Arbuscular Mycorrhizal Fungi Alleviate Ozone Stress on Nitrogen Nutrition of Field Wheat

The nitrogen (N) nutrition, crop yield, and responses of wheat to arbuscular mycorrhizal fungi (AMF) were tested in an experimental field under free-air ozone concentration [O-3] enrichment (FACE) conditions. The experiment included three treatments: ambient [O-3] (Ambient), elevated [O-3] (FACE, targeted at ambient [O-3]x1.5), and elevated [O-3] inoculated with an AMF consortium consisting of several Glomus species (FACE+AMF). AMF inoculation responsiveness of wheat was estimated by comparing plants grown in unsterilized soil inoculated with the exogenous AMF and in untreated soil containing indigenous AMF. Compared with the Ambient, relatively higher N contents but lower shoot biomasses of wheat plants were observed in the FACE treatment without AMF inoculation from the tillering stage in February and heading stage in April, respectively, which significantly (P< 0.05) decreased grain yield by 28% at harvest in June. Under the FACE condition, compared with the non-inoculated treatment, AMF inoculation significantly (P< 0.05) increased root colonization rates both at the tillering stage and heading stage, and also significantly (P< 0.05) increased shoot biomass at the heading stage and, hence, significantly (P< 0.05) increased grain yield by 40% at harvest. However, AMF inoculation significantly (P< 0.05) decreased total N content in wheat shoots at the tillering stage, suggesting that AMF consortia may enhance plant tolerance to elevated [O-3] by elevating root colonization rate rather than plant total N content at early growing stages.