Zinc deficiencies in plants

ZINC IN PLANTS

Zinc (Zn) is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.

In plants, zinc is a key constituent of many enzymes and proteins. It plays an important role in a wide range of processes, such as growth hormone production and internode elongation.

ZINC DEFICIENCY

Zinc deficiency is probably the most common micronutrient deficiency in crops worldwide, resulting in substantial losses in crop yields and human nutritional health problems.

Deficiency in zinc might result in significant reduction in crop yields and quality. In fact, yield can even be reduced by over 20% before any visual symptoms of the deficiency occur!

The cost to the farmer, associated with loss of production, is by far higher than the cost of testing the soil and plant tissue and applying zinc fertilizers.

The mobility of zinc in plants varies, depending on its availability in the soil or growing media. When zinc availability is adequate, it is easily translocated from older to younger leaves, while when zinc is deficient, movement of zinc from older leaves to younger ones is delayed.

Therefore, zinc deficiency will initially appear in middle leaves.

Symptoms of zinc deficiency include one or some of the following:

  • Stunting – reduced height
  • Interveinal chlorosis
  • Brown spots on upper leaves
  • Distorted leaves

As mentioned above, the visual symptoms usually appear in severely affected plants. When the deficiency is marginal, crop yields can be reduced by 20% or more without any visible symptoms.

Zinc deficiency in corn Zinc deficiency in cotton

In order to identify a zinc-deficient soil, the soil and the plant should be tested and diagnosed. Without such tests, the soil might remain deficient in zinc for many years, without the farmer identifying the hidden deficiency, as visual symptoms may not occur.

Zinc deficiency is common in many crops and on a wide range of soil types. It affects the main cereal crops: rice, wheat and maize as well as different fruit crops, vegetables and other types of crops.

Soil conditions that can result in zinc deficiency include:
  • Low total zinc level in the soil (available + unavailable zinc)
  • Low organic matter content or too high organic matter content (e.g. peat soils)
  • Restricted root growth (e.g. due to hardpan, high water table etc.)
  • High soil pH
  • Calcareous soils or limed soils
  • Low soil temperature
  • Anaerobic, waterlogged conditions
  • High phosphorus level in the soil

DIAGNOSING ZINC DISORDERS

Visual observation can be a quick diagnostic tool to identify zinc deficiencies. However, it requires knowledge and expertise, as symptoms may be confusing. In addition, once visual symptoms appear, yield loss has already occurred.

Regular soil or plant testing is the best practice to determine if zinc application is required and to ensure that zinc does not accumulate in the soil to undesirable high levels.

DTPA-extraction is the most commonly used soil test to determine available zinc levels in soils.

Zinc toxicity is quite rare and under normal conditions, most soils will have either normal or deficient level of zinc.

Interpretation of zinc levels in plant tissue of various crops

CORRECTING ZINC DEFICIENCY

Zinc fertilizers can be applied to zinc-deficient soils, once deficiency is identified. The most common fertilizer sources of Zinc are Zinc chelates (contain approximately 14% zinc), Zinc Sulfate (25-36% zinc) and zinc oxide (70-80% Zinc), where Zinc Sulfate is the most commonly used source of zinc.

Foliar zinc applications – foliar applications of zinc are not as effective as soil-applied zinc. The foliar application can overcome visual symptoms but it is less effective in increasing the yield.

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Zinc And Plant Growth: What Is The Function Of Zinc In Plants

The amount of trace elements found in soil is sometimes so small that they are barely detectable, but without them, plants fail to thrive. Zinc is one of those essential trace elements. Read on to find out how to tell if your soil contains enough zinc and how to treat zinc deficiency in plants.

Zinc and Plant Growth

The function of zinc is to help the plant produce chlorophyll. Leaves discolor when the soil is deficient in zinc and plant growth is stunted. Zinc deficiency causes a type of leaf discoloration called chlorosis, which causes the tissue between the veins to turn yellow while the veins remain green. Chlorosis in zinc deficiency usually affects the base of the leaf near the stem.

Chlorosis appears on the lower leaves first, and then gradually moves up the plant. In severe cases, the upper leaves become chlorotic and the lower leaves turn brown or purple and die. When plants show symptoms this severe, it’s best to pull them up and treat the soil before replanting.

Zinc Deficiency in Plants

It’s hard to tell the difference between zinc deficiency and other trace element or micronutrient deficiencies by looking at the plant because they all have similar symptoms. The main difference is that chlorosis due to zinc deficiency begins on the lower leaves, while chlorosis due to a shortage of iron, manganese or molybdenum begins on the upper leaves.

The only way to confirm your suspicion of a zinc deficiency is to have your soil tested. Your cooperative extension agent can tell you how to collect a soil sample and where to send it for testing.

While you wait for the results of a soil test you can try a quick fix. Spray the plant with kelp extract or a micro-nutrient foliar spray that contains zinc. Don’t worry about an overdose. Plants tolerate high levels, and you’ll never see the effects of too much zinc. Foliar sprays provide zinc for plants where it is needed most and the rate at which they recover is amazing.

Foliar sprays fix the problem for the plant but they don’t fix the problem in the soil. The results of your soil test will give specific recommendations for amending the soil based on the zinc levels and the construction of your soil. This usually includes working chelated zinc into the soil. In addition to adding zinc to the soil, you should add compost or other organic matter to sandy soil to help the soil manage zinc better. Cut back on high-phosphorus fertilizers because they reduce the amount of zinc available to the plants.

The symptoms of zinc deficiency are alarming, but if you catch it early the problem is easy to fix. Once you amend the soil, it will have enough zinc to grow healthy plants for years to come.

MINI REVIEW

Strategies to increase zinc deficiency tolerance and homeostasis in plants

Ariadne Ribeiro HenriquesI; Antonio Chalfun-JuniorI, ; Mark AartsII

ILaboratório de Fisiologia Molecular de Plantas, Departamento de Biologia, Universidade Federal de Lavras, Lavras, MG, Brasil
IILaboratory of Genetics, Wageningen University, The Netherlands

ABSTRACT

Zinc deficiency is a global problem of considerable importance for agriculture and human health. Under zinc deficiency conditions, many essential zinc-dependent physiological functions are unable to operate normally, and the cellular homeostasis is adversely affected. This paper described the potential damages that low-zinc bioavailability in soil can have for plants, humans, and animals. In addition, current knowledge on physiological and molecular aspects of zinc homeostasis in plants and strategies used to increase zinc deficiency tolerance were discussed.

Keywords: biofortification, mineral nutrition, plant abiotic stress, plant nutrition

INTRODUCTION

Plants are autotrophic organisms that have the ability to use solar energy to synthesize vital components from carbon dioxide, water, and nutrients. Minerals, macro and micronutrients are essential to plant growth and development. Their absence prevents plants from completing their life cycle (Ramesh et al., 2004), due to their essential role in physiological and metabolic processes in the plant. Zinc is a vital micronutrient for all organisms. This element is involved in many reactions of the cellular metabolism, including biological processes, such as antioxidative defense, protein synthesis, carbohydrate metabolism, auxin metabolism, and stability of genetic materials (Clemens, 2006; Broadley et al., 2007). Thus, zinc deficiency stress in plants, normally due to low-zinc bioavailability in soil, causes significant decreases in the productivity and nutritional quality of food.

