Influence of Microbial Consortium in the Production of China Aster and Gaillardia Seedlings

Floriculture is an age-old farming activity in India, now emerging as an important commercial trade in agriculture. A lot of importance has been given to this sector because of its multiple uses, satisfying the aesthetic needs of the people and creating self-employment among small farmers and gardeners. In India, the floriculture industry comprises flower trade, production of nursery and potted plants, seeds and bulbs. These plants are also being used as raw materials in the manufacture of essence, perfumes, medicines and confectioneries for direct consumption. Because of the increase in demand for these plants, many new modern techniques are being adapted to improve the quality and quantity of plant resources (Kumar et al. 2012). Inoculation with beneficial microorganisms, like arbuscular mycorrhizal fungi (AMF), including nitrogen fixers, phosphate solubilizers and plant growth-promoting rhizobacteria (PGPR), can promote plant growth and shorten time to flower (Sohn et al. 2003; Desai et al. 2020).

AMF constitute a group of obligate biotrophs that form symbiotic association with 80% of plants. The influence of AM symbiosis on plant growth depends mainly on the ability of fungi to take up and transfer diffusion-limited nutrients, especially P, Zn, Cu, etc., to plant roots in exchange for plant photosynthates (Jansa et al. 2011; Bagyaraj 2018; Desai et al. 2020). AMF benefits plants in several ways: improving the nutritional status, growth and development of plants, protecting plants against root pathogens and conferring resistance to drought and soil salinity (Jyothi et al. 2018). In recent years, there has been a considerable interest in PGPR. The PGPR is another group of beneficial soil microbes that stimulate plant growth directly or indirectly. The direct mechanism involves supplying plants with nutrients and phytohormones, and indirect mechanism reduced susceptibility to disease by producing antifungal metabolites, like HCN, phenazines, pyrrolnitrin, 2,4-diacetylphloroglucinol, etc. (Das et al. 2013; Shakuntala et al. 2019). The synergistic interaction of AMF and PGPR in soil resulting in plant growth acceleration was previously described (Anuroopa et al. 2017; Behera et al. 2019).

Microbial consortium is a mixture of beneficial microorganisms that benefits plant growth (Woo & Pepe 2018). They are known to enhance germination and growth of seedlings when inoculated in multipots (pro trays) (Desai et al. 2020) and nursery beds (Cely et al. 2016; Jyothi et al. 2018). The advantages of this technology are: shorter time in the nursery, which shortens the entire production cycle, homogeneity, easy operation, possibility of mechanization, labor saving, cheaper transport and more reliable yield prediction after transplanting to the main field. Inoculation of the planting medium or seeds with a beneficial microbial consortium is a biotechnological approach for producing healthy, vigorously growing seedlings (Desai et al. 2020).

China aster [Callistephus chinensis (L.) Nees] is one of the most popular and valuable seasonal flowers, used as a cut flower as well as in landscaping. Cultivation of China aster is a profitable enterprise. The flowers produce rich nectar, which attracts insects, thus helping in pollination of many flowering plants. The compounds of the plant are of pharmacological importance, as antiallergic, antiinflammatory, antiviral, antiproliferative, and anticarcinogenic (Bhargav et al. 2018). Gaillardia or blanket flower (Gaillardia pulchella Foug.) are known for their attractive, bright-colored flowers, used for various purposes – cut flowers, floral decorations, and garlands. The leaves have medicinal properties and are used as a diuretic when taken internally to get relief from pain during urination and applied externally for treating gout disease (Kim et al. 2015). The objective of the present study was to evaluate the ability of the microbial consortium (Bacillus sonorensis + Funneliformis mosseae) in enhancing the growth of flowering plants China aster and gaillardia seedlings raised in pro trays under polyhouse condition.

The polyhouse experiment was conducted at the Centre for Natural Biological Resources and Community Development (CNBRCD), Bengaluru. The seeds of China aster and gaillardia used in the present study were obtained from the University of Agricultural Sciences, GKVK campus, Bengaluru, India.

