. 

In order to document differences in root trace element uptake and foliar spray, young nursery-grown apple trees were grown in pot experiment on quartz sand. Spraying increased the Cu content of the components that were directly exposed, but hardly increased the Cu content of other tree components, like roots. Differences in plant growth were marginal. The levels of N, P, K, Ca, Mg and Na in the leaves were about the same after sufficient spraying or soil additions of Cu, Mn, Zn and B [12]. 

Within a field trial at the experimental orchard Jedlersdorf (Vienna/Austria), run by the University of Natural Resources Vienna, apples of „Topaz“ cultivar have been grafted upon different rootstocks, beneath a lot of other fruit items. Rootstocks M9 with and without „Rubinola“ as interstem, M26, M7 grafted at 25 cm and at 55 cm, MM111 and Bittenfelder seedlings, were used, trained as spindles. Growth, yields and mean fruit weights have been reported elsewhere [13]. This enabled us to investigate the effect of rootstocks and the year of growth upon the composition of fruits and leaves at the same site and fertilization regime [14]. In 2012, young leaves were sampled in early June, which had got no Cu-treatment, whereas in 2015 and 2016, young leaves from the same trees were sampled after Cu treatment. At the same site, apple leaves of Golden delicious variety were available with and without Cu treatment, grown in 2016. 

For Cu treatment, Cuprofor liquid, containing copper oxichloride, was used, diluted at 0,03% solution (%v/v).