Effects of mycorrhizae, established from an existing intact hyphal network, on the growth response of capsicum (Capsicum annuum L.) and tomato (Lycopersicon esculentum Mill.) to five rates of applied phosphorus

Abstract

The growth response of 2 vegetable crops to 5 rates of applied phosphorus (P)in the presence or absence of an existing network of extraradical mycorrhizalmycelium was determined in 2 greenhouse pot experiments (Expt 1, autumnwinter; Expt 2, summer autumn) using a low-P growth medium (6 or 5 mgNaHCO₃-extractable P/kg for Expt 1 or 2,respectively). In both experiments, capsicum(Capsicum annuum L.) and tomato(Lycopersicon esculentum Mill.) plants were grown at 0(P₁ ), 9.2 (P₂), 27.5(P₃ ), 82.5 (P₄ ), or 248(P₅) mg P/kg oven-dry soil (spot-placed at sowing)within a nylon mesh (pore size 44 µm). The mesh excluded roots from theoriginal sunflower (Helianthus annuus L.) host plants,to which either live (VAM+) or killed (VAM–) mycorrhizal[Glomus etunicatum Becker & Gerdemann andGlomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] inoculum was added at sowing. The mesh did allow fungal hyphae togrow into the growth medium contained by the mesh.

Whereas VAM+ plants generally had higher P concentrations in indextissues than VAM– plants at low P rates, a concomitant increase in drymatter yield was restricted to the P₁ rate. AtP₁ in Expt 2, the increase in the dry weight of wholeplants as a result of VAM colonisation was as large as 91.7-fold and 17.9-foldfor capsicum and tomato, respectively. Root starch analysis indicated that thelower dry matter yields of VAM+ plants than of VAM– plants at≥P₂ could be attributed to insufficient photosynthateproduction by VAM+ plants to meet the carbon (C) demand of both host andendophytes within the relatively low-light environment of the greenhouse(average daily solar irradiance of 8.4 MJ/m² forExpt 1 and 13.4 MJ/m² for Expt 2).

The growth response of vegetable crops grown within the greenhouse fromcolonisation by an established mycorrhizal mycelium appears to depend on acritical balance of P and C supply; i.e. at P₁, P wasmore limiting than C, and the increased uptake of P as a result ofcolonisation of plant roots by VAM resulted in a growth response. At higher Prates, C was more limiting than P due to low light in the greenhouse, and theadditional demand for photosynthate imposed by the endophytes on the hostresulted in a growth depression relative to non-mycorrhizal plants.