Utilization of plant growth promoting rhizobacteria as root dipping of seedlings for improving bulb yield and curtailing mineral fertilizer use in onion under field conditions
Introduction
Onion (Allium cepa L.), a member of family Alliaceae, is reported to be grown in about 175 countries of the world (Barakade et al., 2011). The root system of onion is shallow and has low absorbing and penetrating abilities (Colo et al., 2014). Therefore, it is highly responsive to application of mineral fertilizers containing nitrogen, phosphorus and potassium (Devi and Ado, 2005). As per an estimate, approximately 120 kg nitrogen, 50 kg phosphorus and 160 kg potassium per hectare were removed from an onion crop that yielded 35 tons per hectare (Tandon, 1987). Injudicious use of mineral fertilizers alone to enhance crop yield may contaminate underground water, degrade soil fertility in the long run (Saxena et al., 2013), reduce the storage life of bulbs (Sankar et al., 2009), enhances cost of crop production thereby reducing profits to growers. Therefore, it is desirable to develop production technologies which may curtail mineral fertilizer use without any loss in bulb yield and improve soil health for sustainable agriculture.
The utilization of plant growth promoting rhizobacteria (PGPR) in combination with mineral fertilizers is a viable alternative in this direction as they contribute towards promotion of plant growth and yield of different crops (Saxena et al., 2013; Colo et al., 2014; Thilagar et al., 2016). The term ‘PGPR’ was coined by Kloepper and Schroth (1979) and refers to a wide variety of soil bacteria which when grown in association with a host plant result in growth stimulation of the host. These PGPRs affect plant growth directly or indirectly by increasing nutrient availability and uptake by plants, bringing hormonal changes in plant, emitting volatile organic compounds and imparting biotic and abiotic stress tolerance in plants (Ruzzi and Aroca, 2015). The genera of PGPRs include Azotobacter, Bacillus, Pseudomonas, Azospirillum, Rhizobium, Burkholderia, Sphingobacterium, Agrobacterium, Xanthomonas etc. (Bishnoi, 2015). Azotobacter species are gram-negative, heterotrophic, free living bacteria. They are mainly responsible for nitrogen fixation but also have the ability to produce various plant growth hormones like indole acetic acid (IAA), gibberellins and cytokinins, and antifungal compounds to fight against many plant pathogens (El-Komy, 2004). Bacillus species fall in the category of phosphate-solubilizing bacteria (PSB) and are also reported to produce phytohormones like IAA, cytokinins and gibberellins (Bishnoi, 2015). Besides, many species are also effective for the control of plant diseases caused by soil borne, foliar and post-harvest fungal pathogens (Saxena et al., 2013). Among several PGPR strains, Bacillus-based products have gained momentum for commercialization since Bacillus species produce endospores which can tolerate extremes of conditions such as temperature, pH and exposure to pesticides and fertilizers (Bishnoi, 2015). Burkholderiaspecies are gram-negative, obligate, aerobic, free living bacteria in the rhizosphere. They improve nitrogen fixation, enhance disease resistance and overall host adaptation to environmental stresses (Balandreau and Mavingui, 2007). They are reported to improve the growth and yield of amaranth plants by fixing N2 and increasing the expression of the nitrate transporter NRT1 (Parra-Cota et al., 2014). Sphingobacterium has been isolated and characterized for K-solubilization under in vitro and in vivo conditions (Suman et al., 2016). Besides, Sphingobacterium species produce siderophores (Tian et al., 2009) which bind to available form of iron in the soil by rendering it unavailable to the plant pathogens (Ahmad et al., 2008). Besides, these PGPRs have been reported to reduce mineral fertilizer use in tomato (Adesemoye et al., 2009), cucumber (Saeed et al., 2015) and chilli (Thilagar et al., 2015) without any significant reduction in yield.
