FLAX (LINUM-USITATISSIMUM L.) DEPENDS ON ARBUSCULAR MYCORRHIZAL FUNGIFOR GROWTH AND P UPTAKE AT INTERMEDIATE BUT NOT HIGH SOIL P LEVELS INTHE FIELD

The contribution of indigenous arbuscular mycorrhizal fungi (AMF) to g rowth and phosphorus (P) uptake by oilseed flax (Linum usitatissimum L .) was examined in two held experiments covering soil P levels from 20 -86 mg kg-l NaHCO3-extractable P. The fumigant dazomet was applied to the soil in half of the plots to obtain control plants with reduced my corrhiza formation. An extensive AMF colonization of up to 48% of the root length was established in untreated soil of both experiments, alt hough P fertilization reduced colonization to 28-39% at the latest har vests. Fumigation markedly decreased or totally prevented AMF coloniza tion throughout the experiments. Root growth responded to fumigation b y increased total and specific root length. Shoot P uptake was decreas ed by fumigation at soil P levels lower than ca. 50 mg kg(-1) whereas shoot growth was reduced by fumigation at soil P levels lower than ca. 40 mg kg(-1). The effects of fumigation were ascribed to the suppress ion of mycorrhiza formation. The effect of the AMF increased with decr easing soil P levels. Phosphorus inflow through roots (based on shoot P uptake) was reduced more strongly by fumigation than total P uptake. The P inflow through fungal tissue in roots was estimated to 4 x 10(- 14) mol P cm(-1) s(-1). We conclude that AMF are essential to flax gro wth at soil P levels below ca. 40 mg P kg(-1), which is representative of the conditions under which most flax is grown.