Effects of a network of mycorrhizae on capsicum (Capsicum annuum L.) grown in the field with five rates of applied phosphorus

Abstract

This field trial determined the importance of mycorrhizae[Acaulospora mellea Spain & Schenck,Gigaspora margarita Becker & Hall,Glomus clarum Nicolson & Schenck,Glomus etunicatum Becker & Gerdemann, andScutellospora pellucida (Nicol. & Schenck) Walker & Sanders], established from an existing intact network ofvesicular-arbuscular mycorrhizae (VAM) propagules, for production of capsicum(Capsicum annuum L. cv. Target) in a low phosphorus(≤14 mg NaHCO₃-extractable P/kg) Typic Paleudalf. The mycorrhizalnetwork was formed previously with sweetcorn (Zea maysL. cv. Snosweet) plants grown in distinct rows. A narrow band of milledsuperphosphate was applied at 0 (P₁), 5(P₂), 15 (P₃), 45(P₄), or 135 (P₅) kg P/ha toplots that were either fumigated (VAM ) or not fumigated (VAM+).

The higher (P < 0.05) concentrations of P in theyoungest mature leaf (blade plus petiole) of VAM+ than those of VAMcapsicum plants at P₁, P₂, andP₃ coincided with a greater (P< 0.05) weight of marketable fruit of VAM+ than of VAM– plants;this finding confirmed the importance of VAM to the enhanced P nutrition ofcapsicum at low P levels. In the non-fumigated soil (1061 non-dormantinfective VAM propagules/g air-dry soil), the inverse sine transformedmeans of percentage colonisation of roots were generally unaffected by Papplication, indicating that addition of P may not reduce VAM colonisation ofroots if the inoculum potential of the soil is high. An alternative hypothesisto account for the relatively undiminished mycorrhizal colonisation ofVAM+ plants at high P rates may relate to a reduction in colonisation forroots growing within the narrow band of applied P, but not for roots growingin the much larger volume of soil outside of this localised P zone. Theabsence of reduced yields and of lower starch concentrations of roots of VAM+ relative to VAM– plants suggested that photosynthateproduction was surplus to the requirements of the plant and fungus for theirradiance encountered during the trial. The gross margin for VAM plants wasmaximal at P₅ ($AU3340/ha), and this amountwas similar to the margin for VAM+ plants grown at all treatments exceptP₁, which had a lower gross margin. As a substitute forP, mycorrhizae have limited potential in intensive vegetable productionsystems since the cost of P is low compared with total costs. However, otherbenefits of VAM such as decreased susceptibility to disease and improvedstructure of the soil need to be fully assessed to determine the full benefitof mycorrhizae in such systems.