Abstract:

Objective: Pinus massoniana is an important afforestation tree species in southern China. Asexual rapid propagation technology is one of the effective ways to solve the problem of high genetic differentiation and low stand productivity of forest trees. Adventitious root induction is the key technical bottleneck to cultivating asexual seedlings of P. massoniana. Endogenous gibberellins (GAs) have dual effects in the formation of adventitious roots of P. massoniana. High levels of GAs are not conducive to the rooting of P. massoniana. In this study, the metabolism of endogenous GAs in the rooting process of P. massoniana under the treatment of GAs biosynthesis inhibitors such as paclobutrazol (PBZ) and mepiquat chloride (DPC) was investigated, in order to reveal the rooting-active GAs and the mechanism, and to provide reference for improving the effectiveness of cultivating asexual seedlings of P. massoniana. Method: An elite tree, GLM-3, selected from over 15-year-old P. massoniana var. ‘Tongmiansong’ stand was used as the propagation object, and the rejuvenated branches obtained by micro-continuous grafting technology were used as the experimental materials. Cutting propagation under the conditions of 200 mg·L^?1 NAA (control), 200 mg·L^?1 NAA+100 mg·L^?1 PBZ (PBZ), and 200 mg·L^?1 NAA+100 mg·L^?1 DPC (DPC) treatment was carried out respectively. Liquid chromatography-tandem quadrupole mass spectrometry (LC-MS/MS) technology was used to detect the variation characteristics of GAs at 0 (rooting initial stage), 10 (root formation stage), 20 (root development stage) and 35 days (root formation stage). With 200 mg·L^?1 NAA as the control, the effects of rooting-active GAs in 200 mg·L^?1 NAA+100 mg·L^?1 GA₃ (GA₃) and 200 mg·L^?1 NAA+100 mg·L^?1 GA₄ (GA₄) on rooting ability, endogenous IAA level, rhizome anatomical structure and small molecules of lignin pathway were analyzed. Result: 1) Both PBZ and DPC were able to improve the rooting ability of P. massoniana, but the rooting time and rooting rate were different under the treatment of the two GAs inhibitors. Compared to the control, the rooting time under PBZ treatment was shorter, while the rooting rate under DPC treatment was higher, but there were not significant differences in root number and survival rate between the treatments and the control (P>0.05), with the overall at a low level. At 10 days of cutting, PBZ/DPC reduced the GA₃ level in the cuttings, while at 20 days of cutting, the GA₄ level under PBZ treatment was significantly increased. 2) GA₃ and GA₄ had inhibitory effect on rooting of cuttings, and the rooting rate, root number and survival rate were significantly lower than those of the control (P<0.05). The endogenous IAA content was lower than that of the control at 10–35 days under GA₃ treatment, but there was no significant difference in IAA content between the GA₄ treatment and the control at 20–35 days (P>0.05). Under GA₄ treatment, the suberization was aggravated, the suberin was increased, the level of small molecule caffeol in lignin was decreased, and the levels of caffeic acid and ferulic acid were both increased in the root and stem tissues of cuttings at 20–35 days of cutting. Conclusion: In the process of P. massoniana cutting, different inhibitors of GAs synthesis have different root-promoting effects and mechanisms. The root-inducing effect of PBZ is stronger than that of DPC, but the rooting stability is less than that of DPC. Both PBZ and DPC lead to a decrease in GA₃ level that inhibites IAA level during adventitious root formation. However PBZ causes an increase in GA₄ level that can induce increased cell suberization during adventitious root development. This study reveals that GA₃ and GA₄ are related to the difficulty of adventitious root formation in P. massoniana.

Key words: Pinus massoniana, adventitious root, gibberellin, lignification, suberization