SURVIVAL OF THE EXTERNAL MYCELIUM OF A VAM FUNGUS IN FROZEN SOIL OVERWINTER

The present investigation examines (1) whether the external VAM myceli um survives winter freezing to act as a source of inoculum in the spri ng, and (2) whether soil disturbance reduces the infectivity of the ex ternal VAM mycelium following freezing of the soil. Sealed pouches of fine nylon mesh were placed in pots containing soil inoculated with a Glomus species. The mesh was impervious to roots but not to hyphae. Fo llowing two 3-week growth cycles of maize in the pots, the pouches wer e transplanted to the field. Pouches were removed from the field once during the 4 months when the soil was frozen, and once after spring th aw. Measurements were made of VAM spore density, hyphal length and via bility in the pouches. Bioassays for infectivity were conducted on all pouches. Some VAM hyphae survived freezing and remained infective fol lowing winter freezing, in the absence of plant roots. Soil disturbanc e did not reduce the infectivity of hyphae following exposure to freez ing temperatures. We observed a change in the distribution of viable c ytoplasm within hyphae over winter, which we hypothesize represents an adaptation allowing hyphae to survive freezing temperatures. We sugge st that the effect of disturbance on hyphal infectivity may be related to this seasonal change in the distribution of hyphal viability.