Abstract: We investigated the alleviating effects of nitrogen (N) application (0, 0.46 and 0.92 g per plant) on Rhododendron  ‘Wucai’ng’ under different high temperature stresses (26 ℃/22 ℃, 34 ℃/30 ℃ and 38 ℃/34 ℃), aiming to provide a scientific basis for enhancing plant stress resistance and adaptation to climate change. The results showed that there was a trend of fork-shaped branch structure formed in the roots of seedlings under high temperature stress. At 26 ℃/22 ℃, there was a significant increase in root bifurcation in the N fertilizer treatment of 0.46 g per plant, but a significant decrease in the N fertilizer treatments of 0 and 0.92 g per plant. Under the same level of N fertilizer, there were significant decreases in leaf area, total root length, root surface area, specific root length, specific surface area, and root-shoot ratio, but significant increases in root topology coefficient, soluble protein content, and free amino acid content in plants at 34 ℃/30 ℃ and 38 ℃/34 ℃, compared to that at 26 ℃/22 ℃. There were significant increases in leaf area, specific root length, root-shoot ratio, soluble protein content and free amino acid content of plants under high temperature stress in low N fertilizer treatment (0.46 g·plant^-1), while there were significant decrease in leaf area, specific root length, root-shoot ratio, except for a significant increase in soluble protein content and free amino acids, in the high N fertilizer treatment (0.92 g·plant^-1). The results of principal component analysis and membership function analysis showed that the best performance of seedlings was recorded in the treatments of T3 (26 ℃/22 ℃ and 0.92 g·plant^-1), T2 (26 ℃/22 ℃ and 0.46 g·plant^-1) and T5 (34 ℃/30 ℃ and 0.46 g·plant^-1), with a score of 0.853, 0.599 and 0.550, respectively, exhibiting better overall performance than that in the T1 control treatment (26 ℃/22 ℃ and 0 g·plant^-1). In conclusion, high temperature stress would restrict root growth of seedlings and reduced soluble protein content. Appropriate application of nitrogen fertilization (0.46 g·plant^-1) could alleviate the inhibition in root growth under high temperature and enhance the resistance and adaptation of plants to high temperature. These findings provide theoretical guidance and reference for adopting fertilization in advance in the cultivation of Rhododendron to avoid summer heat damage with high temperature and promoting the application of excellent varieties of Rhododendron in gardens.

Key words: Rhododendron, nitrogen fertilization, summer heat damage, root growth, physiological characteristics