The main function of manganese is to activate various metabolic functions within the plant enzymatic system. It is a constitutive element of the enzyme pyruvate carboxylase. Pyruvate is a key intersection in several metabolic pathways. Manganese is involved in the oxidation-reduction process of photosynthesis and is necessary for the protein complex of photosystem II. The complex employs light photons to energize electrons and participates in photolysis. In addition, manganese activates indole acetic acid oxidase, which thereby oxidizes indole acetic acid in plants.

The importance of microelements in the production of fruit trees and vines of high quality and condition

Manganese (Mn) is an indispensable element for the growth and production of plants. It is classified as a micronutrient because the amounts required by the plant are small, but higher than any other micronutrient such as Cu and Zn⁶. Absorption by the plant is active and occurs through the root and leaves¹⁰.  Within the plant mobility is scarce, and transport is upward by the xylem ⁶.

The Mn is relatively abundant in the soil, however the plants absorb it basically as Mn ion²⁺. There is high availability of Mn in acid soils, with optimum pH for abosorption between 4.5 – 5.5. Likewise, this microelement is limited to soils with the presence of free carbonates and is insoluble in soils with alkaline pH¹.

Mn participates in numerous oxidation-reduction enzyme systems, participates in photosynthesis as part of the mangano-protein that is responsible for the photolysis of water and production of O₂, involved in protein synthesis as it participates in the assimilation of ammonium (NH_{4+), acts on the formation of lateral roots, and regulates the metabolism of fatty acids and activates the growth influencing the long growth of cells⁹.}

Mn is one of the elements that contribute most to the functioning of biological processes, such as the transport of electrons in photosynthesis and the formation of chlorophyll ^{9 y 1}.  Due to this function, the deficiency symptoms of this element generally include leaf yellowing or chlorosis, because chloroplasts lose chlorophyll and starch grains and eventually disintegrate ^{7 y 10}. These deficiencies are similar to those of the Fierro, however they differ because in this case a green aura appears around the ribs. These deficiencies occur more frequently in soils with high levels of organic matter, in soils with neutral to alkaline pH, and in soils that are naturally deficient in Mn content ⁷.

In certain crops, such as Cranberry, the pH decrease by acidification of the irrigation water or by natural product of the mineralization of the organic matter, increases the solubility of the Mn in the soil, leaving it available for the roots. Also, the excess moisture effect generates conditions that promote the passage of the oxidized Manganese Mn ³⁺ to Manganese reduced Mn ²⁺ which is more soluble³.  Also the substrate temperature above 24°C promotes the release of Mn available for absorption. These factors generate toxicity in the plant, observing leaf distortion and dark spots¹. In severe cases, there is a necrosing of the edges of the leaves moving inwardly of them as the severity increases⁸. Symptoms are more visible in young plants.

The response of the cultures to Mn applications can be very profitable in soils where low levels of Mn have been detected or where the pH is high⁷, this previous analysis. Studies have shown that adequate levels of leaf Mn in Palto (Hass variety) cultivation is 30-500 ppm ⁴, in Blueberry the appropriate levels are 50 – 350 mg / kg, in citrus the optimal values are between 25-100 ppm⁵, and in vid the appropriate values are between 41 – 100 ppm². In addition, in the case of grapevine, the optimum values of Mn in the berries when performing a 100g analysis of dry matter is 0.04-0.08mg.

Nutritional management is one of the most important factors in crops, so proper management will ensure success.

Leaves with manganese deficiency show mild interventional chlorosis. In its early stages, it can be mistaken for iron deficiency. Symptoms begin with mild chlorosis in younger leaves, while the veins of adult leaves remain green (the phenomenon is more visible if observed under transmitted light). The leaves then acquire a grayish metallic luster and develop dark speckled and necrotic areas along the nerves. A purple tone may also appear on the upper side of the leaves. Some cereals, such as oats, wheat and barley, are extremely susceptible to manganese deficiency. They develop mild chlorosis along with gray specks that lengthen and join together. At the end of the process, the leaf completely withers and dies.