There are a large number of reports on the beneficial effects of these PGPRs in onion (Sankar et al., 2009; Kumar and Shivay, 2010; Yeptho et al., 2012; Colo et al., 2014; Thangasamy and Lawande, 2015). However, the influence of PGPRs varies from site to site and year to year. This may be due to the reason that the inoculated bacteria will have to compete with the often better adapted native soil microflora for nutrients and space (Colo et al., 2014; Bishnoi, 2015). To achieve the maximum beneficial effect of these rhizobacteria, the proper PGPR strain needs to be selected in each soil-plant-PGPR system and the mode of inoculation must be optimized in both greenhouse and open-field experiments (Ruzzi and Aroca, 2015). The previous researchers have used PGPRs in onion as seed treatment (Colo et al., 2014), soil application (Sankar et al., 2009; Thangasamy and Lawande, 2015), or as dipping of bulblets to be used as planting material (Yeptho et al., 2012). There has been a dearth of literature on the utilization of various PGPRs as root dip of seedlings in onion.
Therefore, the present investigations were undertaken to study the effect of six PGPR combinations applied as root dip of seedlings, two levels of mineral fertilizers (100% NPK and 75% NPK) and their interaction on plant growth, bulb yield, bulb quality, physiological weight loss in storage, and soil microbial and chemical properties. The other aims were to identify PGPR cultures that can act as potential supplements to recommended dose of mineral fertilizers (100% NPK) in improving bulb yield of onion and to identify PGPR cultures that can reduce the use of mineral fertilizers in onion by 25% without any loss in bulb yield.
Section snippets
Experimental site
The field trials were conducted during rabi seasons of 2016−17 and 2017−18 at Jodhpur Romana Farm, Regional Research Station, Punjab Agricultural University, Bathinda, India (30° 9´ 36″ North latitude, 74° 55´ 28″ East longitude and 211 m altitude). The experimental site is characterized by semi-arid climate with an average annual rainfall of 421 mm. The weekly agro-meteorological observations were recorded during the crop season of 2017 and 2018 (Supplementary Tables S1 and S2). The soil
Plant growth parameters
Irrespective of the PGPR treatment, the levels of mineral fertilizers significantly (P ≤ 0.05) influenced the plant height (at 90 DAT) and number of leaves (at 60 DAT) during both the years but exerted non-significant effects on plant height (at 60 DAT) during 2016−17 and number of leaves (at 90 DAT) during both the years (Supplementary Table S3). Significantly (P ≤ 0.05) higher plant height (at 90 DAT) (41.54 cm and 41.21 cm) and number of leaves (at 60 DAT) (5.54 and 5.47) were registered
Discussion
The present study was conducted to assess the effect of six PGPR combinations, two levels of mineral fertilizers (100% NPK and 75% NPK) and their interaction on plant growth, bulb yield, bulb quality, physiological weight loss in storage, and soil microbial and chemical properties. Here, the minimum plant height and number of leaves at both 60 and 90 DAT were recorded in uninoculated control (T0). The integrated use of PGPR treatments (T1 to T6) along with mineral fertilizers recorded low to
Conclusions
The integrated use of PGPRs along with mineral fertilizers in onion resulted in improved plant growth, bulb yield and bulb quality parameters except ash content, and also reduced the physiological weight loss of bulbs in storage. The triple inoculation treatment T6 (Azotobacter sp. + Sphingobacterium sp. + Burkholderia sp.) gave the best results followed by dual inoculation treatment T1 (Azotobacter sp. + Bacillus sp.). The interaction effects between mineral fertilizers and PGPRs were
CRediT authorship contribution statement
Diksha Tinna: Investigation, Data curation, Formal analysis. Naveen Garg: Conceptualization, Formal analysis, Supervision, Project administration, Writing - original draft. Sandeep Sharma: Supervision, Resources. Gulab Pandove: Conceptualization, Supervision, Resources, Visualization. Neena Chawla: Supervision, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Editor and two anonymous reviewers for their constructive comments which were helpful in bringing necessary improvement in the manuscript. This research work was funded by the Punjab Agricultural University, Ludhiana, India, as a part of the scheme, "Establishment of Regional Station, Bathinda RES 58 (PC-1068.2)".
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