Effect of foliar and soil application of plant growth promoting bacteria on kale production and quality characteristics
Introduction
As the world population is growing rapidly, there is an urgent need for more and better quality of food. Kale (Brasicca oleracea L. var. acephala) is an important vegetable crop of the Brasicaceae family, mainly produced in North and Central Europe and North America as well as in China, Russia, Japan and the Republic of Korea (Neugart et al., 2016). One of the major advantages of kale is its high nutrition value, as it is mainly consisted of water, proteins, and low molecular weight carbohydrates (USDA, 2019). Environmentally friendly and sustainable ways for providing larger amounts of food play major role in agriculture. The use of biostimulants is a method which reduces environmental risks, offers high quality food and does not have negative effects on human (Mannino et al., 2020).
Biostimulants are substances or microorganisms that are used at plant cultivations, that are capable to enhance the nutrition content of the plants and improve the quality characteristics, while many of them also offer protection against abiotic stress, (Campobenedetto et al., 2020). According to Zhang and Schmidt (2000), they are materials that in small quantities benefit the plant growth. A major category of biostimulants are the bacteria (du Jardin, 2012; Halpern et al., 2015; Jardin, 2015). A wide variety of bacterial species have been used in agricultural experiments, including Azotobacter spp., Pseudomonas spp. and Bacillus spp. (Drobek et al., 2019; Zhao et al., 2018). Studies have shown that PGPB increase the nutrients intake and thus these nutrients can be easier utilized by plants. Moreover, plant tolerance to abiotic and biotic stress is enhanced (Jardin, 2015; Povero et al., 2016). Furthermore, PGPB are associated with the improvement of some physicochemical soil properties due to the increase of microbial activity and the growth of additional microorganisms (Calvo et al., 2014). Lastly, the use of such products leads to greater and higher quality productivity (Jardin, 2015; Van Oosten et al., 2017).
PGPB can be applied at the leaves of the cultivation, at the soil next to the planting rows or with inoculation on the seeds or on the roots of seedlings. One common application method is the foliar one, where bacteria are sprayed over the developed leaves of the plants. In a study that strain Nostoc sp. LS04 used with foliar application in lettuce plants, the results indicated that it increased shoot length, root length, fresh biomass, dry biomass, Chl a, Chl b and carotenoids compared to control, while also improved the concentrations of macro and micronutrients, and the biochemical compounds in the leaves (Silambarasan et al., 2021). The foliar application of Azotobacter Chroococcum and Azospirillum brasilence on the variety Co 86,032 Saccharum officinarum, when it was cultivated on clay-sandy soil after the 30th, 60th, 90th day of planting, provided high yield and affected positively some agronomic and morphological characteristics, such as height, weight and sugar yield (Jayanthl and Ramarethinam, 2015). In a research on winter wheat cultivation, the use of bacteria increased plant growth and the yield was 70% higher than control (Pati and Chandra, 1981).
Another important application method is the inoculation of bacteria. The inoculation of the plant growth promoting bacteria Methylobacterium komagatae (strain ZM) on the crambe seeds provided a significant yield increase (108%). Also, Bacteria Rhizobium sp. (strain 8.1.2.1), Azomonas sp. (strain 4.3.1.2) and Methylobacterium komagatae had positive effects on the root area of crambe where they provided an average of 102% greater root area compared to inoculated crambe (G. S. de Aquino et al., 2018). Application of Azotobacter Chroococcum and Azospirillum brasilence and a mix of them, was found to have positive effects on the physiological and biochemical characteristics of Mentha pulegium, while it helped them avoid the negative consequences of the lack of water (Asghari et al., 2020). A Bacillus was inoculated in a cultivation of strawberries and the results indicated that the treatments had higher yield, plant growth and chemical elements concentration (Esitken et al., 2010). Root inoculation of Azotobacter stains on maize phylogenetically related to Azotobacter chroococcum, under salinity conditions, was found to have positive effects on the chlorophyll content at zero salinity. Also, better results on the shoot length and dry weight were observed under salinity conditions (Rojas-Tapias et al., 2012).
Recent years, studies are focused to further investigate the effect of PGPB not only at plant growth and yield, but also at the quality of the final product. It is very important, the use of PGPG to have either positive effect on quality characteristics, such as color of lettuce (Moncada et al., 2020), total soluble solids of industrial tomato (Katsenios et al., 2021), total solids and crude fibers of maize (Efthimiadou et al., 2020), increase of macro and micronutrients in lettuce (Silambarasan et al., 2021), or at least to have no negative effect on them (Kopta et al., 2018).
The aim of this study was to investigate the effects of plant growth promoting bacteria (Azotobacter chroococcum, Bacillus subtilis, Priestia megaterium) and one mix of A. chroococcum with B. subtilis and the method of application (foliar or soil) of these bacteria, on kale production. Measurements of plant growth, physiology and yield of the plants were conducted, as well as quality characteristics measurements in order to investigate possible effect of PGPB on quality. The use of PGPB as biostimulants could contribute as a new and sustainable cultivation practice to increase productivity and quality of kale. Moreover, in order to implement this new cultivation technique, the application method should be evaluated, as the results of similar research present significant differences regarding the foliar application and the soil application of the microorganisms.
Section snippets
Experimental site and design
The experiment was established at Hellenic Agricultural Organization «Demeter» (ELGO) at Lycovrissi, Attica, Greece. The trial was conducted in the greenhouse of Department of Soil Science of Athens, Institute of Soil and Water Resources. The daily maximum, daily minimum and the daily average temperature are presented at Figure S1. The experiment duration was from November 2020 until April 2021. Seeds of Redbor F1 (Brassica oleraceae var. acephala) kale hybrid (Bejo Zaden B.V., Warmenhuizen,
Results and discussion
The use of plant growth promoting bacteria had a positive effect on plant growth and physiology measurements of kale, while it did not have any effect positive or negative at the quality of the harvested product.
Conclusions
The PGPB that were used as biostimulants (Azotobacter chroococcum, Bacillus subtilis, Priestia megaterium and the mix of Azotobacter chroococcum and Bacillus subtilis) enhanced plant growth and the physiology of kale, while the method of application, either foliar or soil, had also affected the response of the plants. More specifically, the results showed that nearly all PGPB used were seemed to have positive effects, with statistically significant differences, on the fresh and dry weight,
Author statement
Georgia Kordatzaki conducted the experiment and wrote the paper, Nikolaos Katsenios conducted the experiment and supervised the whole procedure, also he did the statistical analysis of the data. Marianna Giannoglou, Varvara Andreou and Sofia Chanioti did the lad analysis as well as contributed to the formal writing. George Katsaros, Dimitrios Savvas and Aspasia Efthimiadou supervised the procedure, did the formal writing, as well as edited the final paper
Declaration of Competing Interest
We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper with the title
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