Mitigation of negative effects of progressive soil salinity gradients by application of humic acids and inoculation with Pseudomonas stutzeri in a salt-tolerant and a salt-susceptible pepper

Humic acids and inoculation with the plant growth-promoting bacteria (PGPB) Pseudomonas stutzeri was used alone and combined to mitigate negative effects of progressive soil salinity gradients in a bell and a chili pepper. Plant height, length of root system, dry weight of stems, leaves and roots, number of leaves, leaf surface area, chlorophyll a and b content, total chlorophyll, and content of Na+, K+, Ca2+, and Mg2+ were measured in a salt-tolerant and a salt-susceptible pepper. We showed that applications of PGPB and humic acids did not have a clear-cut effect. Some plant parameters, such as leaf and root parameters, were positively affected at certain salinity gradients and others, such as plant height and number of leaves, did not. However, it appears that more positive effects by either treatment were more apparent in the salt-resistant cultivar. No synergism on plant growth parameters and salt mitigation was detected when humic acids and PGPB were applied together. The K+/Na+ and Ca2+/Na+ ratios showed that single applications of humic acids and the PGPB enhanced these ratios in several salinity regimes. More increases in these ratios were detected in the susceptible cultivar. In several salinity regimes, metabolic synergism, leading to enhancement of these ratios, was obtained when humic acids and the PGPB were applied together. In summary, under increased salt gradient, application of the PGPB or humic acids improved some plant growth parameters. Central to those are some improvements in the K+/Na+ and Ca2+/Na+ ratios. Combined application of PGPB and humic acids indicate a potential to use this strategy to combat salinity.