Abstract
The sustainability of ornamental crop production is of increasing concern to both producers and consumers. As resources become more limited, it is important for greenhouse growers to reduce production inputs such as water and chemical fertilizers, without sacrificing crop quality. Plant growth promoting rhizobacteria (PGPR) can stimulate plant growth under resource-limiting conditions by enhancing tolerance to abiotic stress and increasing nutrient availability, uptake, and assimilation. PGPR are beneficial bacteria that colonize the rhizosphere, the narrow zone of soil in the vicinity of the roots that is influenced by root exudates. In this study, in vitro experiments were utilized to screen a collection of 44 Pseudomonas strains for their ability to withstand osmotic stress. A high-throughput greenhouse experiment was then utilized to evaluate selected strains for their ability to stimulate plant growth under resource-limiting conditions when applied to ornamental crop production systems. The development of a high-throughput greenhouse trial identified two pseudomonads, P. poae 29G9 and P. fluorescens 90F12-2, that increased petunia flower number and plant biomass under drought and low-nutrient conditions. These two strains were validated in a production-scale experiment to evaluate the effects on growth promotion of three economically important crops: Petunia × hybrida, Impatiens walleriana, and Viola × wittrockiana. Plants treated with the two bacteria strains had greater shoot biomass than untreated control plants when grown under low-nutrient conditions and after recovery from drought stress. Bacteria treatment resulted in increased flower numbers in drought-stressed P. hybrida and I. walleriana. In addition, bacteria-treated plants grown under low-nutrient conditions had higher leaf nutrient content compared to the untreated plants. Collectively, these results show that the combination of in vitro and greenhouse experiments can efficiently identify beneficial Pseudomonas strains that increase the quality of ornamental crops grown under resource-limiting conditions.
Highlights
Greenhouse-grown ornamentals are largely produced in containers using soilless growing mixes (Ball, 1998)
Ten microliters of the bacteria cultures were transferred to a microtiter plate prefilled with tryptic soy broth (TSB) or yeast extract mannitol broth (YEM), each amended with 30% PEG8000 (w/v)
A total of 44 Pseudomonas strains were screened for their ability to withstand osmotic stress in vitro by growing independently in Yeast Extract Mannitol (YEM) or Tryptic Soy Broth (TSB) media, both containing 30% polyethylene glycol (PEG)
Summary
Greenhouse-grown ornamentals are largely produced in containers using soilless growing mixes (Ball, 1998). Little attention has been given to the development of beneficial microbial communities within these containerized growing systems and to determining how these plant-bacteria associations can be used to improve ornamental crop quality. Plant growth promoting rhizobacteria (PGPR) comprise many taxonomic groups with diverse plant hosts (Kloepper et al, 1989) These bacteria colonize the rhizosphere, a narrow zone of soil that is associated with the roots and influenced by root exudates. In these beneficial plant-microbe interactions, the host plant secretes compounds into the rhizosphere that are used as a food source by the bacteria, which in turn stimulate plant growth and mediate stress responses through multiple mechanisms (Lugtenberg and Kamilova, 2009). PGPR can directly promote plant growth and abiotic stress tolerance by facilitating the acquisition of essential nutrients or by modulating the production of plant hormones (Goswami et al, 2016)
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