Vesicular-arbuscular mycorrhizae (VAM) improve phosphorus and zinc nutrition and growth of pigeonpea in a Vertisol

Abstract

In a glasshouse trial, pigeonpea (Cajanus cajan) was grown in a Vertisol from the Darling Downs, Qld. The experimental design included two rates of inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi (nil and inoculated), three rates of phosphorus (P) application and two of zinc (Zn), and inoculation (nil and inoculated) with a recently discovered pathogen of pigeonpea, Phytophthora drechsleri Tucker (Pdr). lnoculation with the pathogen was included in the factorial design to investigate any effect of VAM on root rot. Plants responded to inoculation with VAM fungi, showing that the growth of pigeonpea is highly dependent upon mycorrhizal colonization of its root system. The mycorrhizal plants yielded, on average, 3.3 times the dry weight of the non-mycorrhizal plants. Shoot dry weight and Zn concentration of the shoots were both exponentially related to per cent VAM colonization of the root length ( R2 values of 0.904 and 0.644 respectively) and P concentration was linearly related to VAM colonization (R2 = 0.888). VAM increased P concentration, P uptake, Zn concentration, Zn uptake and P/Zn ratio, indicating enhanced growth through improved P and to a lesser extent Zn nutrition. Zinc fertilizer (15 mg kg-1 soil) without Phytophthora inoculation was fungitoxic to the mycorrhizae, decreasing per cent VAM colonization and depressing plant growth. lnoculation with Pdr did not result in infection and root rot symptoms. However, it did overcome the Zn toxicity, possibly through organic chelation effects, and thereby enhanced VAM colonization. Greatest VAM colonization and best plant nutrition and growth were obtained with the combination of VAM inoculation, Pdr inoculation, Zn (15 mg kg-1) and P (10 mg kg-1). Our results indicate the importance of maintaining adequate levels of VAM fungi in soil through appropriate agricultural practices in order to maximize pigeonpea growth.