The Role of Hormones in Inhibiting Wheat Growth at Increased Planting Density and Possibility of Reducing This Effect by Plant Growth-Promoting Rhizobacteria

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Abstract
High planting density is an effective strategy for increasing crop yields per unit of sown area. However, increasing the number of plants per unit area inhibits the growth of individual plants. The aim of the present work was to identify the importance of hormones in the growth response of wheat plants to the presence of neighbors and to assess possibility of reducing this growth-inhibitory effect using rhizosphere bacteria strains (Pseudomonas plecoglossicida 2,4-D and Pseudomonas veronii IB K11-1). We found that increased sowing density of wheat plants from 6 to 20 per pot elevated the level of abscisic acid (ABA) both in plants and soil solution bringing about stomatal closure and inhibition of their growth. Introduction of bacteria capable of catabolyzing ABA reduced the content of this hormone both in soil solution and plants, which led to a decrease in the degree of inhibition of wheat growth with increasing sowing density. High planting density reduced indolyl acetic acid (IAA) content in the roots accompanied by inhibition of root growth. Bacterial strains capable of producing IAA increased its concentration in the roots and their growth rate. The obtained results confirm the hypothesis about the ability of rhizosphere bacteria that destroy ABA and synthesize IAA to reduce the inhibitory effect of dense planting on plant growth.

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