BIBLIOGRAPHIC REFERENCES

1. Garcia, E. 2013. Deficiencies of Iron and Manganese in Bean Leaves (Phaseolus vulgaris L.) identified by textural analysis, color of digital images and artificial neural networks. Institute of teaching and research in agricultural sciences. Mexico.
2. Gaspar, L. Fertilization of vine cultivation. Agro-strategies consultants.
3. INIA Chile. Toxicity of manganese in blueberry plants in the north.
4. Sierra, C. 1998. Fertilization of the Palto. INIA Intihuasi.
5. INTA Argentina. Manual for orange and tangerine producers.
6. Plant nutrition, the role of essential nutrients part II, Micronutrients. Institute for Technological Innovation in Agriculture. Mexico.
7. International Plant Nutritiun Institute (IPNI). Know the deficiency of manganese ipni.net/ppiweb/iamex.nsf/$webindex/F9CC2B73823D9E1F06256AD1005E1257/$file/Conozca+la+deficiencia+de+manganeso.pdf
8. Schulte, E. E. and Kelling, K A. 1999. Soil and applied manganese. Publication A2526. Wisconsin county Extension office. University of Wisconsin. WI, USA.
9. Nutrition and Plant Physiology. Continuing training program. 2016
10. National University of Madrid (UNM). Relevant aspects of the Cinc in the plant. Zinc

Zinc is necessary for the synthesis of tryptophan, which in turn is necessary for the formation of indole acetic acid in plants. The indole acetic acid is one of the auxins and is responsible for elongation and cell division. Said acid is an essential component of several vegetable metalloenzymes (specifically the dehydrogenase variety) and is therefore indispensable for several differentiated functions of plant metabolism. In fact, the enzyme carbonic anhydrase is specifically activated from zinc. This element has, on the other hand, a function in RNA and protein synthesis.

The importance of microelements in the production of fruit trees and vines of high quality and condition

Zinc (Zn) is one of the essential micronutrients for the growth of the plant and its reproduction, being necessary only in small quantities. Its availability for absorption is related to the pH of the substrate: the high pH of the soils causes the retention of this micronutrient, fixing it in forms not available to the plant²; in substrates with acidic pH the Zn is more available.
This nutrient accumulates in the roots, through which it is actively absorbed as a divalent cation (Zn²⁺), and can also be absorbed as a monovalent cation (ZnOH⁺) to be distributed in the plant and fulfill a series of processes^{7 y 8}. The absorption of Zinc increases with the presence of arbuscular mycorrhizae, and is reduced drastically in low temperatures and by antagonism with other elements⁷.

The Zn within the plant catalyzes the synthesis of the amino acid Serine, which is precursor of the amino acid Tryptophan, which in the leaf is converted to indoleacetic acid. This auxin is responsible for the growth of the shoot and the leaf, so it is normal for both to decrease their size when the Zn becomes deficient, stopping the terminal growth and forcing the lateral buds to grow weakly. This situation can be observed when the plant presents the symptom of rosette ³. There is little Zn re-translocation within the aerial biomass, particularly in plants with nitrogen deficiency, so the symptoms of the deficiency of this micronutrient are more common in young or middle-aged leaves. After sprouting, Zn concentration drops due to a dissolution effect caused by growth and increase in leaflet density¹³. In the younger leaves, there are chlorotic intervening zones¹¹; the internodes are shortened in the buds, forming rosettes of yellowish leaves; the old leaves appear tanned and fall easily.

If the concentrations of Zn increase above the tolerance threshold, toxicity effects, including chlorosis and reduced plant growth, become evident; inhibition of CO fixation₂, the transport of the carbohydrates in the phloem and the alteration of the permeability of the cellular membrane¹.

It is necessary to consider a correct fertilization plan according to previous analyzes to optimize the resources and to project an efficient yield of the crop. Zinc sulfate (ZnSO₄) is a widely used source of Zn, whose composition is 36%¹⁰.  In the cultivation of avocado (Hass variety) the appropriate range of Zn in leaves is 30 – 150 ppm¹⁰, in blueberry the appropriate values range from 8-30 mg / kg⁵, while in citrus the optimum values are 25 – 100 ppm⁶ and in vine they are of 26 – 40 ppm⁴.  In addition, in the case of vine, the optimal values of Zn in the berries when performing an analysis of 100g of dry matter is 0.04-0.08mg.