When facing zinc shortage, plants undergo a range of physiological and molecular adjustment in order to maintain cellular homeostasis and to avoid abrupt changes in the dynamic and complex process of development (Grusak, 2002). As the natural response mechanism to overcome a low zinc level in the cell, plants increase the expression of several genes encoding zinc transporters and metal chelator biosynthesis enzymes involved in zinc uptake from the soil (van de Mortel et al., 2006). Since there appears to be genetic variation for this ability, plant breeding and genetic engineering approaches can be used to develop new high-zinc content crop genotypes. They are expected to increase the crop production in areas with low-zinc bioavailability and alleviate human malnutrition problems due to zinc deficiency (Cakmak et al., 2010; Gómez-Galera et al., 2010).

This manuscript aimed at providing an overview about zinc deficiency problems in the world, with emphasis on soil-plant-human health interactions. We also described the current strategies that have been used to mitigate such problem.

ZINC DEFICIENCY, A GLOBAL PROBLEM

Zinc deficiency in soils is a serious global problem that affects many agricultural soils. It is estimated that about half of the cultivated soils in the world contains reduced amounts of bioavailable zinc (Figure 1). This problem is aggravated mainly in arid and semi-arid regions, due to low organic matter and soil moisture as well as high levels of pH and CaCO3 (Cakmak, 2008; Gonçalves Junior, 2010). The low availability of this metal in the soil limits zinc uptake by plants, resulting in significant decreases in both productivity and nutritional quality of food.

Reduction in both growth and in yield happens at low-zinc concentrations in the shoot (Cakmak, 2011). In it, there is a range from 30 and 100 µg zinc g-1 dry weight (DW) to be sufficient to support adequate plant growth, reaching a critical point when the Zn concentration is below 100 μg g-1of dry weight in shoot meristems of rice, due to the disintegration of the 80S ribosomes. In addition to that, the same concentration, or most likely even a higher one, is required to maintain protein synthesis in other meristematic tissues. This required amount is five to ten-fold higher when compared to mature leaves, which shows the importance of the nutrient to keep right growth (Marschner, 1995), and even for optimal growth, plants need to keep a tight control over zinc homeostasis.

Zinc is vital for the functionality of more than 300 enzymes. It can act as a functional, structural, or regulatory cofactor of a large number of enzymes. Some enzymes, such as alcohol dehydrogenase, Cu-Zn superoxide dismutase, and carbonic anhydrase, have zinc in their structure. Other enzymes, such as aldolases, enolase, isomerases, peptidases, transfosforylases and RNA and DNA polymerases, require zinc for enzymatic activity (Guerinot and Eide, 1999; Buchanan et al., 2000). The structural and functional integrity of cell membranes is also influenced by zinc, which acts in stabilization of bio-membranes by interaction with phospholipids and sulphydril groups of membrane proteins. Zinc deficiency in plants causes biochemical changes in membranes, which modify the permeability and architecture of biological membranes. The protection against the peroxidation of membrane lipids and proteins has been shown to be the major role of zinc in membranes (Marschner, 1995; Cakmak, 2011).

Delayed and reduced growth, small and malformed leaves, short internodes and yellowing effects are common symptoms of plants growing on low-zinc supply, all largely attributed to the disturbance of auxin metabolism. However, the way in which zinc deficiency affects indole-3-acetic acid (IAA) metabolism has not been clear yet. It is believed that zinc is involved in both IAA biosynthesis and in protecting IAA from oxidative degradation by reactive oxygen species – ROS (Robson, 1994; Cakmak, 2011).

As a result of its vital importance to plants, zinc deficiency affects the plant as a whole, causing serious problems in the metabolism of carbohydrates, mainly by the severe decline in photosynthesis and in sugar metabolism, and the synthesis of proteins, attributed to a sharp reduction in transcription, deformation, and reduction of ribosomes (Marenco and Lopes, 2007). The main damage of low-zinc supply on protein metabolism acts through gene expression by affecting RNA and DNA structure and the stability and function of zinc finger proteins, which are required for the expression and regulation of gene expression (Marschner, 1995).

The regions with zinc-deficient soils are strongly correlated with the ones with high incidence of human Zn deficiency. The cultivation of food crops on Zn-deficient is problematic, leading to low yields, low zinc content in food, and low income to farmers, preventing them to invest in zinc fertilizers. Especially when the crops in question constitute the main food and feed sources for the local population, dietary zinc deficiency will be inevitable in both humans and domestic animals (Erenoglu et al., 2010). Cereals-based foods are the most important source of calories in most developing countries. As an example, wheat cultivation on Zn-deficient soils in Turkey lead to the production of wheat (Triticum aestivum, L.) grains with approximately 50% reduction in zinc content compared to wheat grown with sufficient zinc supply (Cakmak, 2011).

Zinc deficiency in humans is a nutritional problem worldwide. It is estimated that one-third of the world population (around 2 billion people) suffers from mild zinc deficiency and over 450,000 children die each year due such deficiency (Welch and Grahan, 2004; Cakmak et al., 2010). According to the World Health Organization (2003), zinc deficiency in humans can result in several undesirable consequences, including diminished learning ability, impaired immune response, dysfunction of reproductive system, and reduced growth rates on infants. Thus, ensuring adequate dietary intake of zinc is essential to reduce illness and to decrease child mortality in developing countries.

STRATEGIES USED TO INCREASE ZINC DEFICIENCY TOLERANCE

The impacts caused by zinc deficiency in agriculture and in human health have been studied for decades. Many physiological mechanisms, such as root uptake and translocation, zinc sequestration in leaves, biochemical utilization of zinc and, more recently, identification and manipulation of candidate genes for proteins zinc transporters and chelator enzymes have been investigated (Hacisalihoglu and Kochian, 2003), and these findings have contributed to unraveling the molecular system of zinc homeostasis in plants.

A fast and easy solution to prevent zinc deficiency plants is Zn supplementation by applying fertilizer (Cakmak, 2008). However, there are costs associated with the application, and even though the benefits are wider, this poses a threshold for application. In addition, zinc fertilizer application effect can be impaired by physical and chemical characteristics of soil, which reduce the availability of Zn to plants, leading to a disappointing experience for farmers. An promising and cost-effective alternative strategy is improving plant zinc use efficiency and grain zinc content by plant breeding or genetic engineering. Such biofortification approach is one of the aims of the international Harvest Plus Consortium, which supports the development of new high-zinc content genotypes for increased zinc content in food and crops (Cakmak et al., 2010; Gómez-Galera et al., 2010). Although biotechnological techniques advances and significant progress in understanding structures involved in metal homeostasis have driven research in this field, still little is known about the regulators of zinc homeostasis network in plants.

ZINC HOMEOSTASIS

Zinc is a transition metal essential for terrestrial life, as already described. However, it is only required at low concentrations, making it function as a micronutrient. Concentrations between 30 and 100 µg zinc g-1 DW are enough to support adequate plant growth, whereas zinc toxicity symptoms are observed in concentrations above 300 µg g-1 DW for species that are not adapted to high-zinc exposure (Marschner, 1995; van de Mortel et al., 2006). Therefore, for optimal growth, plants need to keep tight control over zinc homeostasis.

