Search

Clonal plants are widely distributed in the riparian zone and play a very important role in the maintenance of wetland ecosystem function. Flooding is an environmental stress for plants in the riparian zone, and the response of plants varies according to the depth and duration of flooding. However, there is a lack of research on the growth response of clonal plants during flooding, and the endogenous hormone response mechanism of clonal plants is still unclear. In the present study, Alternanthera philoxeroides, a clonal plant in the riparian zone, was used to investigate the time-dependent stem elongation, the elongation of different part of the immature internodes, and the relationship between growth elongation and the phytohormone gibberellin (GA) under a series of submergence depths (0 m, 2 m, 5 m, and 9 m). The results showed that stem elongation occurred under all treatments, however, compared to 0 m (control), plants grew more under 2 m and 5 m submergence depth, while grew less under 9 m water depth. Additionally, basal part elongation of the immature internode was the predominant factor contributing to the stem growth of A. philoxeroides under different submergence depths. The phytohormone contents in basal part of the mature and immature internodes showed that GA induced the differential elongation of internode. Plant submerged at depth of 2 m had the highest GA accumulation, but plant submerged at depth of 9 m had the lowest GA concentration. These data suggested that GA biosynthesis are essential for stem elongation in A. philoxeroides, and the basal part of the immature internode was the main position of the GA biosynthesis. This study provided new information about the rapid growth and invasion of the clonal plant A. philoxeroides around the world, further clarified the effects of submergence depth and duration on the elongation of the stem, and deepened our understanding of the growth response of terrestrial plants in deeply flooded environments.

Complete submergence, especially deep submergence, poses a serious threat to the growth and survival of plants. One study previously showed that Alternanthera philoxeroides (a herbaceous perennial plant) submerged at depth of 2 m presented fast stem elongation and reduced stem elongation as water depth increased. In the present study, we aimed to figure out from the morphological and anatomical perspective how the differential growth response of the plant to water depth was achieved. We investigated the elongation of different stem parts and the relationship of stem elongation to cell size and number in A. philoxeroides by conducting experiments using a series of submergence depths (0 m, 2 m, 5 m, and 9 m). The results showed that, in comparison with unsubmerged plants, completely submerged plants exhibited enhanced elongation at depths of 2 m and 5 m but suppressed elongation at depth of 9 m in immature stem internodes, and displayed very little elongation in mature stem internodes at any depths. The stem growth of A. philoxeroides at any submergence depth was chiefly caused by the elongation of the basal parts of immature internodes. The elongation of the basal parts of immature internodes was highly correlated to both cell proliferation and cell enlargement, but the elongation of the middle and upper parts of immature internodes correlated nearly only with cell enlargement. This study provided new information on the growth responses of A. philoxeroides to heterogeneous submergence environments and deepened our understanding of the growth performance of terrestrial plants in habitats prone to deep floods.

Global climate change has resulted in an increase in intensity and frequency of flooding, plants living in lowlands, and shore areas have to confront submergence caused by flooding, submergence-tolerant plants usually respond by adopting either escape or quiescence strategies. While certain plants exhibit a changeover from escape strategy upon partial submergence to quiescence strategy under complete shallow submergence, it remains unknown whether plants completely submerged at different water depths would adjust their strategies to cope with the change in submergence depth. Alternanthera philoxeroides is an ideal species to explore this adjustment as it is widely distributed in flood-disturbed habitats and exhibits an escape strategy when completely submerged in shallow waters. We investigated the responses of A. philoxeroides in terms of morphology, anatomy, and non-structural carbohydrate metabolism by conducting experiments using a series of submergence depths (0, 2, 5, and 9 m). During the submergence treatment, environmental factors such as light, dissolved oxygen, and temperature for submerged plants were kept constant. The results showed that A. philoxeroides plants submerged at depth of 2 m presented an escape strategy via fast stem elongation, extensive pith cavity development, and small biomass loss. However, the retarded stem elongation, reduced pith cavity transverse area, and increased biomass loss along the water depth gradient indicated that A. philoxeroides altered its growth response as water depth increased from 2 to 9 m. It is found that the changeover of response strategies occurred at higher submergence depths (5–9 m). Based on the results of our experiments, we demonstrated that water depth played an important role in driving the change in strategy. The water-depth-dependent growth performance of A. philoxeroides would benefit the species in habit exploration and exploitation. Further studies should focus on the performances of plants when submerged at varied water depths with different light climates and dissolved oxygen content, and how water depths drive the response behaviors of the submerged plants.