Study On Effect Of Rare Earth Elements Nd³⁺ On Adventitious Roots Regeneration Of Dendrobium Densiflorum Lindl. Ex Wall.

Rare earth elements (REEs) have important biological effects even though they are not the essential elements for the organism. The present results showed that the REEs at a lower concentration stimulate germination of seeding, promote root growth, increase root length and induce adventitious rooting. The effects of Nd³⁺ on adventitious rooting of Dendrobium densiflorum shoot cuttings were studied to explore the mechanism of effects of REEs on plant growth.Addition of Nd(NO₃)₃ (5μmol/L) to the culture medium promoted the adventitious rooting of D. densiflorum: significantly increased the rooting percentage, which was enhanced by 40.0% and 32.6% in contrast to the control, and also increased the lengths of adventitious roots and the number of roots per plant, at the 18 and 30 days after inoculation (DAI).Histological modifications occurring during the developmental process were also observed. The root primordium was formed between the 3 to 6 DAI both in the treatment and the control. Endogenous levels of IAA and total cytokinins (CTks) (iP+ iPR and ZT+ZR) were quantified by means of enzyme-linked immunosorbent assay (ELISA). At the 6 DAI. a marked increase in the level of auxin (Indole-3-acetic acid, IAA) of the rooting zones of cuttings was detected and this coincided with the beginning of root initiation in the root cuttings. Furthermore, this increase was 458.3% higher in the treatment. IAA/ total CTKs ratio of the 5μmol/L Nd³⁺ treatment in the rooting zones of cuttings were much higher than those of the control at the 6 DAI throughout the process of the adventitious root initiation, but had no significant differences between them on the other days. These suggested that IAA and IAA/total CTKs ratio have played important roles in the root primordium development, and Nd³⁺ (5μmol/L) promote the the adventitious rooting by increasing the level of IAA and IAA/ CTKs ratio in the adventitious root initiation phase.Although IAA oxidase activity (IAAox) showed the same activity changes in the Nd³⁺ treatment as in the control, and there was no significant difference between them in the process of adventitious rooting of D. densiflorum, the IAA level of the Nd³⁺ treatment was much higher than that of the control at the 6 DAI. These results suggested that the enhanced accumulation of the endogenous IAA by Nd³⁺ should not be related to inhibition of IAA decomposition by IAAox, and may be related to promotion of IAA synthesis or translocation of IAA. The activity of cytokinins oxidase (CKX) was usually induced by CTKs. In this paper, compared to the control, the total CTKs level was higher in the Nd³⁺ treatment at the 6 DAI. and the CKX activity was also higher in the Nd³⁺ treatment correspondingly, while the ratio of IAA/total CTKs in the Nd³⁺ treatment was still higher than that of the control, which would favor root primordium formation. Poxidase (POD) is relevant to adventitious rooting and development. During the whole process of adventitious rooting, POD activity of the treatment was much higher than that of the control.Besides, the Nd³⁺ enhanced the GSH/GSSG ratio in the process of adventitious rooting, but did not affect DHA/ASC ratio significantly in this paper. These suggest that the promotion of Nd³⁺ on adventitious rooting of D. densiflorum is correlative with the level of redox status.