Zinc homeostasis requires a complex network of cellular or tissue specific functions to control metal uptake, accumulation, trafficking, and detoxification. The ability to take up Zn of higher plants depends much more on its bioavailability from soil than on the absolute soil concentrations. Zinc bioavailability is modulated by various physical and chemical soil factors. Zinc solubility in soil decreases due to high levels of calcium carbonate, metal oxides, and pH and low levels of organic matter and soil moisture as well as high amounts of phosphate (Robson, 1994; Cakmak, 2011). When available in the soil solution, zinc is absorbed and transported in the divalent ion form (Zn+2) from roots to shoots through the xylem, being easily retranslocated by phloem (Clemens, 2001). This transport of ions and molecules from epidermal and cortical cell to xylem can occur through the symplastic or apoplastic route.

Following the apoplastic route, zinc and other minerals (essential and nonessential) traverse with water through cell walls and intercellular spaces outside the cell membrane to the endodermis, where Casparian strips force all solutes that need to be transported through xylem to enter endodermis cells by crossing the plasma membrane. The symplastic route consists of a continuous system of cytoplasmatic transport interconnecting cells by plasmodesmata. Upon entering the stele of the plant root, ions are transported symplastically until they reach the xylem parenchyma cells, which act in loading minerals into the xylem (Taiz and Zeiger, 2004; Broadley et al., 2007).

Regardless of the chosen path, the solutes that reach the xylem parenchyma cells are transferred to the xylem elements in a tightly-controlled process mediating membrane transport. Via the xylem sap, the minerals are released in the apoplast of the leaves and are subsequently distributed intracellularly (Clemens et al., 2002).

Zinc distribution, transport, and accumulation are affected by the level of zinc supply to the plant, as well as by mycorrhiza fungi, reducing the distance across which nutrients have to be transported in the soil (Marschner, 1995). At low or adequate zinc supply, roots, vegetative shoots, and reproductive tissues have higher zinc concentrations in young growing tissues than in mature ones. When exposed to toxic zinc levels, tolerant plants can accumulate zinc in the root cortex cells and in leaves, specifically in the cell wall or vacuoles (Robson, 1994).

GENES INVOLVED IN ZINC HOMEOSTASIS IN PLANTS

Accumulation of zinc or other metallic minerals depends on the uptake capacity and on intracellular binding sites. The metal accumulation rates are affected by the concentration and affinities of chelating molecules and by the presence and selectivity of transport activities (Clemens et al., 2002). Metal transporters are required for metal uptake and efflux or intracellular metal transport in the plant, and metal chelators contribute to metal detoxification by buffering free cytosolic metal concentrations. Therefore, both play a major role in metal homeostasis. Several studies have focused on the identification and manipulation of genes for zinc transporter proteins and chelator biosynthesizing enzymes.

The natural resistance associated macrophage protein (NRAMP) is a family of proteins whose function is to take up and transport metal. Research in Arabidopsis thaliana (L.) Heynh (Arabidopsis) showed that NRAMP transporters have limited metal specificity. AtNRAMP3 and AtNRAMP4 are localized to the vacuolar membrane and are involved in the intracellular iron transport (Thomine et al., 2000). These genes show similar expression levels in most tissues (Grotz and Guerinot, 2006), and only double mutants show a mutant phenotype, i.e., they are functionally redundantly (Palmer and Guerinot, 2003). Thomine et al. (2003) reported that AtNRAMP3 controls the accumulation of zinc and manganese in roots upon iron starvation.

Another family of transporters involved in zinc efflux is the P1B-ATPase. Arabidopsis has eight genes encoding P1B-ATPases that differ in their structure, function, and regulation (Eren and Argüello, 2004). Among these, HMA1, HMA2, HMA3, and HMA4 are involved in zinc transport (Hussain et al., 2004). AtHMA1 localizes in the chloroplast envelop and can contribute to Zn detoxification under excess zinc conditions (Kim et al, 2009). The AtHMA2 gene encodes a Zn+2-ATPase located in the plasma membrane. Expression is induced by cadmium and zinc (Eren and Argüello, 2004). The AtHMA3 protein possibly mediates zinc hyperaccumulation, since zinc hyperaccumulator species show higher expression of HMA3 than the non-hyperaccumulator ones, such as Arabidopsis (Becher et al., 2004; van de Mortel et al., 2006; Hassan and Aarts, 2011).

Recently, HMA3 has been cloned from Thlaspi carulescens Alpine Penny-cress (Ueno et al., 2011), and rice (Ueno et al., 2010), therefore it is a vacuolar influx transporter, important for cadmium tolerance in both species. This suggests that different homologs of HMA3 may have different metal-substrate specificity. AtHMA4, similar to AtHMA2 and acting alike, plays an important role in translocation of zinc, specifically in loading of zinc into the xylem (van de Mortel et al., 2006; Waters and Sankaran, 2011). Hussain et al. (2004) reported that both HMA2 and HMA4 are essential to zinc homeostasis and they show a functional redundancy.

Members of the cation diffusion facilitator (CDF) family play an important role in living organisms, as they control cation concentrations in cells through sequestration into internal compartments and efflux from cell (Gustin et al., 2011). MTP1 and MTP3 seem to be involved in the sequestration of zinc in root vacuoles and can act to limit its translocation to the shoot (Arrivault et al., 2006). In addition, when MTP1 is overexpressed in Arabidopsis, an increased resistance to zinc and higher zinc content in roots are observed (Kobae et al., 2004). MTP2 is also involved in zinc homeostasis. Under zinc deficiency, MTP2 expression increases suggesting it to play a specific function in counteracting the effect of zinc deficiency (van de Mortel et al., 2006). MTP8 is another member of the CDF family, which in addition to mediating manganese transport may also function in zinc uptake (van de Mortel et al., 2006).

The zinc regulated transporter (ZRT), iron-regulated transporter (IRT)-like protein (ZIP) family contains many members thought to be involved in transporting zinc into the cytosol across the plasma membrane, which is an important process for plant zinc uptake (Palmer and Guerinot, 2009; Song et al., 2010). ZIP transporters have eight transmembrane domains and a histidine-rich variable loop between transmembrane domains III and IV that appears to be conserved among all family members (Colangelo and Guerinot, 2004). In Arabidopsis, there are 15 ZIP gene members, ZIP1-12 and IRT1-3, of which at least ten members (ZIP1, 2, 3, 4, 5, 9, 10, 11, 12 and IRT3) appear to play a role in plant zinc uptake. Approximately half of the ZIP genes (ZIP1, 3, 4, 5, 9, 10, and IRT3) is induced in response to zinc deficiency (Wintz et al., 2003; van de Mortel et al., 2006; Assunção et al., 2010).

Metal chelators are also important for metal homeostasis. Nicotianamine (NA) is a metal-chelating compound, made by the action of NA synthase that binds zinc, as well as iron, copper, and nickel (Curie et al., 2009). NA is thought to be involved in long-distance transport, perhaps also playing a role in the entry of metals into the phloem or xylem through metal-NA chelate transporters of the Yellow Stripe-like (YSL) family (Gendre et al., 2007). NA synthase is encoded by four genes in Arabidopsis, AtNAS1-AtNAS4, which act functionally redundant, although they show different expression patterns, suggesting that each NAS gene may have a specialized function (Klatte et al., 2009). As an example, only AtNAS2 and AtNAS4 are highly expressed in roots under zinc deficiency (van de Mortel et al., 2006).

