INTRODUCTION Mungbean or green gram is an important pulse crop in cereal-based cropping systems of South Asia, East Asia and South-East Asia. In India, it is an important subsistence crop adding essential nutrients to the diets especially protein, iron, zinc, phosphorus and potassium. Though, India is the largest producer, accounting for 30 percentage of the global mungbean production (Nair and Schreinemachers 2020), it still imports mungbean from other countries (Mynmar, Kenya, Mozambique, Australia and Tanzania) to meet the huge consumption demand. Indian mungbean productivity (538.65 kg ha-1 in 2020-21) is 1.34 times below the global average productivity (721 kg ha-1) (Nair and Schreinemachers 2020), which is mainly attributed to poor crop establishment (Naseem et al., 1997; Bjelica, 2016; Rahmianna et al., 2000; Ashraf and Foolad 2005). Like any other pulse crop, mungbean is cultivated in marginal lands of arid and semiarid areas, where the crop suffers from water deficit due to erratic rainfall and soil nutrients deficiencies specially of zinc. Under such situations, inclusion of seeds with high initial seedling vigour in crop cultivation leads to better crop establishment (Kumar et al., 2002). Seed being a living entity, its quality is bound to deteriorate from harvesting to till its sowing in field, if not handled properly (Jacob et al., 2016). Micronutrient seed treatments were found to be effective in enhancing early seedling vigour, crop establishment, crop growth and yield, besides overcoming the particular soil nutrient deficiency in crop (Farooq et al., 2012). Zinc seed treatment has been found to enhance the early seedling vigour; thereby crop establishment and also could overcome soil Zn deficiency resulting in higher yields and grain Zn fortification in many crops (Rehman et al., 2015, 2018a, 2018b; Rehman and Farooq 2016; Farooq et al., 2018; Ullah et al., 2019a, 2019b, 2019c). In mungbean cultivation areas, as initial crop establishment problem is also associated with soil Zn deficiencies, the present study was undertaken to standardize a suitable Zn-seed treatments that could enhance the early seedling vigour of mungbean seeds. MATERIAL AND METHODS Plant material. Seeds of mungbean variety PUSA Vishal were collected from Seed Production Unit, IARI, New Delhi for conducting the present investigation. Variety PUSA Vishal was selected because of its bold seeded nature and resistance to Mungbean Yellow Vein Mosaic Virus (MYMV). Zinc Fertilizers. The required Zn fertilizer for seed priming (ZnSO4.7H2O) was procured form Sigma-Aldrich®, USA. Zn-loaded Nano Clay Polymer Composite (Zn-NCPC), which was used for seed coating experiment was obtained from Division of Soil Science and Agricultural Chemistry, ICAR-IARI, New Delhi. Amino acid chelated Zn fertilizer and EDTA chelated Zn fertilizer, which were used for seed coating experiment were purchased from local Delhi market. Zn content in all the Zn sources used differed. It was 22.74 per cent in ZnSO4.7H2O, 10 per cent in Zn-NCPC fertilizer, 12 per cent each in Amino acid chelated Zn fertilizer and EDTA chelated Zn fertilizer. Treating seeds with Zn solution. To identify the optimum Zn concentration for mungbean seed priming, batches of 100 seeds in 3 replicates were placed in the circular shaped boxes of size 16 cm diameter and 5 cm height, which contained two layers of filter papers that were submerged with 20 ml of ZnSO4.7H2O solutions of different Zn concentration (300 ppm, 450 ppm, 600 ppm and 700 ppm) as per the treatment. Then each box was incubated in an incubator maintained at 25 ± 2°C for 9 hours duration (duration of incubation as per the standardized hydropriming treatment of lab. Data not shown). At the end of incubation duration, three boxes representing three replicates of a particular Zn concentration was taken out and seeds were dried back to original seed moisture content (9 per cent) by spreading them on the blotter paper under the fan. For hydropriming, seeds were treated in same manner using only distilled water without any Zn supplementation in water. Seeds without any treatment (hydropriming and Zn-priming) were used as control. Thus, treated seeds along with control were subjected to germination test following the method recommended by ISTA (2018) and observations were recorded at the