Winter survival of extraradical hyphae and spores of arbuscular mycorrhizal fungi in the field

The ability of the extraradical mycelium (EM) of arbuscular mycorrhizal (AM) fungi to colonize spring-sown seedlings in temperate regions requires that this EM survive winter conditions. We studied the production and winter survival of such an EM and its associated spores at its site of synthesis in a field soil. Pouch-like nylon mesh bags of sieved and pasteurized soil were buried in a field below where a maize (Zea mays L.) crop was then grown. A mesh size of 45 mu m prevented the entry of roots into the pouches. Extraradical hyphae and spores of AM fungi were formed in these pouches. The density of trypan-blue staining hyphae in the pouches increased from 16.3 to 60.0 m g(-1) between planting and maize-harvest time, but it then fell to one-third of this value over the following winter. The density of fluorescein diacetate staining hyphae in the pouches were also highest at maize-harvest time, at 38.1 m g(-1). In contrast, the density in soil of newly produced AM spores with cytoplasm was maximal earlier, reaching 28.8 g(-1) during the grain-filling period; this density in soil had been halved by the time of maize harvest. At various times before and after winter, pouches were excavated from the field and used for growth-chamber bioassays. The extraradical component of the AM fungi retained over winter its capacity to colonize roots. Arbuscular colonization of sudangrass (Sorghum sudanense Staph.) bioassay plants at 28 days ranged from 36.5% to 69.3% root length, and colonization of plants in bioassays after the winter was similar to that for bioassays before the winter. Disturbance of pouch soil after excavation reduced the P content of bioassay shoots, relative to pouches with undisturbed soil. We show here that extraradical structures of AM fungi can survive the winter in the soil at their site of synthesis. Reductions in soil densities of spores in autumn, and of hyphae over winter, did not affect the capacity of the extraradical mycorrhizal system to colonize plants swiftly in the spring, even after the elimination of any potential contribution from the part of the mycorrhizae inside the roots. However, disturbance of soil was a potent means of hindering the subsequent development by the plant of a functional mycorrhizal P-acquisition system. (C) 1999 Elsevier Science B.V. All rights reserved.