Changes in endogenous hormone levels and redox status during enhanced adventitious rooting by rare earth element neodymium of Dendrobium densiflorum shoot cuttings

The effects of neodymium nitrate (Nd3+) on the adventitious rooting of Dendrobium densiflorum shoot cuttings were studied. The addition of Nd3+ (5 μmol/L) to culture medium significantly increased rooting frequency. Histological investigation showed that Nd3+ did not change the process of root initiation. Nd3+ did not influence total endogenous cytokinin levels, but significantly increased the level of endogenous indole-3-acetic acid (IAA) in the base of shoot cuttings. Compared to the control, the ratio of IAA/cytokinins was very high in the Nd3+ treatment. These results suggested that the enhanced rooting frequency may be related to the increase in endogenous IAA level in Nd3+ treatment. Analysis of enzyme activities showed that the enhanced accumulation of the endogenous IAA by Nd3+ should not be attributed to inhibition of IAA decomposition by IAA oxidase or promotion of cytokinin decomposition by cytokinin oxidase. Besides, Nd3+ increased the ratio of reduced glutathione (GSH) and oxidized glutathione (GSSG) in the process of adventitious rooting while the ratio of ascorbate (ASC) to dehydroascorbate (DHA) was not affected.