Beneficial Effects of Azospirillum brasilense and Paenibacillus dendritiformis on Melon Under Salinity Stress

Soil salinity is a major environmental stress that limits plant growth and yield worldwide. In the present study we assessed the response of different varieties of melon to individual or combined inoculation of two plant growth promoting rhizobacteria, Azospirillum brasilense (strain Sp7) and Paenibacillus dendritiformis (strain T), under saline conditions. Experiments were performed in vitro (in filter paper) as well as in the greenhouse and in the field. In vitro and greenhouse experiments revealed strong inhibition effects of saline stress on plant growth. However, bacterial treatments, and particularly the co-inoculation treatment with strains Sp7 and T, were able to significantly attenuate this stress. Positive effects on root and shoot parameters were consistently observed in three different varieties of Ananas-type melon varieties, Ophir (Zeraim Gedera), Raymond (Hazera Genetics) and OR-AN305 (Origene Seeds). Efficient colonization of melon roots by both strains was confirmed by classical bacterial counts as well as by quantitative real time-PCR. A field experiment with variety OR-AN305 was carried out at the Zohar agricultural station (Central and Northern Arava-Tamar R&D) during Fall 2017. The experiment consisted of twenty plots per treatment, with 10 plants per plot. Plants were irrigated with local, saline irrigation (EC~3.5). The treatments included the inoculation of melon plants twice with individual strains or with combined inoculation: at sowing in the nursery and before planting in the field. Controls were non-inoculated plants. The three bacterial treatments enhanced melon yield relative to non-inoculated controls by 11 to 15%. Positive effects of bacterial treatments were also observed on melon average size. Overall, findings from this study support that combined inoculation of A. brasilense and P. dendritiformis has the potential to enhance plant growth and yield of melon, especially under saline irrigation.