Plant responses to stress in acid environments: An assessment of the role of mycorrhizal fungi

The purpose of the research was to gain further understanding of the responses of plants to stress in acid environments and the role of mycorrhizal fungi in stress tolerance. Acidic soils of the southeastern coastal plain typically have low fertility, but weeds remain problematic even in soybean and peanut fields where no fertilizer nitrogen is applied. Field experiments using 15N natural abundance examined whether nitrogen might be transferred between the N2-fixing crop species and neighboring weeds. A five year field study demonstrated that substantial nitrogen transfer did occur. Because nitrogen transfer was largely dependent on the presence of arbuscular mycorrhizae, the results strongly suggested that N moved from plant to plant through mycorrhizal hyphae that connected plant root systems. Another characteristic of acid soils is the presence of high levels of aluminum. When soil pH is below 5.0, the presence of Al+3 in soil solution can cause aluminum toxicity to occur in many plants. Aluminum toxicity inhibits root growth and predisposes crop plants to drought and nutrient deficiencies. Loblolly pine was determined to be extremely tolerant to aluminum. In a series of experiments, we examined the ability of loblolly pine to exclude aluminum from root tip meristems which are known to be the main sites of aluminum toxicity. The primary and secondary roots of pine exhibited high degrees of Al tolerance. Tolerance was associated with Al exclusion from the root tips and, of the Al accumulating in the root, exclusion from the root meristem. Ectomycorrhizal colonization was found to contribute to aluminum tolerance, evidently by providing an extra barrier to Al entry into the root. Additional experiments examined Al relations of arbuscular mycorrhizal fungi. The root systems of more than 80% of all plant species are colonized by arbuscular mycorrhizae. In the presence of aluminum, colonization by arbuscular mycorrhizal fungal species was inhibited. The results suggested alterations in root function caused lower colonization, as mycorrhizal infection potential appeared unaffected. Aluminum also inhibited fungal spore germination, but only at very high Al levels. Much of the function of the mycorrhizal fungi was unaffected even as aluminum accumulated in fungal structures.