Plant Growth Promotion by IAA-producing Rhizobacteria in Ornamental Plant Propagation

Abstract

ABSTRACT Intensive containerized-plant production systems in the ornamental nursery industry require high levels of water and chemical inputs to meet market demands and produce high quality plants over short periods of time. Inoculants based on plant-growth promoting rhizobacteria (PGPR) are applied extensively to agricultural crops to improve plant growth and at the same time reduce chemical inputs including fertilizers and pesticides which can cause environmental degradation. However, PGPR application in the ornamental industry has not been widely studied. The overall aim of this project was to evaluate the effectiveness of PGPR on the growth and development of ornamental plants of high value to the nursery industry by selecting the most responsive PGPR and ornamental host plant combination. This project also investigated the efficacy of several different formulations of PGPR to improve root growth parameters of cuttings and investigated the development of suitable formulations for IAA-producing PGPR. The development of roots from cuttings was an effective method to measure PGP effects of PGPR in ornamental plants and for evaluating the most responsive combination of PGPR-ornamental plant. The highest IAA producer, Azospirillum brasilense Sp245 and the most responsive plant Lavandula stoechas (lavender) were selected to further evaluate the potential of PGPR in cutting propagation. When grown in sand and water media, Sp245 culture grown with tryptophan showed comparable effects to commercial rooting hormone in stimulating root growth of L. stoechas cuttings. This formulation was more effective than other Sp245 treatments and commercial biofertilizer product. There was a positive relationship between increased IAA concentration contained in the immersion solution and improved root growth parameters and this was especially apparent in sand media. Less observable PGP effects in water medium may have been a result of an IAA dilution effect. There was only a weak positive relationship observed between numbers of viable cells recovered from cuttings with root growth (only 0.6-19% variability in root growth parameters could be explained by number of viable cells recovered from cuttings). iv However, more consistent plant growth promotion was observed when cuttings were treated with IAA-producing viable cells than with cell-free extracts of IAA in the supernatant of Sp245 cultures. This indicates that improved root growth parameters were not only due to the IAA concentration alone but may be partly due continuous secretion of IAA after colonisation of the plant. Peat cultures Sp245 and freeze-dried commercial biofertilizer resulted in low IAA production and were ineffective for L. stoechas propagation. Future work on the development of inoculant technology that may be useful to the ornamental industry should focus on the minimum number of viable PGPR required as well as stimulation of IAA production.