New strains of plant growth-promoting rhizobacteria in combinations with humic acid to enhance squash growth under saline stress

The study aims at assessing the potentials of some new salt-tolerant isolates as plant growth-promoting rhizobacteria (PGPR) under saline condition. Three of the 165 isolates that grew on the presence of 2-20% NaCl were high salt-tolerant and had many features of PGPR. They were identified by using 16S rRNA gene sequencing. The nearest species to our isolates were Paenibacillus polymyxa (GQ375783.1), Ochrobactrum intermedium (MG309678.1) and Enterobacter cloacae (MG309676.1) with nucleotide similarity 99, 97 and 99%, respectively. In 2017, a greenhouse trial was carried out to assess the efficiency of these novice isolates combined with humic acid and doses of inorganic fertilizers on squash (Cucurbita pepo L.) growth and productivity. Data showed that fertilizing the soil with a full dose of chemical fertilizers only lead to decrease the values of dehydrogenase, alkaline phosphatase and nitrogenase activity at all determination periods. While, soil inoculation with PGPR strains combined with NPK 50% and humic acid spraying gave the higher records of all enzymes activity. Moreover, data showed that the highest values of peroxidase and polyphenol oxidase activity were observed in squash that sprayed with humic acid and inoculated with salt-tolerant PGPR strains combined with half dose of mineral-NPK. Generally, inoculating squash with salt-tolerant PGPR strains has a positive effect on nutrients uptake, growth characteristics and yield and yield components as well as fruits quality. So, it could be recommended as biofertilizers to promote plant growth, increase crop production under salinity condition, decrease production costs and reduce pollution