ZINC HOMEOSTASIS REGULATION

The growth and development of all organisms depend on adequate gene expression regulation. Transcription factors play an essential role in modulating gene expression by controlling transcription initiation rates. Thus, strategies that seek to modify the transcription expressions factors have shown a more efficient and promising approach than the modification of a single structural gene in Genetic Engineering (Yang et al., 2009). Currently, there are few groups focusing on this approach, however they obtained very promising results with regards to engineering environmental stress tolerance. In Arabidopsis, overexpression of the transcription factor DREB1A, driven either by a constitutive (CaMV 35S) or conditional promoter (rd29A), increases tolerance to drought, cold, and salt stress (Kasuga et al., 2004). Overexpression of an ERF transcription factor TSRF1 enhances the osmotic and drought tolerances (Quan et al., 2010).

Regarding micronutrient response, experiments carried out on rice showed that OsIRO2 overexpression, a transcription factor involved in the regulation of iron homeostasis genes in rice (Oriza sativa L.), increases iron deficiency tolerance by improving growth and yield of rice plants (Ogo et al., 2011). Recently, two transcription factors, bZIP23 and bZIP19, which are involved in the regulation of zinc deficiency response in Arabidopsis, were identified. These transcription factors recognize 8 to 10 bp palindromic motifs called zinc deficiency response elements, found in tandem in promoters of several zinc homeostasis genes, activation of which constitutes the primary response to zinc deficiency (Assunção et al., 2010). Modification of these transcription factors to control zinc deficiency tolerance and accumulation is in progress (Henriques and Aarts, unpublished results). The simultaneous expression of a set of genes implicated in the same process is a promising strategy to enhance productivity and stress tolerance in plants, when activation of multiple genes at the same time is necessary to obtain an effective stress response. Still, although the listed findings offer interesting options to try and modify plant micronutrient deficiency response, the control of post-transcriptional and -translational regulation also deserves attention.

CONCLUSIONS

In terms of understanding the roles of genes involved in uptake and translocation of zinc in plants, a lot has been achieved, but information on where in the plant each transporter functions and how each one is controlled in response to nutrient availability remains still unclear. The identification of transcription factors involved in the control of zinc deficiency response offers interesting opportunities to modulate zinc deficiency responsive gene expression to make plants less sensitive to zinc deficiency or to confer a constitutive zinc deficiency response, which can induce plants to over-accumulate metals. Therefore, understanding the regulator identity of the zinc homeostasis network in plants should provide new insights for the development of crops in areas suffering from low zinc bioavailability and for biofortification strategies. Furthermore, understanding how zinc interacts to other metals, such as cadmium and lead, during its absorption is important to be known, avoiding undesirable accumulation of such heavy metals in plants.

Acknowledgements: We would like to thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for their financial support to Ariadne Ribeiro Henriques.

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Received: 13 December 2011
Accepted: 19 June 2012

Corresponding author: [email protected]

Zinc in Soils and Plant Nutrition

The zinc (Zn) content of soils, according to rather extensive surveys, is generally in the range of 10-300 ppm. Certainly Zn, because of its concentration, can be considered as a trace element in soil. It occurs most frequently in the lithosphere as the mineral ZnS (sphalerite). Zn appears to be scattered throughout the mineral fraction of soils. It is probably held in crystal lattices, by isomorphous substitution and as occluded ions. Since it is a trace element, it is usually surrounded, by many other solid phases. Zn can also be held, by exchange sites, and adsorbed to solid surfaces. Crops differ in their sensitivity to zinc deficiency. Zn deficiencies are frequently found on soils, with restricted root zones. The movement of Zn to plant roots is dependent on the intensity factors (concentration) and on the capacity factors (ability to replenish). Increasing the pH decreases the solubility of zinc in soils, and thereby reduces the concentration, the concentration gradient, and, hence, the uptake and availability of Zn to plants. Zn plays an important role in auxin formation and in other enzyme systems. Presently, Zn is recognized as an essential component in several dehydrogenases, proteinases, and peptidases.

Abd-El-Wahab M.A. 2008. Effect of some trace elements on growth, yield and chemical constituents of Trachyspermum ammi L. (AJOWAN) plants under Sinai conditions. Research Journal of Agricultural and Biology Sciences, 4(6): 717-724.

Akhtar N., Abdul-MatinSarker M., Akhter H., Katrun- Nada M. 2009. Effect of planting time and micronutrient as zinc chloride on the growth, yield and oil content of Mentha pipeta. Bangladesh Journal of Scientific and Industrial Research, 44: 125-130; DOI: 10.3329/bjsir.v44i1.2721

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Gee, G.W., Bauder, J.W.C., 1986. Methods of soil analysis, Part1, Physical and mineralogical methods (2end Ed.). Am. Soc. Agron., USA.

Hendaway S.F., Khalid K.A. 2011. Effect of chemical and organic fertilizers on yield and essential oil of chamomile flower heads. Medicinal and Aromatic Plant Science and Biotechnology, 5 (1): 43-48.

Hoelz J., Demuth G. 1975. Influence of ecological factors on the composition of essential oil and flavonoids in Matricaria chamomilla of different origin. Planta Medica, 27: 46-52.

Kalidasu G., Sarada C., Yellamanda-Reddy T. 2008. Influence of micronutrients on growth and yield of coriander (Coriandrum sativum) in rainfed Vertisols. Journal of Spieces and Aromatic Crops, 17 (2): 187- 189.

Letchamo W. 1993. Nitrogen application effects on yield and content of active substances in chamomile genotypes. In Janick J. and Simon, E. (eds) New Crops. Willey. New York.

Marschner H. 1995. Mineral nutrient of higher plants. Edition 2, Academic Press Limited. Harcourt Brace and Company Publishers, London.

Misra A., Sharma S. 1991. Critical Zn concentration for essential oil yield and menthol concentration of Japanese mint. Fertilizer Research, 29: 261-265; DOI: 10.1007/BF01052394

Said-Al-Ahl H.A.H., Omer E.A. 2009. Effect of spraying with zinc and / or iron on growth and chemical composition of coriander (Coriandrum sativum L.) harvested at three stages of development. Journal of Medicinal Food Plants, 1: 30-46.

Towfighi H. and N. Najafi. 2001. Changes in recovery and availability of native and applied zinc in waterlogged and non- waterlogged conditions in paddy soils of north of Iran. pp. 382-384. In: Proceedings of the 7th Iranian Soil Science Congress, 26-29 August, Shahrekord University, Shahrekord, Iran.

Zehtab-Salmasi S., Heidari F., Alyari H. 2008. Effects of microelements and plant density on biomass and essential oil production of peppermint (Mentha piperita L.). Plant Science Research, 1: 24-26.

Zehtab-Salmasi S., Saeideh B., Ghassemi-Golezani K. 2012. Effects of foliar application of Fe and Zn on seed yield and mucilage content of psyllium at different Stages of maturity. Pp.63-65. In: International Conference on Environment, Agriculture and Food Sciences (ICEAFS), August 11-12, Phuket, Thailand.

