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

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 mycorrhizal mycelium was determined in 2 greenhouse pot experiments (Expt 1, autumn-winter; Expt 2, summer-autumn) using a low-P growth medium (6 or 5 mg NaHCO3-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 mu m) The mesh excluded roots from the original 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 to grow into the growth medium contained by the mesh. Whereas VAM+ plants generally had higher P concentrations in index tissues than VAM- plants at low P rates, a concomitant increase in dry matter yield was restricted to the P-1 rate. At P-1 in Expt 2, the increase in the dry weight of whole plants as a result of VAM colonisation was as large as 91.7-fold and 17.9-fold for capsicum and tomato, respectively. Root starch analysis indicated that the lower dry matter yields of VAM+ plants than of VAM- plants at greater than or equal to P-2 could be attributed to insufficient photosynthate production by VAM+ plants to meet the carbon (C) demand of both host and endophytes within the relatively low-light environment of the greenhouse (average daily solar irradiance of 8.4 MJ/m(2) for Expt 1 and 13.4 MJ/m(2) for Expt 2). The growth response of vegetable crops grown within the greenhouse from colonisation by an established mycorrhizal mycelium appears to depend on a critical balance of P and C supply; i.e. at P-1, P was more limiting than C, and the increased uptake of P as a result of colonisation of plant roots by VAM resulted in a growth response. At higher P rates, C was more limiting than P due to low light in the greenhouse, and the additional demand for photosynthate imposed by the endophytes on the host resulted in a growth depression relative to non-mycorrhizal plants.