Response of Tracheid Structure Characteristics and Lignin Distribution of Taxodium Hybrid Zhongshanshan to External Stress

Abstract

The Taxodium hybrid Zhongshanshan fast-growing species is susceptible to environment and gravity to form reaction wood. In this study, individual growth rings of reaction wood are used as subjects, and an individual growth ring is divided into three zones: compression zone (CZ), lateral zone (LZ), and opposite zone (OZ). The microanatomical structure and chemical properties of the tracheids in CZ, LZ, and OZ forms by the inclined or bent growth of T. Zhongshanshan are comparatively analyzed by using optical microscopy, scanning electron microscope, laser confocal microscopy, and Raman imaging techniques. In CZ, the length and diameter of compression wood (CW) tracheids decreased, and the shape of cross-sections became rounded as compared to the OZ and LZ tracheids. More notably, threaded fissures appeared on the cell wall of tracheids, and the thickness of the cell wall increased in CW. The analysis of tracheids’ cell wall structure showed that CW tracheids had a complete outer secondary wall middle (S2L) layer, but had no secondary wall inner (S3) layer. In the transition zone (TA) between CW and normal early wood, tracheids were divided into compressed and normal tracheids. Despite the compressed tracheids having a similar cell morphology to normal tracheids, they had a thin secondary wall S2L layer. Tracheids in LZ had a thin S2L layer only at the angle of the cell. No S2L layer was seen in the cell wall of OZ and CZ late wood tracheids. It can be concluded that the response of lignin deposition location to external stress was faster than the change in cell morphology. The above results help provide the theoretical basis for the response mechanism of T. Zhongshanshan reaction wood anatomical structures to the external environment and has important theoretical value for understanding its characteristics and its rational and efficient usage.