From the Lab to the Field: Combined Application of Plant-Growth-Promoting Bacteria for Mitigation of Salinity Stress in Melon Plants

Soil salinization is a major and increasing problem adversely impacting plant growth and crop production. Accordingly, coping with this problem has become a central topic in agriculture. In this study, we address this issue by evaluating the potential effectiveness of two bacterial spe-cies, Azospirillum brasilense and Paenibacillus dendritiformis, in enhancing growth and yield of melon and tomato plants under salinity stress. In vitro laboratory experiments indicated that these bacteria can efficiently colonize plant roots, and increase root length (25–33%) and root biomass (46–210%) of three melon plant varieties under saline stress. Similarly, greenhouse experiments showed that these bacteria significantly induced root (78–102%) and shoot weights (37–57%) of the three melon varieties irrigated with saline water. Tomato plants grown under the same conditions did not ex-hibit growth deficiency upon exposure to the saline stress and their growth was not enhanced in response to bacterial inoculation. Interestingly, saline-stressed melon plants inoculated with P. dendritiformis and A. brasilense exhibited lower total antioxidant activity compared to un-inoculated plants (80% vs. 60% of DPPH radical scavenging activity, respectively), suggesting that the inoculated plants experienced lower stress levels. These positive effects were further manifested by an increase of 16% in the crop yield of melon plants grown in the field under standard agricultural fertilization practices, but irrigated with saline water. Overall, these results demonstrate the beneficial effects of two plant-growth-promoting rhizobacteria, which can significantly alleviate the negative outcome of salt stress.