University of Agriculture, Faisalabad
Approximately 1/3 of total world’s population has become victim of Zn malnutrition which poses serious health hazards to human and causes thousands of deaths, annually. 37% population of Pakistan is suffering from Zn malnutrition. In Pakistan, micronutrient deficiency in plants is due to arid to semiarid climate, high temperature and alkaline soils with low organic matter. Some other reasons for the malnutrition of micronutrients are use of high yielding crops, high cropping intensity, imperfect cover of crop residues and excessive use of synthetic fertilizers.
Zinc deficiency makes plant susceptible to light, fungal infections and heat. It also affects pollen formation, water uptake and transport, root development and grain yield. Specifically, in plants, zinc is involved in carbohydrate metabolism, auxin metabolism. Plants absorb zinc as divalent cation but only insignificant amount of total soil zinc is available in soil solution for plants uptake. Remaining portion of total soil zinc is in the form of insoluble compounds and minerals.
Deficiency symptoms of zinc differ among species of plants but most of the symptoms are common. Zinc is an immobile nutrient, so deficiency symptoms mainly appear on the young leaves. The newly emerging leaves are smaller with close and curling margins. Leaves turn yellow and commonly interveinal area but main veins of the leaf remain green. With the passage of time, chlorotic spots change to yellowish-brown color and necrotic spots begin to form from the edges. Zinc deficient plants have short internodes.
Nitrogenous fertilizer damages the soil, ground water and environment. The synthetic fertilizers must be substituted with natural fertilizers to conserve the environment. Organic fertilizers improve soil structure, soil health, water holding capacity, cation exchange capacity, nutrient uptake efficiency and microbial population.
There are various method to alleviate zinc deficiency. Zinc fertilizers used as zinc sulfate or Zn-EDTA but their utilization impose an environmental and economical pressure and these fertilizers are converted into insoluble complex forms within 7 days of application. In various areas, intercropping and crop rotation have been used to enhance uptake of zinc by plants. Further methods are conventional breeding, transgenic approaches and genetic engineering. However, these methods are high priced, difficult and time consuming. A superior alternative to all these approaches is the use of zinc solubilizing bacteria.
In this decade, bio-fertilizers proved to be a key element of integrated nutrient management in agriculture. Zinc-solubilizing microorganisms make available zinc from different organic and inorganic pools of total soil zinc and these can be efficiently utilized to enhance zinc availability to plants. However, in the literature, few bacterial species of the genera Acinetobacter, Gluconacetobacter, Bacillus and Pseudomonas have been reported. Zn application alone or in collaboration with the biocontrol agent Pseudomonas aeruginosa notably decreased the invasion of the root knot nematode Meloidogyne javanica in tomato. Species of Meloidogyne are considered as one of the most detrimental nematode pests of sugarcane, reduced the cane yield by 9– 15 t/ ha.
There are various mechanisms by which zinc solubilizing microorganisms solubilize zinc and acidification is one of those. In soil, zinc solubilizing microbes yield organic acids which sequester the zinc cation and lower the pH of closely associated soil. The anions can also enhance zinc solubility by chelating zinc. Other mechanisms include yielding of siderophore and proton, chelated ligands and oxido-reductive systems on cell membranes. Dhaked et al. (2017) revealed that the solubilization zone for zinc oxide is varied from 16mm to 6mm and maximum solubilization zone showed by isolate ZnSB-3 and minimum solubilization by ZnSB-7.
Zinc solubilizing bio-fertilizers control diseases, activate hormones, enhance crop yield as well as its quality, improve plant growth, raise photosynthetic activity, absorb residues of pesticides and heavy metal, make plant able to tolerate stresses, fulfill the requirement of zinc in the economic part of plant, reduce the use of synthetic zinc fertilizers, improve soil fertility.
Soil may possess sufficient amount of zinc but, in some cases, it may not be accessible to plant because of its fixation. To avoid from this situation, zinc solubilizing bacteria should be used. These microbes provide native zinc to plant to fulfill nutritional gap (zinc deficiency) in human diet as well as saving of additional cost of production. Zinc solubilizing bio-fertilizer also conserve soil fertility and environment.