Characteristics and occurrence

Zinc (Zn) deficiency may occur on acidic soils which are low in zinc, or on alkaline soils in which the solubility of zinc is reduced. Due to the declining availability of zinc with increasing soil pH, applying lime or dolomite to acid soils may induce zinc deficiency. Applications of copper fertiliser, or large amounts of phosphorus fertiliser, may also exacerbate zinc deficiency.

Crops vary considerably in their sensitivity to zinc deficiency. Sweetpotato appears to be of moderate sensitivity, and is less susceptible than, for example, cassava or citrus. These species may provide good indicators, if present in the vicinity of a sweetpotato crop suspected of zinc deficiency, as they develop clear and distinctive symptoms in situations where symptoms on sweetpotato are mild or not evident.

Sweetpotato seems to suffer most severely from zinc deficiency at early stages of establishment, and in cool weather. It is common for the crop to “grow out of it”, or to recover as the season warms. However, the effect on final yield of an early zinc stress has not been determined.

Symptoms

The most distinctive symptom of zinc deficiency in sweetpotato is a reduction in the size of young leaves. The leaves are thickened but usually not distorted, and may be as small as 1-3 cm in length. After the onset of this symptom, plant growth is severely limited. In some cultivars, internodes are also shortened, but in others this occurs to a far lesser extent than the reduction in leaf size. General chlorosis of the young leaves is usual, but may vary from mild to almost complete bleaching. Increased purple pigmentation of the shoot tips may occur in some cultivars. Characteristic changes in leaf shape are narrowing of the blade, and repositioning of the lateral lobes (if present) to point towards the leaf tip more acutely than normal.

Interveinal chlorosis on mature leaves is often the first sign of zinc deficiency, preceding obvious symptoms in the young leaves. However, in some cultivars it may not appear at all. The pattern of chlorosis is diffuse (fading gradually with distance from the veins), and minor veins retain green pigment to some extent, giving the leaf a mottled appearance.

It has been reported that the storage roots of zinc-deficient sweetpotato plants are of normal shape and size, but may display a brown discolouration of the flesh.

Possible confusion with other symptoms

The appearance of a fine interveinal mottle on mature leaves may be mistaken for early signs of potassium deficiency. On potassium-deficient plants, however, the oldest leaves are usually the most affected, and leaves do not respond to painting with zinc sulfate.

Some cases of zinc deficiency may be mistaken for iron deficiency. However a number of distinguishing features will be evident. With zinc deficiency, chlorosis on older leaves is more diffuse, while in iron deficiency, the veins contrast sharply with the interveinal tissue. In young leaves, the change in leaf shape and thickening of the leaf blade are not observed in iron deficiency, and necrosis of young leaves or the shoot tip is not typical of zinc deficiency. Again, leaf painting may be used to separate the two deficiencies.

Little leaf, or witches’ broom, is a disease of sweetpotato caused by a mycoplasma-like organism. Affected plants have small, thickened and sometimes chlorotic young leaves which may be mistaken for zinc deficiency. As it rarely affects all the plants in a plot, the observation of healthy plants adjacent to severely stunted, small-leaved plants indicates little-leaf, while zinc deficiency tends to affect adjacent plants similarly. Little-leaf also induces proliferation of side shoots from leaf axils, and latex is absent from cut stems or roots. Symptoms of little-leaf often appear or intensify following a dry period.

Diagnostic soil and plant tissue tests

Suspected cases of zinc deficiency can be confirmed by a positive response to painting the leaf surface with a solution of 0.5% zinc sulfate plus 0.25% calcium hydroxide. Normally, one half of a leaf blade is painted, so the response can be compared directly with the untreated half. After a few days, this should result in regreening of chlorotic tissue on either mature or young leaves, and may increase expansion of the treated area of young leaves. It is important to label the painted leaf clearly so that it can be identified on later inspection.

A critical leaf concentration of 11 mg Zn/kg was determined in experiments using solution culture (7th to 9th youngest leaf blades at 4 weeks). However, field data suggest that this underestimates the actual critical concentration. A concentration of 17 mg Zn/kg was associated with severe symptoms in young plants growing on granitic sand in northern Australia. These plants responded positively to leaf painting with zinc sulfate solution. Data from other sweetpotato crops in the same region suggest that the critical concentration may lie between 20 and 30 mg Zn/kg, and may increase with age of the crop. Leaf analyses from crops in a wide range of situations indicate a normal range of approximately 30 – 60 mg Zn/kg.

Various extractants have been used to estimate plant-available zinc in soils, including hydrochloric acid, dithizone and DTPA. These tests are influenced to varying degrees by the soil pH, free lime content and phosphate concentration, and reported critical concentrations for zinc deficiency in a range of crops vary widely (1.0-7.5 mg Zn/kg with hydrochloric acid, 0.3-2.3 mg Zn/kg with dithizone). An extractant containing 0.01 M EDTA and 1 M ammonium carbonate has been found to be suitable over a range of soils, including alkaline, calcareous soils (Trierweiler and Lindsay, 1969). Using this test, a critical concentration of 1.4 mg Zn/kg was determined for Zn deficiency in maize, a crop regarded as being sensitive to low Zn supply.

Management

Foliar spraying is probably the most convenient method of supplying zinc to a zinc-deficient crop, and is particularly recommended on alkaline soils, where soil-applied zinc may have low availability. Rates for sweetpotato have not been optimised, but guidance may be taken from the rate used for ginger (0.5% zinc sulfate heptahydrate with 0.25% calcium hydroxide) and cassava (1-2% zinc sulfate heptahydrate solution).

On neutral and acid soils where zinc deficiency is known to occur, soil application at or before planting is likely to be more effective than foliar sprays after crop establishment. Soil application rates of 3-10 kg Zn/ha as zinc sulfate heptahydrate (23% Zn) or zinc oxide (60-80% Zn) are typical for vegetable crops. The lesser amount may suffice on light-textured acidic soils, whereas clayey soils may require the higher amount. Zinc oxide should be broadcast and incorporated into the soil before planting. Zinc sulfate heptahydrate is more soluble, and band application at the time of planting is acceptable. Zinc applications may be effective for several years.

Prevention of zinc deficiency during establishment of cassava has been achieved by dipping the cuttings in 2-4% zinc sulfate solution for 15 minutes prior to planting. A similar strategy may be effective with sweetpotato cuttings.

Burying small pieces of scrap galvanised iron in the mound or ridge may provide an effective source of zinc to the crop. This is a particularly useful strategy where zinc fertilisers are not available or are poorly effective due to high alkalinity of the soil.

Maintenance of a high soil organic matter content increases the availability of zinc to plants.

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Chapman, H.D. 1966. Zinc. In: Chapman, H.D. (ed.) Diagnostic criteria for plants and soils. Dept of Soils and Plant Nutrition, University of California Citrus Research Centre and Agricultural Experiment Station, Riverside, California.

Landon, R.J. 1991) (ed.) Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics. Booker Tate Ltd; Longman, London.

O’Sullivan, J., Loader, L., Asher, C., Blamey, P. 1997b. Troubleshooting nutritional problems in a new industry: sweet potato in North Queensland. Proceedings of the First Australian New Crops Conference, Gatton, July 1996. Rural Industries Research and Development Corporation, Australia.