It is very important to know the best way of applying this micronutrient. Foliar applications are not as effective as Zn applied to soil, since foliar application can overcome visual symptoms but is less effective in increasing yield. This was demonstrated by Salazar et al, 2008, who argued that leaf spraying with ZnSO₄ were not effective to correct foliar deficiency of Zinc and neither increased the fruit production nor the size of the same. The growth and production of plants is determined in certain periods by the limiting elements, such as Zn, so that with proper nutritional management we will ensure the success of our crop.

Leaves with Zinc deficiencies develop inter- nectal necrosis. In the early stages, the younger leaves turn yellow. Mature leaves, on the other hand, develop small holes in their superior intervening surfaces. The phenomenon of gutting (bleeding) is also recurrent. As the deficiency progresses, the above symptoms evolve toward intense, operative necrosis. However, the main nerves remain green, as if the plant were recovering from an iron deficiency. In a large number of plants, and especially in trees, the leaves acquire a very small size and the internodes are shortened, which gives rise to that they acquire a form of rosette (circular).

BIBLIOGRAPHIC REFERENCES

1. Efroymson, R.A., M.E. Will, G.W. Suter y A.C. Wooten. 1997. Toxicological benchmarks for screening contaminants of potential concern for effects on terrestrial plants. Department of Energy, Office of Environmental Management Activities at the East Tennessee Technology Park. 123 p.
2. Fancelli, AL. 2006. Micronutrients in the physiology of plants. Pp 11-27. En: M Vázquez (ed). Micronutrients in agriculture. Argentine Association of Soil Science. Buenos Aires, Argentina.207pp
3. Flores, M.; Anchondo, M.y E. Sanchez. 2009. Acidification in band, winter tillage and Zinc in walnut pecan. Chihuahua, Mexico.
4. Gaspar, L. Fertilization of vine cultivation. Agro-strategies consultants.
5. Hirzel, J. blueberry Fertilization. INIA Quilamapu. Chile.
6. INTA, Manual for Orange and Mandarin Producers. Argentina.
7. Plant nutrition, the role of essential nutrients part II, Micronutrients. Institute for Technological Innovation in Agriculture. Mexico.
8. Perea, E.; Ojeda, D.; Hernandez, O.; Escudero, D.; Martinez, J. y G. López. 2010. Zinc as a promoter of growth and fructification in walnut pecan. Technoscience chihuahua Vol IV N° 2.
9. Salazar, S.; Cossio, L. y L. González. 2008. Correction of chronic deficiency of Zinc in Hass avocado. Rev. Chapingo Ser. Hortic. Vol N° 2. Mexico.
10. Sierra, C. 2003. Fertilization of crops and fruit trees in the north. INIA, Ministry of Agriculture of Chile.
11. Sierra, C. 1998. Fertilization of the Palto. INIA Intihuasi.
12. National University of Madrid. Relevant aspects of Zinc in the plant.
    uam.es/docencia/museovir/web/Museovirtual/fundamentos/nutricion%20mineral/micro/Zinc.htm.
13. Wood, B. 2007. Correction of Zinc deficiency in pecan by soil banding. Dep. of Agriculture. HortScience 42: 1554-1558.

Iron is essential for the enzymatic system in plant metabolism (photosynthesis and respiration). Among the enzymes involved are catalase, peroxidase, cytochrome oxidase and other cytochromes. In addition, iron is part of the protein ferrodoxin, is necessary for reductions of nitrate and sulfate and is essential for the synthesis and maintenance of chlorophyll in plants. Finally, iron is closely related to protein metabolism.

Iron in the plants

Iron (Fe) is an essential micronutrient, required by plants. It constitutes approximately 5% in the terrestrial crust and is associated with hematite, siderite and in combinations with organic matter⁶. Fe is absorbed as Fe²⁺ or in any of its forms if chelated, either naturally or artificially. However, there is a low availability to be used by the roots due to factors that limit the presence of this microelement.

