Several hypotheses have been put forward for the mechanisms of plant growth promotion by PGPF, including (a) hormone production, (b) substrate degradation (mineralization), and (c) suppression of deleterious microorganisms.
Culture filtrates of certain fungal species promote plant growth, due to the production of plant growth hormones by these fungi (Ram 1959). Growth promotion has been seen in some plants after treatment with mycelial exudates from PGPF strains of Trichoderma and SRF, and a gibberellin-like substance was reported to be involved (Gillespie-Sasse et al. 1991; Windham et al. 1986). Some strains of Phoma species have been found to produce abscisic acid, and this compound is also reported to promote plant growth. However, in general terms, these appear to have little relationship between the production of plant growth hormones and the ability of PGPF to promote plant growth.
Close relationships have been shown between the reduction of barley grain weight due to PGPF, the subsequent growth promotion effect of PGPF, and their cellulase and starch degration activity (Hyakumachi 2000). Production of NH4-N and NO3-N in soil can also be increased by amendment with PGPF-infested barley grains.
The total amount of nitrogen in PGPF infected-barley grains remains the same despite which PGPF isolate is used, however, the amount of NH4-N varies depending on the isolate, with the highest level being seen in grains infected with Phoma. The NH4-N levels later decreased in the order Phoma > Fusarium > Penicillium > Trichoderma > control. The amount of NH4-N was about 7.8 times higher in Phoma infested-barley grains than that of control (Hyakumachi 2000). Hyakumachi (2000) also demonstrated correlations between reduction of barley grain weight and cellulase activity, starch degradation activity of starch, and the dry weight of bentgrass. These results suggest that the mineralization of organic substrates by PGPF relates to the plant-growth promoting effect of those PGPF. The PGPF may therefore provide the plant with necessary mineral nutrients in an easily assimilating form.
A remarkable plant growth promotion effect has been reported for field-grown cucumbers, and this was attributed to the suppression of indigenous pathogenic Pythium spp. in the soil by PGPF (Hyakumachi 1994). The suppression of deleterious microorganisms by PGPF may therefore be one of the mechanisms of plant-growth promotion.
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