Contributed by: Jane O’Sullivan

Zinc Deficiency In Cannabis Plants

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Zinc is a mineral and one of many essential micronutrients necessary for a healthy plant diet. Despite cannabis requiring an astonishingly small quantity of it, it is nonetheless crucial for numerous physiological activities. Zinc is used by the cannabis plant to build proteins and macromolecular structures like membranes. It also fundamental to regulate enzyme function. Zinc is also co-factor of gene expression by stabilising both DNA and RNA structures. The growth hormone auxin requires zinc to operate.

So if you have ever had a cannabis clone that for some unknown reason was not up to par with her sisters, it could have been a zinc imbalance. Most of the time you will notice a deficiency rather than excess. While excess zinc certainly does harm the plant, it is a much rarer occurrence, and the plant can deal with this situation better.

Excess zinc will primarily lockout iron which is easier to spot. On much rarer occasions, zinc levels may become so high it becomes toxic. If this occurs, the cannabis plant will quickly die off. This last scenario is almost implausible to think of!

So we will focus on zinc deficiency. A direct zinc deficiency is not a common occurrence and usually a side effect from tertiary causes. More often than not, it is pretty straightforward to fix.

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HOW TO SPOT A ZINC DEFICIENCY IN THE CANNABIS PLANT

Zinc is an immobile element. This means that once deposited it can no longer be relocated to other parts where it is needed the most. When a deficiency happens, older parts of the plant cannot distribute zinc reserves from one place to another, as it can with nitrogen or phosphorous. Therefore, early signs of zinc deficiency occur at the newest growth zones, generally at the top.

You may start noticing a plant is not growing as vigorously as it should. Internodal distance is progressively shortening. New growth shoots look a little different than earlier ones, like if they were shy to open up. Shoot tips will congregate, wrinkling up close together. Once they finally do open and start to stretch out, leaves will begin to yellow from the veins out.

From here on, if the issue is not quickly addressed the results could be very damaging. The yellowing will lead to some rust-like spotting. The tips and outer margins of the leaves will start shrivelling. Clear signs of irreversible chlorosis will be present.

By this point, the leaf is completely yellow, reddish and brown – becoming crumbly and crisp. Buds will contort, start drying up and will eventually die off. This is a doomsday scenario when no corrective measures are taken. With a little knowledge and care, it is something relatively simple to deal with.

CAUSES FOR ZINC DEFICIENCY

Unless you are growing in an entirely new or unknown type of soil, the most common cause of zinc deficiency in marijuana is water pH imbalance. As pH becomes too alkaline, the roots become incapable of absorbing this mineral trace element. Other micronutrients like manganese and copper quickly become unavailable. Nitrogen and calcium start getting affected too.

If growing organically, your pH range is much more permissible than when using chemical fertilisers. Nevertheless it very important to control – something organic growers tend to disregard.

The main disadvantage of organic growing is that correcting deficiencies can take a lot more time. So the best solution is prevention. Keep monitoring your pH! Since organic growing is so permissive regarding pH, growers tend to stop checking their primary water source. In some regions, be it municipal or well water, pH can fluctuate up to 3 to 4 points within the same year.

A sudden influx of phosphorus can also cause a zinc lockout in weed. Are you a hydroponic expert grower that cannot explain why you are getting this deficiency all of a sudden? pH meters routinely calibrated, everything seems dialled in as always? It could be as simple as you having run out of nitric acid and switched over to phosphoric acid as pH-down. In excessively hard water regions, a high limestone and chalk content is calcium and magnesium rich, making for a strong pH buffer. Your once nitric acid stable system now uses significant amounts of phosphoric acid to deal with that strong alkaline water. This type of Phosphorous is readily available for intake by the plant, potentially pushing it into a zinc deficiency.

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ROAD TO RECOVERY

As a very last resort, adding any of these zinc sources can help out: zinc sulphate, chelated zinc or zinc oxides. There are numerous sources for these, depending if you want to stay completely organic or use chemical fertilisers. Keep in mind the golden rule of cannabis growing; less is more.

Tap water contains around 50% of all cannabis zinc requirements. If you know your soil is of good quality, then that rules out the most critical potential issues.

If you have just started using reverse osmosis water, beware you should be adding back what you remove. Start by adding a Cal-Mag mix to buffer your water. Check the numerous options commercially available, and you will find it is not difficult to find a good Cal-Mag product that also contains trace elements.

Once that potential cause for zinc depletion has been ruled out, you should perform a good flush. A good flush cleans out the roots of any salt build-up and removes stale water-pockets. Reset the feed, only a little lighter. Check your pH meter is calibrated and functioning well.

Because zinc is immobile, do not expect the marijuana plant to start looking better any time soon, even in hydro. In fact, once a zinc deficiency sets in and becomes visible, those affected leaves or buds will not look much better ever again. You will have to wait for new shoots to open and flourish to evaluate. This is easier during the vegetative phase when growth is explosive rather than mid to late bloom.

The trick is to learn how to spot these things as early on as possible. Keep a habit of constant monitoring the leaves from top to bottom. Anything seems out of the ordinary? The very first sign of trouble is almost always discoloration of the lush green tone.

The best growers will maintain strict diaries and perform continuous and rigorous analyses. From registering day and night temperatures, relative humidity, checking light timers, checking drip systems for clogs or efficiency of pumps, all the way to water pH – it is imperative to keep things in check regularly. Everything is of crucial importance and it all adds up in the end.

So when you encounter a growing problem, remember – it all boils down to finding the cause and not just dealing with the symptoms.

Rice (Oryza Sativa L.)

Description of Symptoms

In rice, the visible symptoms of zinc deficiency vary with soil, variety, and growth stage. Usually, the midrib at the base of the youngest leaf of zinc-deficient rice becomes chlorotic 2-4 weeks after sowing or transplanting. Brown spots then appear on the older leaves. The spots enlarge, coalesce, and give the leaves a brown color. Some varieties exhibit an yellow-orange discoloration of older leaves, spreading from the tip, instead of the brown spots. In severe deficiency, the entire leaf turns orange or brown and the rice plant dies ( Plate 1(0) and Plate 2(0)). Zinc deficiency results in stunted growth and depressed tillering. In soils with moderate zinc deficiency, plants may recover after 4-6 weeks, but maturity is delayed and yields of susceptible cultivars are reduced.

Symptoms can easily be confused with those of tungro, a disease in rice caused by virus. The disease symptoms, however, are brighter orange in color, and infestation spreads from one plant outward to other plants around it. Zinc deficiency damage is observed in the field in patches, with plants in depressions showing more severe symptoms. The patchy appearance of a zinc-deficient field often observed around six weeks after transplanting is due to the recovery of some plants and the death of others.

Soil Conditions Likely to Produce Zinc Deficiency

Zinc deficiency is the most common disorder in wetland rice soils, next to nitraogen and phosphate deficiencies. Zinc deficiency in wetland rice occurs on soils with a pH greater than 7.0, soils with low available zinc or a low total zinc content, and on soils with a high organic matter content. The following soils are likely to be deficient in zinc: calcareous soils, sodic soils, volcanic ash soils, scraped soils, sandy soils and, regardless of pH, soils which are continuously wet. Zinc deficiency is also associated with a high bicarbonate content, a magnesium to calcium ratio in soils which is greater than 1, and high levels of available phosphate and silica. The use of high levels of fertilizers, intensive cropping, the use of high-yielding varieties, prolonged submergence, and irrigation with alkaline water, all tend to induce a state of zinc deficiency in rice.

