Colonisation of wheat in southern New South Wales by vesicular-arbuscular mycorrhizal fungi is significantly reduced by drought

Abstract

Colonisation of wheat crops by vesicular-arbuscular mycorrhizal (VAM) fungi was examined on a pair of adjacent organic and conventionally managed farms in southern New South Wales during 1993-94. Although intended to be part of a larger study on the roles of VAM fungi in organic and conventional farming systems, the severe drought of 1994 allowed an examination of the effects of drought on VAM fungi. In 1993, rainfall was above average, crop growth was good and VAM fungi colonised between 40 and 70% of crop root length. In 1994, low rainfall resulted in poor crop growth, and colonisation by VAM fungi was significantly lower than in 1993, ranging between 5 and 16% in the crops most affected by the drought. Wheat plants adjacent to tree lines exhibited particularly poor growth and low VAM colonisation, presumably due to the trees competing with the crop for water. The lower colonisation of crops by VAM fungi in 1994 resulted in reduced inoculum levels in the soil which could affect growth of a subsequent VAM-dependent crop. In contrast to the results of research carried out under conditions of milder drought stress, the low level of VAM colonisation found in this study suggests that the VAM fungi had no significant role in alleviating the drought stress experienced by the crop.