## 4 Discussion  
Zinc, being a micronutrient, is involved in a wide variety of physiological processes [2]. It is required for optimum growth, yield, and quality of crop produce. Different approaches have been adopted to supply Zn to the plants such as soil, foliar, and soil + foliar application of chemical fertilizers, organic fertilizers, etc. The main issues with chemical fertilizers are their detrimental effects on soil ecology and lower solubility in soil, which make these approaches not feasible for poor farmers. On other hand, the use of PGPR (plant growth-promoting rhizobacteria) is an economical and environment friendly approach. The use of bio-inoculants such as bacteria could be a viable option to enhance Zn bioavailability and to achieve the objective of low input and sustainable agriculture to overcome Zn deficiency in soils and plants [42]. In soils, about 90% of Zn is present in an insoluble form, which is not available for plant uptake. On the other hand, metal salts solubilization is an important feature of so-called PGPR. Normally, PGPR mobilizes the nutrients by the secretion of organic acids, chelating agents (siderophores), and through exchange reactions [43]. Various reports have confirmed that PGPR secretes siderophores, gluconate, or the derivatives of gluconic acids, e.g., 2 and 5 ketogluconic acid, and various other organic acids for the mobilization of Zn and iron in soil [44; 45; 46; 47]. Use of Zn solubilizing PGPR to enhance the growth and yield of crop plants is expected to become an emerging trend in contemporary agriculture in the near future. In the same way, bio-activation of an insoluble source of Zn i.e., ZnO to enhance Zn bioavailability and then the coating of this bio-activated Zn (ZnO) on urea could be an effective and sustainable approach to enhance Zn bioavailability in soil and biofrification of Zn in wheat grains, thereby improving physiology and quality of crops. [PERSON] and his coworkers reported in 2013 that the major benefit of Zn coated urea is saving the amount of Zn to be applied, only 2.83 kg Zn ha\({}^{-1}\) was required with Zn coated urea as against 6 kg Zn ha\({}^{-1}\) in the case of soil + foliar application of ZnSO\({}_{4}\). The Zn coated urea is therefore a favorable fertilizer in developing countries with small landholiding farmers [48]. ZnO is sparingly soluble and not readily available to crop plants but through bio-activation and coating of ZnO at the level of 1.5%, the maximum response was recorded regarding growth, yield, and physiological parameters of wheat under both field conditions in the present study. This problem could be overcome via bio-activated Zn coated urea as it controls the formation of insoluble Zn compounds due to the presence of organic acids. In the previous studies, the effective results were achieved by the application of 2% ZnO coated urea in the wheat-rice cropping system in the present investigation, the maximum increase in most of the growth, physiological, yield, and quality parameters of wheat recorded was observed with the application of 1.5% bio-activated (Zn) ZnO coated urea. The physiological parameters such as carbonic anhydrase (CA) activity in which Zn act as a cofactor improved significantly with the application of Zn in Zn deficient soils, especially in rice-wheat cultivated areas [49]. carbonic anhydrase activity decreases in many plants under Zn deficit conditions [50]. CA activity is directly related to the Zn concentration in plants. Under severe Zn deficient conditions, no activity of CA was observed [51]. For more activity of CA in the mesophyll cells, Zn application is necessary. The activity of CA is an indicator for the levels of physiologically active Zn [50]. Due to the involvement of CA in CO\({}_{2}\) fixation during photosynthesis, Zn application significantly improved photosynthesis, which automatically increased all other growth, physiological, and yield parameters [52].