Enhancing growth and graft success of shea (Vitellaria paradoxa C.F. Gaertn) through targeted inoculation with indigenous arbuscular mycorrhizal fungi

Abstract

Vitellaria paradoxa is a keystone species in West African agroforestry systems, but its regeneration is constrained by slow juvenile growth and low graft success. Arbuscular mycorrhizal fungi (AMF) can enhance seedling vigor, yet their contribution to the production of graftable rootstocks remains poorly documented. This study evaluated six indigenous AMF-containing inocula—M1 ( Vigna unguiculata rhizosphere), M2 ( Sclerocarya birrea rhizosphere), M3–M5 ( Vitellaria paradoxa rhizosphere), and M6 ( Senegalia macrostachya rhizosphere)—for their effects on mycorrhizal colonization, early growth and graft success of V. paradoxa under nursery conditions. Seedlings were amended with one of six indigenous AMF-containing inocula or maintained as a non-amended background-soil control. Because the nursery substrate was not sterilized, the control was not considered to be a non-mycorrhizal treatment. Mycorrhizal colonization was assessed at 9 and 16 months after inoculation, growth parameters were measured after 9 months, and plants were grafted using terminal cleft grafting. Graft success was evaluated 90 days after grafting. Mycorrhizal colonization differed among inocula, with M3 and M2 showing the highest colonization frequency and intensity at 16 months. Seedling height and collar diameter also varied among treatments, with M1 and M3 producing the largest rootstocks. Graft success was highest in M3-amended plants and lowest in the non-amended background-soil control. However, these differences are interpreted as treatment-associated responses under nursery conditions rather than definitive evidence of AMF-specific causal effects. Logistic analysis indicated that graft success was primarily associated with treatment identity, whereas collar diameter was not an independent predictor after treatment effects were accounted for. These findings suggest that selected indigenous AMF-containing inocula may improve the production of graftable shea rootstocks in this nursery system. However, because the inocula consisted of mixed biological materials, the substrate contained background microbial communities, and retrospective spore counts may not fully reflect infective propagule potential at inoculation, further studies using molecularly characterized inocula, standardized infective propagule potential and multi-site validation are required before broad operational recommendations can be made.