All the plant parameters studied showed significant difference between the inoculated and uninoculated plants. For most of the parameters tested, the effect was medium to high. It was observed that shoot, root, and the whole seedling length of inoculated plants showed significant increase in China aster by 133, 96, and 111%, respectively, compared to uninoculated seedlings (Table 1). A similar trend was observed in gaillardia, where shoot, root and the whole seedling lengths increased by 57, 192, and 131%, respectively, in inoculated compared to uninoculated seedlings. The increased length of shoots, roots and cuttings due to the inoculation of AMF + PGPR has been reported previously on other plants, such as tomato and pepper grown in pro trays (Behera et al. 2019; Desai et al. 2020). Regarding stem diameter, inoculated plants showed significant increase by 58% in China aster and 68% in gaillardia seedlings, compared to uninoculated control, supporting our earlier study on flowering plants (Sukeerthi et al. 2020). The BI of the plants, which indicates the quality of seedlings, showed 270% increase in China aster and 167% increase in gaillardia compared to uninoculated seedlings (Table 1). Enhanced BI of seedlings due to AMF + PGPR inoculation has been reported earlier for French bean (Chauhan & Bagyaraj 2015) and chilly (Thilagar et al. 2016).

Seedlings inoculated with microbial consortium (B. sonorensis + F. mosseae) showed a significant increase in all the plant growth parameters. Germination rate, plant strength and SVI in inoculated treatment showed significant increase in both China aster and gaillardia seedlings compared to uninoculated control seedlings (Table 1). This indicates that the consortium helps in improving the germination of seeds and the seedling emergence (Paredes-Páliz et al. 2016). The total plant dry weight in inoculated seedlings was 140% higher in China aster and 172% higher in gaillardia compared to uninoculated seedlings (Table 1). This is in conformity with the results obtained for tomato, pepper, zinnia and balsam (Behera et al. 2019; Desai et al. 2020, Sukeerthi et al. 2020).

The content of N, P, and K macronutrients was respectively 13, 71, and 68% higher in the inoculated China aster seedlings as compared to uninoculated seedlings, and in gaillardia these increments were 6, 67, and 12%, respectively (Table 2). Micronutrients followed a similar trend. This could be mainly because of the activity of inoculated AMF and PGPR. Improved uptake of diffusion-limited nutrients like P, Zn, Cu, etc. is well documented (Bagyaraj 2014). Similar results of enhanced nutrient uptake because of dual inoculation with PGPR + AMF have been reported earlier in French bean and chilly (Chauhan & Bagyaraj 2015; Thilagar et al. 2014; Anuroopa et al. 2017). PGPR may change the morphology of roots by the phytohormones that are synthesized, which results in an increased root surface area (Chauhan et al. 2015).

In China aster and gaillardia, 99% roots were colonized by AMF, while root colonization of uninoculated seedlings was only 2%. The mycorrhizal spore numbers were 41 and 45 in the inoculated substrate of China aster and gaillardia, respectively, while uninoculated substrate did not harbor any spores (data not given). The significantly higher percent mycorrhizal root colonization and spore number in the substrate of seedlings inoculated with microbial consortium, as compared to uninoculated control, indicates the better proliferating ability of F. mosseae with China aster and gaillardia as also in other flowering plants – zinnia and balsam (Sukeerthi et al. 2020). Enhanced root colonization and spore number in the root zone because of inoculation with AMF + PGPR have been reported earlier in different crops (Anuroopa et al. 2017; Pan et al. 2020). The CFU of B. sonorensis in the root zone substrate was found to be 4 × 10⁴·g⁻¹ substrate in China aster and 3 × 10⁴·g⁻¹ substrate in gaillardia, while these bacteria were not detected in the substrate of uninoculated treatment (data not given). The presence of inoculated PGPR in the root zone at the end of seedling stage was reported earlier in other crops (Desai et al. 2020, Sukeerthi et al. 2020). Synergistic interaction between F. mosseae and B. sonorensis promoting higher plant growth compared to single inoculation with either of them has been reported earlier in chilly (Thilagar et al. 2014). Considering the results obtained for all plant parameters in our study, it can be concluded that inoculation of China aster and gaillardia with the microbial consortium (F. mosseae + B. sonorensis) resulted in healthy vigorously growing seedlings in pro trays. The hypothesis that inoculated plants will grow faster and will have better quality also during the entire production cycle remains to be verified in further experiments.