Effects of arbuscular mycorrhizal fungi on root architecture and ultrastructure of <i>Hordeum jubatum</i> under the interactive impact of nitrogen deposition and cold stress

Hordeum jubatum L. is a perennial herb with high ornamental value and strong stress tolerance. Nitrogen deposition and cold stress are key environmental factors that affect stability of ecosystems in cold regions of northeast China. These factors significantly affect plant growth and development. Arbuscular mycorrhizal fungi (AMF) are symbiotic soil fungi that can increase plant resistance and growth. However, research on impacts of nitrogen deposition and cold stress on roots of H. jubatum-AM symbionts remains limited. Root biomass (dry and fresh weight), architecture (length, surface area, volume, forks, number of fourth-order roots, and root fractal dimension), and ultrastructure of H. jubatum were assessed, both in the presence and absence of AMF, under conditions of nitrogen deposition and cold stress. Cold stress inhibited all indicators of root architecture and disrupted root ultrastructure, with greater inhibition shown in the N2 (NH₄ ⁺/NO₃ ^- = 1:1) treatment under cold stress, indicating nitrogen deposition increased sensitivity of H. jubatum to cold stress. Inoculation with AMF significantly reduced damage caused by nitrogen deposition and cold stress on H. jubatum roots compared with the non-inoculation treatment. Our results demonstrate different effects of the interaction of nitrogen deposition and cold stress versus single stress (nitrogen deposition or cold stress) on plant root development and provide a scientific basis for the use of mycorrhizal technology to improve resistance and productivity of cold-tolerant plants in cold regions under stress conditions.