Background The restoration of the vegetation in the water-level-fluctuating zone is widely valued by the public. This work is aimed to explore the physiological and structural adaptation mechanism of Cynodon dactylon and Morus alba surviving on the water-level-fluctuating zone.

Methods The natural surviving C. dactylon and the two-year native M. alba (plant height in 1.5-2.0 m) at 171 m and 0, 10 and 70 d after re-emergence of the water-level-fluctuating zone were sampled as the study materials. The C. dactylon and M. alba of the same age at 177 m without submergence were sampled as controls (CK). Their growth and physiological indexes (number of fibrous roots, diameter of root, stem length or height of a tree, internode length, number of branches, area of a single leaf, chlorophyll content, soluble sugar content, soluble protein content, free proline content, SOD activity, CAT activity, and root vitality) as well as the anatomical structure changes were analyzed.

Results 1) The root system of C. dactylon after flooding decreased, and the growth of the aerial parts was inhibited. Most of the growth indexes were recovered to the level of land growth after 70 d of re-emergence. The stem diameter and plant height of M. alba decreased after 70 d of re-emergence, and there was still difference compared to the control. 2) There was no difference in the ratio of chlorophyll a/b between C. dactylon and the CK after 70 d of re-emergence, and the ratio of M. alba was 2.07 at the same time, which was still different from the CK. 3) The recovery rate of osmoregulation substance from roots of C. dactylon to the CK was faster than that of leaves, and there was no difference after 70 d of re-emergence. Some indexes from roots of M. alba in the same period did not restore to the normal level. 4) SOD and CAT of new leaves of C. dactylon and M. alba decreased gradually after re-emergence, and there was no significant difference between them after re-emergence of 70 d. The recovery rate of antioxidant enzyme and TTC content in C. dactylon root was faster than that of M. alba. 5) Most of the cells in the roots and stems of C. dactylon had become larger after submergence, and irregular atmospheric cavity fusion occurred. The structure of the roots was restored to the CK after 70 d of re-emergence. Some of the cells in the cortex of M. alba roots were dissociated, and the number of vessel cells significantly increased. After 70 d of re-emergence, there was still significant difference between the roots of flooded gorup and the CK of M. alba.

Conculsions The structure and physiological indexes of roots and stems of C. dactylon actively are started the compensation effect quickly after re-emergence, while the M. albais has a slow start compensation mechanism, which is also a reason that the adaptability range of M. alba is narrower than C. dactylon.