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

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 mgNaHCO 3 -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(P 1 ), 9.2 (P 2 ), 27.5(P 3 ), 82.5 (P 4 ), or 248(P 5 ) 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 and Glomus 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 P 1 rate. AtP 1 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≥P 2 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/m 2 forExpt 1 and 13.4 MJ/m 2 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 P 1 , 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. Australian Journal of Agricultural Research 50(2) 223 - 238 Full text doi:10.1071/A97167 © CSIRO 1999