Depressed root morphology and altered hormone signaling in Toona sinensis brought about by different forms of excessive nitrogen

Toona sinensis, a precious tree species native to China, faces severe excessive nitrogen (N) stress due to escalating nitrogen deposition. The effects of excessive N (NO₃⁻, NH₄⁺ and NO₃⁻ + NH₄⁺) on root morphology and physiology were investigated in 1-year-old T. sinensis seedlings. Under excessive NH₄⁺, root morphology traits like root surface area and volume were inhibited, and under excessive NO₃⁻, root length, surface area, and volume were more significantly suppressed. However, a minimal decrease in branch number, apical number, specific root area, and specific root length was observed under excessive NO₃⁻ + NH₄⁺. Under excessive NO₃⁻ or NH₄⁺, the activity of N-metabolizing enzymes such as glutamine synthetase (GS), glutamate synthetase (GOGAT), nitrate reductase (NR), and nitrite reductase (NiR) in roots decreased. Under excessive N, the indole acetic acid (IAA) content in roots decreased, while the contents of abscisic acid (ABA) and cytokinins (CTK) increased. Overall, excessive NO₃⁻ or NH₄⁺ induced nutrient overload, which inhibited root development in T. sinensis. On the other hand, hormone signaling was also modulated in response to excess N, and root morphology changed. These findings enhance our understanding of the effects of excessive N stress on T. sinensis and provide guidance for its plantation production.