Mixed rhizobia and Herbaspirillum seropedicae inoculations with humic acid-like substances improve water-stress recovery in common beans

Abstract Background Common beans are a staple food in many developing countries. However, changes in global precipitation patterns, particularly short droughts during the rainy season, have affected crop production because nodulation is greatly affected by water shortages. Plant growth-promoting bacteria, together with humic substances, can alleviate the negative effects of soil stresses, including drought. Aims This work aimed to evaluate the effects of co-inoculations of rhizobia and Herbaspirillum seropedicae in the presence of humic acid-like substances isolated from vermicompost on the recovery of common beans after a controlled water stress. Methods Two independent experiments were conducted in a greenhouse using Phaseolus vulgaris cv. Grafite and cv. Bonus, which originated in Brazil and Mozambique, respectively. The soil humidity was maintained at field capacity in pots, and the water suppression was induced at the pre-flowering stage. After 12 days, the water was restored, and the beans were evaluated. Results In the first experiment with ‘Grafite’ beans, the relative water content of the foliar disks was significantly higher in the co-inoculated treatment, as were the numbers and masses of nodules. The phenylalanine ammonia lyase activity was induced by drought, and its activity was higher in co-inoculated plant leaves. In the second experiment, the rate of net photosynthesis and stomatal conductance were also evaluated. The recuperation of common beans after water stress was improved by co-inoculation with rhizobia and H. seropedicae in the presence of humic acid-like substances. Graphical abstract The relative water content increased (RWC%) with co-inoculation