Formation Mechanism and Anatomical Structure of the Knee Roots of Taxodium Ascendens

Abstract Aims: Flooding seriously limits the growth and distribution of plants. Taxodium ascendens is a typical tree species with high flood tolerance, and it can generate knee roots in the wetlands. This study was conducted to understand the formation mechanism of the knee roots.Methods: The number and size of knee roots and soil flooding conditions were investigated in this study. Furthermore, physiology, biochemical responses, and the anatomical structure of knee roots and underground roots were measured at different developmental stages. Results: The results show that the formation of knee roots was significantly affected by the soil water table (P < 0.05). Moreover, knee root formation was affected by ethylene and indole-3-acetic acid (IAA) concentrations in the roots. The 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were significantly lower in the knee roots than in the underground roots. The ethylene release rate was significantly higher in the knee roots than in the underground roots (P < 0.05), and IAA content first increased and then decreased with knee root development. The cells of the periderm at the apex of the knee roots were dead and had a large number of intercellular spaces, which was beneficial for the growth of T. ascendens. Conclusions: Seasonal flooding induced the production of endogenous hormones, resulting in the formation of knee roots, which improved root respiration and ventilation. The results obtained can gain a basis for the formation mechanism of knee roots and provide scientific evidence for the afforestation and management under wetland conditions.