Fe is limited among other factors by soil pH, being more available in soils with acid pH (6.5) and less available in soils with alkaline pH (7.8). For each unit of pH decrease (between 4-9), the Fe is reduced 1000 times; while for Mn, Zn and Cu the availability is reduced by 100 times^3,6. Also, soil texture influences the availability of this microelement: clay soils have more available Fe, while sandy soils have less availability; organic matter forms organic complexes called chelates, which facilitate the availability of Fe to the plant. Likewise, the low soil temperatures limit the availability of this element, taking a lot of importance in perennial crops such as fruit trees (avocado, citrus, vines, etc.)⁶.

It has been shown that iron absorption occurs only during root growth⁴. However, this is affected by factors such as poor irrigation management, as excess moisture in the soil decreases the presence of oxygen, thus limiting root growth and consequently Fe absorption. Likewise is increased the CO₂, which generates the appearance of carbonates (HCO_3-) which restrict absorption⁴.  The absorption process begins when the plant releases H⁺ by the roots which generates a reduction of the pH in the rhizosphere and facilitates the solubilization of Fe³⁺, chelation and reduction to Fe²⁺. Finally, ferrous iron enters the roots probably by a mechanism of specific conveyors³.

Fe plays an important structural role in several enzymatic systems where hemin functions as a prosthetic group. These include catalases, peroxidases and various cytochromes^6,9. These allow the respiratory mechanism of cells. Other enzymes such as ferredoxin are important in the oxidation-reduction reactions in the plant (reduction of nitrite and sulfate). El Hierro is not part of chlorophyll but is indispensable for its biosynthesis (by supplying Fe to plants a good correlation is observed between Iron and chlorophyll content⁹). In addition, this micronutrient is an enzymatic catalyst of several biochemical reactions⁶.

Fe is a very little moving element inside the plant, so deficiency symptoms first appear on the young leaves at the top of the plant². In these leaves the deficiency manifests itself as a chlorosis (pale green color) while the ribs remain green, observing a sharp contrast. In severe deficiencies, this chlorosis is observed throughout the plant as a yellowish or whitish² which may be accompanied by marginal necrosis in both young and adult leaves. Chlorosis occurs because Fe is necessary for the synthesis of chlorophyll, which is responsible for the green color of the leaves. Its deficiency does not affect the size of the leaves⁴. A mild and moderate deficiency affects production and quality; a severe deficiency leads to death to the plant⁴. Excess toxicity of this element is not common, with the exception of rice cultivation⁶.

According to the foliar analysis, there are adequate values of Fe that should be kept in mind to avoid problems of crop failure. In blueberry the appropriate level of Fe is 60-120 mg / kg⁵, in Palto (Var. Hass and Fuerte) is 50-200 ppm^{7 y 8}, in Tomato 141 – 250 ppm^7y8, and in Vid the appropriate values of Fe are between 40 – 100 ppm¹. Foliar applications are a good alternative to improve plant nutrition. If it is via fertigation, it must be taken into account that in soils with pH values higher than 7.8, it is recommended to apply EDDHA-Fe chelated iron, whereas in soils with a pH lower than “7,8” should be used as EDTA-Fe or DTPA-Fe⁶. A correct use of nutrients depends on the knowledge of each one of them.

BIBLIOGRAPHIC REFERENCES

1. Gaspar, L. Fertilization of vine cultivation. Agro-strategies consultants.
2. Know the deficiency of Iron. International Plant Nutritiun Institute. ipni.net
3. Iron chlorosis in crops. Institute for Technological Innovation in Agriculture. Mexico.
4. Ferreyra R. y Ruiz, R. 2008. Feral Chlorosis of the Palto and irrigation management. INIA Tierra Adentro. INIA La Platina – INIA La Cruz. Chile.
5. Hirzel, J. blueberry Fertilization. INIA Quilamapu. Chile.
6. Sierra, C. 2017. An intense relationship: El Hierro, the soil and the plant. El Mercurio Journal, Santiago de Chile. Chile.
7. Sierra, C. 2003. Fertilization of crops and fruit trees in the north. INIA, Ministry of Agriculture of Chile.
8. Sierra, C. 1998. Fertilization of the Palto. INIA Intihuasi.
9. Nutrition and Plant Physiology. Continuous training program. 2016

Copper (Cu) is an essential micronutrient for plants, as it is required to complete its life cycle, including the production of viable seeds¹. This element in the soil is mainly presented as chalcopyrite, in organic combinations and as an interchangeable cation in soil colloids².

