Response of Lewis flax seedlings to inoculation with arbuscular mycorrhizal fungi and cyanobacteria

Forbs comprise an important though understudied component of western rangelands. Little is known about the dependence of forb species on associated soil microorganisms such as arbuscular mycorrhizal fungi. In 1996 and 1998, we conducted two experiments examining the response of Lewis flax (Linum lewisii Pursh) to inoculation with arbuscular mycorrhizal fungi under a variety of soil conditions. Lewis flax was found to be highly dependent on mycorrhizae when grown in sandy soils, regardless of soil nutrient levels. Mycorrhizal plants had 10 times the root biomass and 16 times the shoot biomass of nonmycorrhizal controls. The addition of mycorrhizae also significantly decreased root:shoot ratios and significantly increased plant tissue concentrations of P, K, and Zn. The addition of cyanobacterial inoculum to the soil increased plant survival at low soil fertility, but had no effect on seedling growth. In the second experiment, three accessions of flax were grown in sand, peat, and a sand and peat mixture fertilized with high levels of a slow-release fertilizer. All three flax accessions responded positively to mycorrhizae in the sand and mixed soils, growing five and three times larger, respectively. In contrast, no significant response was observed in the peat soil. These findings indicate that mycorrhizal dependence is complex, and can be influenced by a number of factors, including soil nutrient status and organic matter content.