end of germination test. Germination percentage was calculated based on number of normal seedlings as follows: Germination percentage = Ten normal seedlings were randomly selected and each seedling’s root length was measured on linear scale from the point of seed attachment to tip of the root. Similarly, shoot length of selected ten normal seedlings was measured on linear scale from the point of seed attachment to the tip of the plumule. Seedling length of the individual selected ten normal seedlings was obtained by adding the root length and shoot length of the respective seedling. The average shoot length, root length and seedling length of ten seedlings was used for the statistical analysis. After taking the individual seedling length of ten randomly selected normal seedlings, the fresh weight of all the seedlings together, was taken using the four digit analytical balance. Then, the seedlings were dried collectively overnight in an oven set at 90°C ± 2°C and the dry weight was measured using four digit analytical balance, which later was used for statistical analysis. Seed vigour index-I and seed vigour index-II were calculated using the following formula (Abdul-Baki and Anderson, 1973): Vigour index I = Average seedling length of 10 seedlings (cm) × germination percentage Vigour index II = Seedling dry weight of 10 seedlings (g) × germination percentage Coating seeds with Zn-fertilizer. In the present study we have attempted to explore the possibilities of delivering the Zn fertilizers (Zn-NCPC fertilizer, Amino acid chelated Zn fertilizer and EDTA chelated Zn fertilizer) as seed coats. Mungbean seeds were coated with respective Zn fertilizers in seed coating machine using gum Arabic as binder as per the manufacturer’s instructions manual of seed coating equipment (model ‘SATEC Concept ML 2000’ of SATEC Equipment GmbH, Germany). Around 250 g of seeds were first coated with gum Arabic solution and then coated with dry powder of Zn-NCPC fertilizer and chalk powder mixture mixed in different proportions viz., 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6 on volume basis. Similarly, another batch of 250 g seeds each, were coated using either Amino acid chelated Zn-fertilizer or EDTA chelated Zn-fertilizer and chalk powder mixture mixed in different proportions (1:1, 1:2, 1:3, 1:4, 1:5, 1:6 and 1:7 on volume basis) separately. After coating with the respective fertilizers, seeds were dried overnight under the fan and subjected to germination test as per ISTA (2018) and observations on germination percentage, seedling root length, seedling shoot length, seedling length, seedling dry weight, Seed vigour index-I and seed vigour index-II were taken as detailed above. Seeds without any coatings were used as control. Statistical analysis. All the experimental analysis were subjected to single factor ANOVA analysis of completely Randomized Design using SPSS 13 and Critical Difference (CD) values was calculated at p=0.05 to compare the difference between the treatments of the respective experiments. RESULTS AND DISCUSSION During the financial year 2018-19, India imported 0.57 million tonnes of mungbean, which accounted for 22.71 per cent of the total pulses imported (Anonymous, 2019), indicating that the mungbean domestic demand is around 24.44 per cent more than the quantity being produced. Thus, productivity deficit and consumption demand, warrants immediate strategies to increase the mungbean productivity, which is lower than global average productivity. In majority of the mungbean cultivation areas, limited crop establishment and soil Zn deficiency are the major constraints for the lower productivity. To address this, enhancing the mung bean seed quality through Zn-seed treatment is the better alternative, which addresses the problems associated with both crop establishment as well as soil Zn deficiency. Hence, in the present investigation, we standardized the Zn-seed treatments viz., Zn- priming and seed coating with different Zn-fertilizers, to identify the best Zn-seed treatment, which could be used during mungbean cultivation. The obtained results during the process are detailed below: Zn-priming. Priming of mungbean seeds with ZnSO4 had a significant effect on the seed quality parameters compared to control (Table 1). Seed priming with 450 ppm Zn at 25 ± 2 °C for 9 hours has manifested in significantly higher germination percentage (98.00), seedling length (37.76 cm), seedling dry weight (0.2786 g), seed vigour index I (3701.95) and II (26.28) when compared to the control and hydro- primed seeds. Better seedling vigour parameters in 450ppm Zn-primed seeds in comparison to control and hydro-primed seeds is attributed to the involvement of Zn in cell division, cell proliferation, protein synthesis and retaining membrane structure (Sarwar, 2011). Similar results enhanced seedling vigour parameters were reported in mungbean for Zn-seed priming (Haider et al., 2020) and Phosphorus-seed priming (Al-Salhy and Rasheed, 2020). With the increase in concentration of Zn (600 ppm and 750 ppm), there was a decrease in seedling vigour parameters, which could be attributed to the possible Zn toxicity at 600 and 750 ppm as reported in case of boron seed priming and Zn-priming (Farooq et al., 2011; Rehman et al., 2013; Haider et al., 2020). Priming the mungbean seeds at Zn concentration affects cellular antioxidant enzymes system leading to decreased seedling vigour parameters and failure of seed to germinate at extreme higher Zn concentration (Khan et al., 2021). As SVI-I and SVI-II values account for the germination s well as seedling growth potential that directly influence the crop stand establishment under field conditions, seed priming with 450 ppm Zn solution for 9 hrs. at 25 ± 2°C is best suited for getting the better crop establishment in mungbean. Seed coating. Coating of mungbean seeds with Zn-NCPC had a significant effect on the seed quality parameters compared to control (Table 2). Seed coating with Zn-NCPC (1:4) has manifested in significantly higher germination percentage (98.33), seedling length (38.18 cm), seedling dry weight (0.2864 g), seed vigour index I (3756.64) and II (28.17) when compared to the control. Germination percentage of seeds coated with Zn-NCPC (1:1) has decreased in comparison to control as number of abnormal seedlings has increased. Whereas all other seedling vigour parameters (seedling length, seedling dry weight, vigour index I and II were found on par with the control. Similarly, seeds coated with amino acid chelated Zn fertilizer (1:7) has resulted in germination percentage (95.67), seedling length (31.96 cm), seedling dry weight (0.2483 g), seed vigour index I (3060.86) and II (23.77) that were on par to the control (Table 3). The seeds coated with higher dose of Amino acid chelated Zn fertilizer (1:1, 1:2 and 1:3) has significantly reduced seed germination in comparison to the control as the number of abnormal seedlings noticed were more. This could be because of the excessive Zn concentration as well as amino acids present in the fertilizer formulation. All the treatments combinations in case of seeds coated with EDTA chelated Zn fertilizer has significantly reduced all the seedling vigour parameters in comparison to control (Table 4). Among the three fertilizers used, coating seeds with Zn-NCPC fertilizer (1:4) found to be the best as significant enhancement of early seedling vigour parameters were observed in comparison to control. Enhanced seedling vigour parameters in case of seeds coated with optimum dose of Zn-NCPC fertilizer could be attributed to the biological role played by the Zn element. At the time of seed germination and subsequent seedling growth, cell cycle, cell division, cell proliferation, cell elongation and seed reserve mobilization should happen at optimum phase and time, so that the resulting seedling will be of high vigour to surpass the suboptimal abiotic and biotic stresses. Zn being an essential structural entity of numerous enzymes; plays a crucial role in cell cycle, cell division, cell proliferation, cell metabolism, cell elongation, protein synthesis, structural integrity of cell, chlorophyll synthesis, water use efficiency, shoot and root structure, abiotic and biotic stress tolerance through suppression of Reactive Oxygen Species (ROS) and nitrogen fixation in legumes (Hassan et al., 2020; Hacisalihoglu, 2020) and seeds with higher intrinsic Zn concentration produce seedlings of high vigour than seeds of lower intrinsic Zn concentration, which facilitates in better crop establishment and in realization of better crop yields (Boonchuay et al., 2012; Cakmak, 2008).