Cu in soil is not always available to be absorbed by plants. Some factors, such as pH, affect its availability as it decreases when the pH>7 and increases with values below 6. Also, of the metallic micronutrients (Fe, Mn, Zn y Cu), the Cu is the one that is usually more linked with the organic matter, forming very stable compounds. This explains why copper deficiency occurs in very organic soils. Sandy soils naturally have low Cu contents, while clay soils usually have a higher availability. The availability of Cu is also affected by antagonistic ions: high levels of Nitrogen and Phosphorus hinder the absorption of copper^2,3, and an excess of Zinc or Manganese may accentuate copper deficiency².

The availability of Cu is also closely related to total reserves. Mineral soils containing less than 6 ppm or organic soils with less than 30 ppm of Cu may be considered to be potentially deficient². Only a small fraction (generally less than 0.001 ppm) is in soluble form and available to be absorbed as a divalent ion (Cu⁺²) in aerated soils, and in its monovalent form (Cu⁺) in moist soils with low oxygen concentrations. Absorption is through the epidermis of the root; the movement of the ions from the epidermis to the root endodermis is through apoplastic diffusion⁴. The mobility of copper in the plant is classified as semi-mobile⁵.

Many proteins that contain Cu play key roles in processes such as photosynthesis, respiration, detoxification of superoxide radicals and lignification⁶. Without Cu there would be no photosynthesis, since this element is necessary for the formation of chlorophyll¹. Superoxide dismutase enzymes (SOD) detoxify superoxide radicals,  the Cu-Zn-SOD for example is located in the chloroplast stroma, where the atom of Cu is involved in the detoxification of O₂ generated during photosynthesis, these superoxide radicals can cause severe damage to cells⁶. Some enzymes that contain Cu are found in cell walls and participate in the biosynthesis of melanocytic substances and lignin: some melanocytic substances such as phytoalexins inhibit spore germination and fungal growth; the formation of lignin forms a mechanical barrier as resistance of the plant to diseases⁶.

Crops such as cereals and citrus are more vulnerable to lack of Cu^1,2. The characteristic symptoms of its deficiency appear first in the extremities of the young leaves, which narrow and curl, while the extremity turns whitish. In addition, the growth of the internodes diminishes and the plants have aspect of “stunted”. In fruit trees the flowering and formation of fruits are very affected². Severe deficiencies of Cu they produce chlorosis and the descending death of the terminal growths. On the other hand, concentrations higher than those required by plants produce toxic effects, such as inhibition of growth in roots and shoots⁴.

BIBLIOGRAPHIC REFERENCES

1. International Plant Nutritiun Institute (IPNI). Know Copper Deficiency. ipni.net
2. Sierra, C. 2016. A look at the relationship between copper, soil and plants.
3. Synergisms and antagonisms between nutrients. Institute for Technological Innovation in Agriculture. Mexico. Tecsup. Nutrition and Plant Physiology. Continuing training program. 2016
4. León, J. y G. Sepulveda. 2012. The oxidation damage caused by copper and the antioxidant response of plants. Inter science Vol. 37, N° 11, pp 805-811. Venezuela.
5. Nutrition and Plant Physiology. Continuous training program. 2016
6. Kirkby, E y Volker, R Micronutrients in Plant Physiology: Functions, Absorption and Mobility. International Plant Nutritiun Institute(IPNI)
7. Sierra, C. 1998. Fertilization of the Palto. INIA Intihuasi.
8. Hirzel, J. blueberry Fertilization. INIA Quilamapu. Chile.
9. Gaspar, L. Fertilization of vine cultivation. Consulting Agro-Strategies.
10. INTA, Manual for Orange and Mandarin Producers. Argentina.