Diagnosis by Soil Analysis

The critical deficiency limit in the soil is 1 mg/kg Zn by extraction with 0.05M HCl. Zinc extracted from the soil solution of deficient paddy fields is generally in the range 0.05 – 0.25 mg/L.

Diagnosis by Plant Analysis

In the rice plant, the critical deficiency limit is 20 mg/kg zinc in the plant tissue at tillering.

Interaction with Other Elements

High rates of phosphorus application may induce zinc deficiency. So may high levels of organic matter, since organic matter inactivates soil zinc and retards its uptake by the rice. Zinc deficiency is more acute in calcareous or alkaline soils where levels of organic matter are high. The increased availability of calcium, magnesium, copper, iron and phosphate depress the absorption, of zinc by rice. Excess aluminum in acid soils decreases the concentration of zinc.

How to Correct the Deficiency

The application of 20 to 50 kg/ha of zinc sulfate to the soil is the most common way of correcting zinc deficiency. Dipping the roots of rice seedlings in a 2-4% suspension of zinc oxide before transplanting in the field is effective ( Plate 3(0)). Likewise, treating seeds with zinc materials before sowing in nursery beds or direct seeded field is recommended.

Fertilizer application

Caution should be used in applying phosphate fertilizer, as excess phosphate may aggravate the disorder. Growers should use ammonium sulfate if the rice deficiency is caused by a high soil pH.

Other cultivation practices

Thorough aeration (drying) of the soil between rice crops often alleviates zinc deficiency. Growers should also plant zinc-efficient varieties, and practice crop rotation ( Plate 4(0), Plate 5(0), and Plate 6(0)).

Examples of wrong correction

On soils which are moderately deficient in zinc, grain yields of zinc-efficient cultivars may decrease if zinc is applied because of zinc toxicity and resulting mineral imbalances in the plant. It is therefore recommended to apply zinc fertilizer to moderately deficient soils only after zinc deficiency occurs. Excessive use of phosphate fertilizers may induce zinc deficiency, or aggravate an existing problem.

Information from Dr. Corinta Quijano-Guerta, International Rice Research Institute, Philippines

Corn (Zea Mays L.)

Zinc deficiency of corn affects the development of the leaves. The leaves are stunted, a condition sometimes known as “little leaf” ( Plate 7(0)). Chlorosis is common in the interveinal areas of the leaves, which have light yellow stripes and are yellowish in color ( Plate 8(0) and Plate 9(0)). The leaves often fall prematurely.

Shortening of the stem internodes results in dwarfism of the plant. Delayed maturity, and the abnormal development of stems, decrease the yield. Under condition of extreme zinc deficiency, the corn yield is almost nil.

Excessive applications of phosphate fertilizer may induce zinc deficiency, and this is aggravated in limestone soils with a high pH (higher than 8).

A high soil pH, or a calcareous soil, means that zinc is less soluble. Corn and other crops may suffer from zinc deficiency under these soil conditions.

This tends to result in stunted growth and small younger leaves. Corn growing in zinc-deficient soils in Taiwan have small, brown spots on their leaves, and the development of the whole plant is poor.

The normal range of zinc in the soil solution extracted with water is generally in the range 0.05 – 0.25 mg/L. Levels below 0.05 mg/L are likely to lead to zinc deficiency in the crop. The level of exchangeable zinc in soil extracted with ammonium acetate appears to be in the range 0.1-2 mg/kg.

How to Correct Zinc Deficiency

The application of 30 to 50 kg/ha of zinc oxide (or 80 to 120 kg/ha of zinc sulfate) is recommended for corn production on soils with low available zinc status.

Fertilizer zinc should be mixed with the topsoil to reach the feeding roots, since zinc is very immobile, and does not spread out from the point of application.

Information from Dr. Zueng-Sang Chen, National Taiwan University

Grape (Vitis Vinifera L.)

The main symptom of zinc deficiency in grapes is that the grape bunches have grapes of uneven size ( Plate 10(0)). Small grapes are mixed with large ones in the same bunches. The smaller grapes are pale yellow in color, and have no seeds.

Soil Conditions Likely to Producd Zinc Deficiency

A high soil pH or a calcareous soil means that zinc is less soluble. Grape may suffer from zinc deficiency under these soil conditions. Excessive applications of phosphate fertilizer may induce zinc deficiency, especially in calcareous soils.

Zinc extracted from the soil solution in normal soils is generally in the range 0.05 – 0.25 mg/L. Grapes growing in soils with levels below 0.05 mg/L are likely to suffer from zinc deficiency. The level of exchangeable zinc extracted by ammonium acetate appears to be in the range 0.1 – 2 mg/kg.

The concentration of zinc in leaves is less than 15 mg Zn/kg.

Thirty to 50 kg/ha of zinc oxide (or 80 to 120 kg/ha of zinc sulfate) is recommended for grape production on soils with a low level of available zinc.

Alternatively, a foliar spray of 0.5-1.5% zinc sulfate (ZnSO4) can be applied repeatedly until the deficiency is corrected. In large grape orchards, foliar application may not be practical. Soil application (80-120 kg/ha zinc sulfate), broadcast and incorporated into the soil, may be preferable. Any fertilizer zinc applied as a dressing should be mixed with the topsoil to reach the feeding roots, as zinc is highly immobile.

Information from Dr. Zueng-Sang Chen, National Taiwan University, Taiwan ROC

Cacao (Theobroma Cacao L.)

Zinc deficiency of cacao produces symptoms in both the leaves and the bean pods. Leaves develop in a deformed rosette shape, with chlorosis of the interveinal areas, which are colored pale green or yellowish. The leaves are smaller than normal, and often fall prematurely.

Trees with zinc deficiency show delayed maturity, with few leaves and branches, which together with the abnormal development of small pods and flat beans results in low yields ( Plate 11(0), Plate 12(0) and Plate 13(0)). If the zinc deficiency is acute the yield may be almost nil. Should multiple nutrient deficiencies occur, diagnosis through symptoms can be confirmed by soil and plant tissue analysis. Moreover, certain diseases of cacao such as VSD (vascular streak die-back) have symptoms similar to those of zinc deficiency. Branches should be split and checked for a “brown streak” to see whether the problem is zinc deficiency or VSD.

Climatic Conditions Likely to Produce Zinc Deficiency

Zinc uptake and availability for plants is considered normal between temperatures of 15oC and 40oC. Zinc deficiency because of reduced plant uptake may occur if temperatures are lower than 15oC, with very limited uptake at temperatures below 5oC. Hence, zinc deficiencies are common during cool periods or wet seasons.

Zinc in soil exists in a number of different forms as part of the mineral structure, as a salt; as soluble and insoluble organic complexes. etc.

Nevertheless, zinc deficiency can be induced by soil pH conditions (lower than pH 4.5 and higher than 7.5). Zinc deficiency is found both in soils with a very high organic matter content (peat and muck soils) and soils with low level of organic matter. Zinc deficiency is also common in sandy soils, soils with very high native phosphate or excessively fertilized with phosphate fertilizers, and in poorly drained soils.

