Prespectives of using zinc impregnated urea for improving yield and biofortification of zinc in cereals

Abstract

Zinc (Zn) is an important micronutrient required for the proper growth and development of plants, animals and humans. Its role is not ignorable. In plants, it plays a vital role in cell division, photosynthesis, enzymatic activity especially carbonic anhydrase, protein synthesis and DNA replication. In humans its importance is well reported for skin and cell division. Its deficiency among humans resulted in skin disorders, cancer, malaria, diarrhea and respiratory disorders. It acts as co-factor in more than 100 enzymes and provides protection against cancer. According to a report about 800000 deaths annually just due to Zn deficiency in poor communities. About 70% of soil in Pakistan is Zn deficient due to less organic matter, high pH, high CaCO3 contents, water logging and saline soils. Due to soil Zn deficiency and farmer’s ignorance toward Zn application in developing countries, the cereals grown on such soils are Zn deficient to fulfill the human requirement. On the other hand the cereals like rice, wheat and maize grains are major source of minerals and caloric intake especially population of rural area. Due to Zn deficient conditions (in plants and humans) bio-fortification of Zn in rice and wheat grains by proper fertilization is a dire need of the time. The new approach i.e. the solublization (with ZSB) of Zn insoluble source (ZnO) and then coating of this bio-activated ZnO on urea was hypothesized as a possible solution to enhance growth, physiology, yield and quality of the above mentioned crops. The Zn source which is mostly used in Pakistan is ZnSO4 contains 33% of Zn concentration. The applied Zn in soils is usually fixed with soil matrix only 4-5% of applied Zn becomes available for plants. ZnSO4 is considered an expensive source that’s why farmers are reluctant to use it. While the other Zn source e.g. ZnO which is cheaper source contains 80% of Zn contents but in insoluble form. The bio-activation of ZnO and then coating of this bio-activated Zn on urea could be a cost effective, eco-friendly and feasible method to improve Zn bio-availability from this source. This approach improves the Zn bioavailability. By keeping in mind the above mention issue, a series of experiments were conducted to produce and evaluate the different levels (0.5, 1 and 1.5%) of bio-activated Zn (ZnO) coated, only Zn (ZnO) coated, and Zn (ZnO) blended urea for improving growth, yield, quality and grains Zn concentration of rice and wheat crops. This bio-activated Zn (ZnO) coated urea was compared with the recommended Zn (ZnSO4) and control (no Zn). For the purpose of bio-activation pre-isolated zinc solubilizing bacterial strain (Bacillus sp. AZ6 Accessio no. KT331633) was taken from Environmental Sciences Laboratory, Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad. The characterization of the ZSB was done and observed that strain having the ability to solubilize the Zn (qualitatively and quantitatively) also contains plant growth promoting attributes such as ACC-deaminase, siderophore, auxine and pH reduction in broth culture media by organic acids production. After the conformation of above mention growth promoting attributes in strain, this strain was used for the bio-activation of Zn (ZnO) and coated this bio-activated Zn on urea at different levels as mentioned above meanwhile the other products such as Zn coated and blended urea was formed with the same levels. Bio-activated Zn (ZnO) coated, Zn blended, without bio-activated Zn coated urea, recommended Zn (ZnSO4) and ZSB were checked for the temporal release of Zn and urease activity in soil. 1.5% bio-activated Zn (ZnO) coated urea showed gradual and constant increase in Zn concentration and urease activity. The maximum increase in Zn bioavailability was observed at 60th day of experiment. Different treatments showed different response with respect to Zn release in soil. So, all treatments were evaluated under pot condition for rice and wheat crops. The pot experiments were conducted to evaluate and compare the effect of different levels of Zn coated, Zn blended and bio-activated Zn coated urea with ZnSO4 xviii and control (no Zn) on rice and wheat crops. Eleven treatments with one control were arranged according to completely randomized design. The application of 1.5% bio-activated Zn coated urea showed best results in all growth and yield parameters. On the other hand with the application of ZnSO4 improved results were observed as compare to control where no Zn was applied. 1.5% bio-activated Zn coated urea significantly improved Zn concentration in the grains of rice and wheat. The use of only ZSB improves all above mentioned parameters as compare to control. In 1.5% bio-activated Zn coated urea, almost 30-35 % increase in grains yield was observed as compare to ZnSO4. Three levels 1.5% bio-activated Zn coated urea, 1.5% Zn coated urea and 1.5% Zn blended urea were selected for further field evaluation on the basis of pot experiments results. The results of field experiments on rice and wheat again showed that best performing level was 1.5% bio-activated Zn coated urea as compared to ZnSO4 and control. Grains yield and grains Zn concentration was increased 30 to 50% and 20 to 30 % in rice and wheat crops respectively. The quality parameters like oil contents, protein and N concentration improves significantly with the application of 1.5% bio-activated Zn coated urea in both crops. The anti-nutrient e.g. phytate concentration was decreased up to 30 percent in rice and wheat grains. Soil analysis was done after crops harvesting of rice and wheat and the residual plant available Zn contents were determine and in all treatment increased concentration of plant available Zn was observed while the maximum value that was 0.82 mg kg-1 after rice and 0.9 mg kg-1 after wheat harvesting with the application of bio-activated Zn coated urea at the level of 1.5%, in rice and wheat crops the % increase in residual Zn concentration was 17.07 and 17.7% respectively as compare to recommended Zn (ZnSO4). The humans Zn deficiency fulfills in this way, in Pakistan the current status of Zn in grains 28 μg g-1 and for humans 45 μg g-1 is required, with the application 1.5% bio-activated Zn coated urea 45 and 45.16 μg g-1 concentration was achieved in rice and wheat respectively and this concentration enhanced by the constant application of this product. As a conclusion we can say that the use of 1.5% bio-activated Zn coated urea is a novel strategy to improve growth, physiology, yield, grains Zn concentration and quality of rice and wheat due to less laborious, less costly and eco-friendly approach as compare to the application of Zn and urea separately. The farmer’s ignorance can be minimized in this way. This is a way of economic supply of Zn to cereals and gets maximum benefits with less labor cost.