Quantitative studies of how plant growth-promoting rhizobacteria mixtures affect plant growth and health
While many studies have investigated mechanisms of plant growth promotion by plant growth-promoting rhizobacteria (PGPR) little is known about nutrient interactions with PGPR. Our first objective was to determine the effect of applying mixtures of two or three PGPR strains and experimental communities (i.e., mixture of more than 10) on plant growth and nutrient status in sunflower, as a way to achieve more consistency in plant growth. In general, plants exhibited more promotive effects in root and shoot growth with mixtures and bacterial communities than with single strains resulting in increased stem caliper, leaf area, stem length, flower diameter, and enhancement of flower quality. Across all bacterial rates greater leaf chlorophyll content and leaf area were associated with higher percentages of nitrogen in leaves.
Objective 2 analyzed root morphological changes by PGPR and the influence of different nitrogen forms on such changes. Growth enhancement was associated with changes in the root system. Detailed analysis of roots carried out by using WinRhizo software of roots exhibiting PGPR-induced growth increases revealed that morphological features such as total length, diameter, number of root tips, root surface area, and volume were promoted. Regarding the effect of nitrogen form, we found that several measurements of root morphology were enhanced with PGPR and CaNO3 because root systems of the plants had greater root weight, root length, root tip number and biomass.
Besides growth promotion, PGPR can lead to reduced pathogen infection through induction of systemic resistance (ISR) that occurs when physical and chemical processes are activated in response to inducing agents. Hence, objective 3, aimed to determine if selected PGPR applied as a foliar spray could act as biocontrol agents and then to determine if some of the control activity resulted from ISR in the pathosystems tomato—P. syringae pv. tomato and X. axonopodis pv. vesicatoria. Seeds or plants were bacterized once 1–2 weeks after transplanting. Although it is accepted that biocontrol by PGPR depends on root colonization, we found that foliar sprays of PGPR reduced disease severity of both pathogens at the same rate as treatments applied prior to seeding. In addition, disease reduction afforded by foliar PGPR treatments was similar if applied either one day or seven days before pathogen challenge, which suggests that systemic protection could be leading to a reduction in disease severity, and increased shoot weight, most likely resulting from the protection elicited by PGPR. Foliar applications of PGPR could constitute a practical way to deliver biological agents in those agricultural systems in which soil or media application is impractical and could offer a way to deliver mid-season applications through enriching the phyllosphere environment eliciting ISR. (Abstract shortened by UMI.)