The total level of zinc in soils ranges from less than 10 to 200 mg/kg. However, the following guide could serve as a reference for DPTA-extractable zinc. A level of less than 0.2 mg/kg zinc is very low, and cacao crops are almost certainly deficient. A level of 0.2-0.5 mg/kg zinc is low, and crops are likely to be deficient; 0.2 _ 0.5 mg/kg zinc is moderate, and crops may be slightly deficient; 0.6-2 mg/kg is high, and crops have a very adequate supply of zinc. More than 2 mg/kg zinc is excessive, and crops may suffer from zinc toxicity.

Plant tissue analysis should be of samples taken from the second or third leaf (from the apical shoot) of the recently mature flush. Normally, levels of zinc in cacao leaves are 80-170 mg/kg. Levels of 20-30 mg/kg are mildly deficient, and less than 15 mg/kg is severely deficient.

Several factors should be considered in the interpretation of results. Younger leaves contain higher levels of zinc than older ones. Furthermore, the zinc concentration decreases with the age of the tree.

High levels of phosphorus (soil or fertilizer application) reduce the levels of zinc in the leaves. Likewise, excessive levels of calcium, iron and manganese tend to depress the leaf zinc concentration. Excessive aluminum is likely to have an antagonistic effect on zinc in highly acidic soils.

For seedlings, young trees and mature trees, a foliar spray of 1% zinc sulfate (ZnSO4) (23% Zn) or zinc oxide (ZnO) (0-70% Zn) is applied repeatedly until the deficiency is corrected. For larger areas, it may be easier to broadcast zinc fertilizer (10-20 kg/ha zinc sulfate) and incorporate it into the soil. Fertilizer zinc should be mixed with the topsoil to reach the feeding roots, as it is not transferred far from the point of application.

Zinc chelates applied in a band are an alternative method of Zn deficiency correction, but are not often used. They include Na2Zn-EDTA (synthetic), applied at a rate of 0.5-1 kg/ha, and Zinc polyflavonoids (natural), applied at a rate of 0.5-4 kg/ha.

Some organic fertilizers such as chicken manure contain 200-1500 mg/kg of zinc (Magat 2000). Applications of chicken manure thus not only improve the humus content and physical and biological condition of the soil, but enrich the level of zinc.

According to experience in the field (Mindanao, Philippines), mature cacao trees exhibiting low levels of leaf zinc (less than 50 mg/kg, dry matter) recovered with the application of 25 g tree of zinc sulfate. The production of normal cacao pods and beans was sustained. In fact, the application of zinc sulfate seemed to reduce the number of trees affected with VSD, as well as the number of trees with “VSD-like symptoms” which were in fact zinc-deficient.

Information from Dr. Severino S. Magat, Philippine Coconut Authority

Citrus (Citrus Senensis, Citrus Grandis)

Zinc deficiency is usually indicated in the leaves. They are deformed in their development, and the interveinal areas are pale green or yellowish ( Plate 14(0), Plate 15(0), Plate 16(0) and Plate 17(0)).

Zinc deficiencies of citrus are related to heavy precipitation.

Zinc deficiency may occur if zinc is present as part of the mineral structure, as ferromagnesium minerals (augite, biotite and hornblende). It is also common on sandy alluvial soils with a low zinc content, calcareous soils with a high pH (close to 8), and poorly aerated soils. The excessive application of N, P, and K to the soils will inhibit zinc uptake by the trees, thus inducing zinc deficiency.

Zinc extracted from the soil solution of normal soil is generally in the range 0.05 – 0.25 mg/L. The level of zinc extracted with ammonium acetate from normal soil appears to be in the range 0.1 – 2 mg/kg. Below the level of 0.05 – 1 mg/kg, symptoms of zinc deficiency are likely to appear.

In seedlings, young trees and mature trees, apply a foliar spray of 0.5-1.5% zinc sulfate repeatedly until the deficiency status has been corrected.

For large areas, a foliar application may not be practical. Thus, the soil application of 80-120 kg/ha of zinc sulfate may be preferred, broadcast and incorporated into the soil. The fertilizer zinc should be mixed into the topsoil so it reaches the feeding roots, as it is immobile from the point of application.

Information from Dr. Zueng-Sang Chen, National Taiwan University,Taiwan ROC.

Index of Images

  • Plate 1 Light Brown Discoloration of Leaves Due to Zinc Deficiency

  • Plate 2 Rice Plants Suffering from Severe Deficiency. Symptoms in the Field Are Patchy.

  • Plate 3 Amelioration of Zinc Deficiency in Rice on Left by Root Dip of Zinc Oxide (4% Zno). the Rice on the Right Has Not Had Any Zinc Treatment

  • Plate 4 Varietal Differences in Tolerance of Zinc Deficiency

  • Plate 5 Zinc Deficiency of Rice in Taiwan Is Usually Indicated by a Rusty Mottled Color on the Leaf Surface, or Small Reddish-Brown Mottles on the Leaf Surface.

  • Plate 6 Zinc Deficiency Shown by Reduced Tillering, Chlorosis in the Leaves, and Reduced Plant Size (Mature Rice Plants Are Only about 50-80 CM High).

  • Plate 7 Zinc Deficiency in Corn Plant with Stunted Leaves (&Quot;Little Leaf&Quot;)

  • Plate 8 Zinc Deficiency Indicated by Light Yellow Stripes in the Interveinal Area of Younger Leaves

  • Plate 9 Zinc Deficiency Indicated by White Stripes in the Interveinal Area of Younger Leaves When the Zinc Deficiency Is Severe

  • Plate 10 Grape Vines in Taiwan with Zinc Deficiency Usually Bear Bunches Where Grapes of Normal Size Are Mixed with Smaller Grapes. These Have No Seeds, and Are Light Yellow in Color.

  • Plate 11 Cacao Trees Deficient in Zinc Growing on Clay Soil (Eutrandept) in Mindanao, Southern Philippines

  • Plate 12 Cacao Trees Deficient in Zinc Growing on Clay Soil (Eutrandept) in Mindanao, Southern Philippines

  • Plate 13 Cacao Trees Deficient in Zinc Growing on Clay Soil (Eutrandept) in Mindanao, Southern Philippines

  • Plate 14 Leaves of Citrus Sinensis Var. Tungkan in Taiwan with Zinc Deficiency. There Is Chlorosis of the Interveinal Areas, Which Are Yellowish in Color.

  • Plate 15 Leaves of Citrus Grandis Osbeck Var. Yaotzu Showing Zinc Deficiency in Younger Leaves. the Intervein Areas Are Yellowish in Color, Although the Yeins Leaf Still Keep Their Green Color.

  • Plate 16 Leaves of Citrus Grandis Osbeck Var. Mato-Wentan with Zinc Deficiency. the Interveinal Area of Younger Leaves Is Bright Yellowish in Color.

  • Plate 17 Leaves of Citrus (Wentan) in Taiwan Showing Different Conditions of Zinc Deficiency. the Green Leaves Are Normal (Lower Right) and the White Leaves Suffer from Serious Zinc Deficiency (Upper Left).

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