The role of micronutrients in the avocado crop

Introduction

Chemical elements are referred to as "micronutrients", or "trace elements", because their concentrations in plants are less than 100 mg/kg (ppm) on a dry matter basis. In reality, many of these elements are present in even lower concentrations. In contrast, macronutrients such as nitrogen and potassium are present in plants at concentrations of about 1 - 3%, i.e. 1000 times higher. Most of the micronutrients of plant/human/animal health and environmental relevance are metals such as cobalt, copper, iron, manganese, molybdenum, nickel and zinc. Other important micronutrients are non-metals, such as arsenic, boron, chlorine, molybdenum, selenium and silicon. Trace elements occur naturally in soils. The most important natural sources include the effects of climate on the soil, erosion and deposition of wind-blown particles, volcanic eruptions, forest fires and biogenic sources. Low concentrations of essential micronutrients can result in inadequate supply to plants, affecting their growth and development, which can ultimately cause deficiency disorders further up the food chain, and this is when specific fertilizers should be included in the mineral nutrition scheme of micronutrient-poor crops. Regardless of the fact that they are biologically essential, trace elements become toxic when absorbed in excessive amounts.

A good starting point for a good fertilization program is crop removal values.

The table above clearly shows that the micronutrients removed by the fruit in the highest amount are boron, zinc, copper and iron. This does not reduce the importance of the other elements, as any deficiency can cause serious damage. Naturally, each micronutrient should be returned to the soil or directly to the tree in the amount that is exported by the fruit, plus an appropriate efficiency coefficient. It should also be remembered that no nutrient can be replaced by another, therefore, a high level of one element in the soil, or even in the plant, cannot compensate for a deficiency of another element.

Another useful tool for the grower is the analysis of leaves, carried out according to strict standards, as follows.

Leaf sampling

Leaf samples (~40 leaves per homogeneous block) are taken in late August - October, from 5-7 month old leaves developed in spring, about 5-6 late period leaves, from non-fruit bearing branches). Ref: A & L, Agronomy Handbook, Ankerman & Large Eds. 2017; Bender, 2016, et al.

The main functions and challenges of the micronutrients mentioned above are as follows

Boron (B)

Avocado trees probably use more boron than any other crop, mainly for good flower formation and fruit set. Boron is essential for Ca transport within the tree and for normal shoot tip (apical meristem) development, especially during pollination, as it stimulates pollen tube growth, which directly increases fruit set.

Boron is also essential for branching, normal flower, fruit and root formation, nucleic acid synthesis and carbohydrate metabolism. Its weak negative charge makes it very sensitive to leaching, hence low use efficiency. Higher use efficiency is obtained when the soil is rich in organic matter, or if boron is applied in combination with humic acid. The sugar-borate complex is mobile in the xylem of the tree, but its mobility in the phloem is limited.

Zinc (Zn)

Zinc is a key structural and catalytic component of a large number of proteins, as a cofactor for more than 100 specific enzymes, transcription factors and protein interaction domains, and nucleic acid synthesis. Zinc is essential for carbohydrate transformation and regulation of plant sugar consumption. It is indispensable for producing auxin AIA and gibberellic acid. Therefore, Zn deficiency results in stunting, and a 'small leaf' and rosette growth pattern. Zn availability to roots is highest at soil pH 5-7.5, and much lower on both sides of this range. Its availability is negatively related in the soil to phosphorus availability. Zn deficiency symptoms: spotted interveinal chlorosis of leaves, leaves are smaller than normal and have a rosette growth pattern. Also, small round fruits.

Copper (Cu)

In most of Cu's functions as a plant nutrient, it is linked to enzymes, which catalyze redox reactions in photosynthesis, respiration, C- and N- metabolism, and protection against oxidative stress. It forms highly stable complexes and participates in electron transfer reactions, in which it continuously changes its valence between +2 and +1. Cu enzymes react in cells directly with molecular oxygen. About 98% of Cu in plants is present in complex forms in the cytoplasm of cells. The availability of Cu to roots is highest at a soil pH of 5-7.5, and much lower on both sides of this range. Its availability is also positively related to soil organic matter. Cu deficiency symptoms: older leaves have a dull appearance. Shoot tips have multiple bud formation. New leaves abort and dry up.

Iron (Fe)

Iron is a component of two important groups of proteins, specifically, heme proteins and Fe-S proteins. These macromolecules are involved in respiratory and photosynthetic activity, essential for numerous plant functions. A central function is, of course, chlorophyll production and function, but other important functions are redox reactions related to respiration, energy transfer and metabolic processes within the plant. Several heme proteins act as cofactors for cytochromes involved in respiratory reactions. Other heme proteins include catalase and peroxidase, converting hydrogen peroxide to water and O2. Fe-S proteins are importantly involved in the light-dependent reactions of photosynthesis. Ferrodoxin, which contains iron atoms, is the end product of photosystem I, and transfers electrons to a number of acceptors. Fe availability to roots is highest at soil pH 4-7, and much lower above 7. The most prevalent symptom of Fe deficiency in avocado is interveinal chlorosis of young, fully expanded leaves.

Manganese (Mn)

Manganese functions primarily in the activation of many enzyme systems and is also a component of certain enzymes. It participates in a variety of redox processes, such as enzymes involved in the breakdown of carbohydrates, and as a cofactor of nitrate to nitrite reducing enzymes. It also plays an important role in photosynthesis, pollen germination and pollen tube growth. Mn is rather immobile within the active phloem transport system. Therefore, its deficiency symptoms will appear first in the newer leaves. Manganese availability to roots is highest at soil pH 5-7.3, and much lower on both sides of this range. The most prevalent symptom of Mn deficiency in avocado is interveinal chlorosis of young, fully expanded leaves.

Molybdenum (Mo)

Molybdenum is vital in the avocado tree for nitrate reduction on the way to protein synthesis. Molybdenum availability to roots is highest at soil pH above 6.5, and much lower below 6.5.

Chlorine (Cl)

Chlorine is required in photosystem II at concentrations of 200-400 ppm in dry matter. But, since it is very common in soil and irrigation water, it causes a yield loss in avocado of 12% for every 35.5 ppm Cl- in irrigation water.

Literature cited

Rosecrance, R., Faber, B., Lovatt, C. 2012. Patterns of Nutrient Accumulation in 'Hass' Avocado Fruit. Better Crops, Vol. 96, pp. 12-13.

Torres, M.D., Farré, J.M., Hermoso, J.M. 2002. Foliar B, Cu and Zn Applications to Hass Avocado Trees. Penetration, Translocation and Effects on Tree Growth and Cropping. Acta Hort. 594: International Symposium on Foliar Nutrition of Perennial Fruit Plants.