Importance of zinc in plant nutrition

Introduction

Zinc (Zn) is an vital essential mineral nutrient needed for plant growth and development. It is a component of several phyto-enzymes, and plays a key role in protein synthesis, gene expression, sugar transformation, photosynthesis pollen development, auxin metabolism, signal transduction and, membrane permeability, plays significant role in plant defense against various diseases and pests. Its deficiency in plants results in stunting, small leaves, delayed maturity, interveinal chlorosis in leaves and necrotic leaf tissue in severe cases. It also makes plants susceptible to disease and pest attack. Therefore, crop plants need adequate Zn supply for higher crop yield and better crop quality.

Soil is the prime source of Zn supply to the plants, they absorb Zn2+ion primarily from soil through their roots. Most of the plant roots present in above soil depth directly or indirectly influence phyto-availability of soil nutrients including Zn. Phyto-availability of Zn is important for plant growth and biomass production; and it is influenced by soil depth. The factors such as soil properties, soil processes and soil microbes influence phyto-availability of Zn, and these factors are also depth-dependent. This necessitates the need for studying soil to a greater depth for better understanding of phyto-availability of Zn.

Functions of zinc in plants 

Zinc is a constituent of three enzymes i.e., Carbonic anhydrase, Alcoholic dehydrogenase and Superoxide dismutase. Zinc is involved in synthesis of IAA, metabolism of GA and synthesis of RNA. Zinc plays an important role in translocation and transport of P in plants. Zinc sufficient plants contain 27 to 150 ppm Zn in mature tissues. Since it does not have variable valency, it has no role in influencing redox processes directly. 

Plant zinc deficiency symptoms

Plants containing less than 15 ppm Zn are regarded deficient in Zn. Common deficiency symptoms of Zn are :

• Interveinal chlorosis.
• Reduction in the size of young leaves.
• Bronzing, and purple, violet reddish-brown coloration of the foliage.
• Shorter internode and decrease in leaf expansion in case of dicots.

Fractions of zinc in soil 

Different fractions of Zn present in soil are 

• Water soluble (WS)
• Sorbed (SORB)
• Easily reducible Mn bound (ERMN)
• Carbonate bound (CA)
• Organic matter bound (OM)
• Fe and Al-oxides bound (feox) and
• Residual (RES)

Determination of zinc fractions and in soil available zinc  

• Water soluble Fraction of zinc: 2.5 g of soil samples and 12.5 ml distilled water added and mixed well. Samples are kept on mechanical shaker for 2 hours. After shaking, samples are centrifuged at 8000 rpm for 10 minutes. Collect the aliquot for the analysis of soluble zinc and keep the residue for next steps. 
• Absorbed fraction of zinc : In residue part 25 ml 1N Ammonium Acetate is added (1% NaCaH EDTA in 1 M NH₄ OAC) pH 8.3 and mixed well by vortex mixture. Keep the tube on mechanical shaker for 2 hours. After shaking samples will be centrifuged at 8000 rpm for 10 minutes and collect the aliquot for the analysis of Absorbed zinc in soil samples.
• Easily reducible Mn zinc: In residue part add 25 ml (0.2% Quinol in 1 M NH₄ OAC) pH 7 and mixed well by vortex mixture. Keep the tube on mechanical shaker for 2 hours. After shaking samples will be centrifuge at 8000 rpm for 10 minutes and aliquot collected for the analysis of Easily reducible Mn zinc in soil samples.
• Carbonate (CA) Zinc: In residue part add 25 ml  of 0.5 M Na acetate (pH 4.74), soak for15 hrs. and keep the tube on mechanical shaker for 3 hours. After shaking, samples are to be centrifuged at 8000 rpm for 10 minutes and aliquot collected for the analysis of Carbonate (CA) Zinc in soil samples.
• Organic matter (OM) zinc: In residue part, add 25 ml of 0.5 M Na acetate + 5 ml of 30 % H₂O₂ pH 4.74, Soak  for 15 hrs and the tube is kept on mechanical shaker for 3 hours. After shaking samples will be centrifuge at 8000 rpm for 10 minutes and aliquot collected for the analysis of Organic matter (OM) zinc in soil samples.
• Fe & Al oxide (FeOx) zinc: In residue part, add 25 ml of 0.175 M (NH₄)₂C₂O₄ 100 M H₂C₂O₄ (pH 3.25), soak for 15 hrs and keep the tube on mechanical shaker for 2 hours. After shaking samples will be centrifuged at 8000 rpm for 10 minutes and the aliquot collected for the analysis of Fe & Al oxide (FeOx) zinc in soil samples. 

Determination of available zinc  

For this purpose, 20 g of air-dried soil and 40 ml of DTPA extractant (0.005 M DTPA + 0.01 M CaCl₂.2H₂O + 0.1 M triethanolamine (TEA) solution adjusted at pH 7.3 (soil: extractant: 1:2) are taken in 250 ml conical flask and shaken for 2 hours in a reciprocating shaker at 25°C. Then the solution is filtered and the filtrate is analyzed for Zn using atomic absorption spectrometer (AAS).

Contributors : Akanksha Sikarwar, Rahul Mishra, Seema Bhardwaj, Vimal Shukla, Mayank Vyas, Rahul Prajapati and Yogesh Sikaniya
                       Indian Institute of Soil Science, Bhopal (Madhya Pradesh)