Trichoderma and bradyrhizobia act synergistically and enhance the growth rate, biomass and photosynthetic pigments of cowpea (Vigna unguiculata) grown in controlled conditions

Abstract

Trichoderma exhibits great ecological and agricultural relevance because it improves plant growth, development and productivity. In this study, Trichoderma isolates (T01, T02, T04, T74, T76 and T96 of T. asperelloides; T77 of T. asperellum; or T44, T78 and T92 of T. harzianum) were evaluated in vitro for their ability to solubilize phosphate and produce indole-3-acetic acid (IAA). Afterwards, a completely randomized experimental design with 12 treatments was used to investigate physiological changes in cowpea plants inoculated with Bradyrhizobium sp. BR 3267 (bradyrhizobia) or coinoculated with bradyrhizobia and those Trichoderma isolates in the greenhouse conditions. All Trichoderma isolates showed the ability to solubilize phosphate and to produce IAA. Cowpeas were positively influenced by coinoculation with bradyrhizobia and Trichoderma, with the highlight being cowpea plants coinoculated with bradyrhizobia and T. asperelloides T02. These plants display significant increases in height; relative growth rate; stem diameter; dry weight of shoots, roots and nodules; total dry weight; and specific root length in relation to other symbiotic pairs and absolute control. In contrast, negative responses were registered in cowpea plants coinoculated with bradyrhizobia and T. asperelloides T04, bradyrhizobia and T. harzianum T44 or bradyrhizobia and T. harzianum T92. These plants, together with the absolute control, display lower values of stem diameter, total dry weight, specific root length, and total chlorophyll and carotenoids; the absence of root nodules; and a higher root length and anthocyanin content in relation to other treatments. Our hypothesis is that the increase in root length may be related to IAA produced by Trichoderma, while anthocyanin accumulation is associated with nitrogen deficiency, suggesting that these plants are under stress. To our knowledge, this is the first evidence of antagonistic relationships between bradyrhizobia and Trichoderma in the cowpea rhizosphere. Our findings demonstrated that Trichoderma promotes positive effects on cowpeas nodulated by bradyrhizobia and acts as stimulators of plant growth, but an adequate microbial consortium of bradyrhizobia-Trichoderma could represent a promising practical method for increasing the productivity of cowpea and other legumes.