Flooding is considered to be a major determining factor in plant growth and species distribution in riparian and lacustrine wetlands (van Eck et al., 2004; Dwire et al., 2006; Luo et al., 2008). Flooding constrains the growth of emergent macrophytes mainly by reducing oxygen availability, ranging from deficiency (hypoxia) to absence (anoxia) of oxygen in highly reduced soils (Vervuren et al., 2003). The strategies of plants to cope with flooding-induced low oxygen can be categorized as the escape strategy and the quiescence strategy (Bailey-Serres and Voesenek, 2008, 2010; Striker et al., 2012). In the escape strategy, the plant grows and/or elongates its shoot to reach the surface and restore contact with the atmosphere (Bailey-Serres and Voesenek, 2008; Manzur et al., 2009). However, the process of shoot elongation is costly under low-oxygen conditions, i.e., the energy required for growing tissues would lead to faster carbohydrate depletion by anaerobic metabolism (Bailey-Serres and Voesenek, 2008, 2010). In contrast, the quiescence strategy is characterized by stable functional traits, such as minimum growth and conservation of carbohydrate reserves (Setter and Laureles, 1996; Sauter, 2000; Bailey-Serres and Voesenek, 2008).

The escape strategy is proposed to be beneficial only in shallow and prolonged floods, where shoot de-submergence is generally possible (Pierik et al., 2009; Chen et al., 2011; Striker et al., 2012). The quiescence strategy is speculated to be beneficial in deep and temporary floods, where shoot emergence might incur higher energy cost and affect eventual recovery when water recedes (Colmer and Voesenek, 2009; Akman et al., 2012; Striker et al., 2012). Along the elevational gradient in wetlands, flood duration and flood depth are highly associated, i.e., plants distributed at high-elevation sites usually experience shallow and temporary flooding, while plants distributed at low-elevation sites often experience deep and long-lasting flooding. This raises the question whether plant populations experiencing deep and prolonged flooding exhibit the quiescent strategy, and plant populations experiencing shallow and temporary flooding use the escape strategy.

Generally, whether plant species exhibit the escape or the quiescence strategy depends on genetics and the flooding regimes in their natural habitats (Akman et al., 2012; Striker et al., 2012). For example, Rorippa amphibia (L.) Besser, occupying habitats with long-lasting and relatively shallow floods, adopts the escape strategy and Rorippa sylvestris (L.) Besser, occupying habitats with deeper and transient floods, adopts the quiescence strategy (Akman et al., 2012). However, some species occur over a large range of flooding conditions. It is estimated that approximately 30% of the common trees of the Amazonian floodplains also grow in non-flooded upland forests (Ferreira et al., 2009). With long-term adaptation to local flooding conditions, plant populations may have evolved divergent survival and tolerance strategies to cope with distinct flooding regimes (Chen et al., 2009; Ferreira et al., 2009; Mollard et al., 2010). However, few experimental studies have found significant differences in flooding tolerance strategies among populations (Lenssen et al., 2004; Li et al., 2016; Zhang et al., 2016).

In the present study, we investigated the effect of flooding gradients on the growth and carbohydrate reserves of Polygonum hydropiper (L.) Delarbre plants originating from two habitats with contrasting flooding conditions in the Dongting Lake wetlands. Polygonum hydropiper grows along flooding gradients from low-elevation, lakeside habitats to high-elevation, landside habitats. We hypothesized that P. hydropiper from low-elevation sites would utilize the quiescence strategy to survive deep and prolonged flooding, whereas P. hydropiper from high-elevation sites would employ the escape strategy to survive shallow and temporary flooding. Hence, morphological traits and the contents of non-structural carbohydrates (i.e., soluble sugars and starch) of P. hydropiper plants collected from these two habitat types were evaluated in relation to four water table levels; specifically, drained (water level at -15 cm), waterlogged (0 cm), semi-submergence (15 cm), and complete submergence (30 cm).

Shoot length did not differ among flooding treatments for plants either from low-elevation or high-elevation habitats, indicating that P. hydropiper did not exhibit an apparent escape strategy to cope with flooding. Shoot elongation is considered an efficient approach for escaping submergence stress, and a key attribute in a low-oxygen escape strategy (Chen et al., 2009). However, elongation growth competes with maintenance processes for carbon used for survival during complete submergence (Ram et al., 2002). Moreover, if air contact cannot be established, high carbohydrate consumption will lead to an energy deficit, severe tissue damage, and mortality (Pierik et al., 2009; Chen et al., 2011). It was found that elongation growth by leaves is negatively correlated with seedling survival under complete submergence in rainfed lowland rice cultivars (Ram et al., 2002). In our flooding treatments, shoots of P. hydropiper emerged from the water. However, in their habitats of origin, flooding was deep and long-lasting even for P. hydropiper populations in the high-elevation habitat (mean flood depth 0.98 m and 71 days). Elongation growth may not be sufficient for shoots of P. hydropiper to regain contact with the air, and therefore the escape strategy was not selected.

Flooding generally reduced the total biomass and carbohydrate reserve of P. hydropiper from both high-elevation and low-elevation habitats. Emergent macrophytes like P. hydropiper have evolved a range of morphological and physiological adaptations, such as stem or petiole elongation, narrower and thinner leaves, short and thick roots, a shallow root system, aerenchyma formation, glycolysis, and fermentation to acquire oxygen or diminish root oxygen demand in flooded environments (Chen et al., 2009; Xie et al., 2009; Pan et al., 2014). However, emergent plants usually lack the ability to sustain photosynthetic activity when submerged in water (Macek et al., 2006). Extended periods of anoxic conditions may eventually result in the reduction of plant growth and total biomass (Edwards et al., 2003).

Although flooding generally reduced the growth of P. hydropiper plants from both habitat types, shoot length and total biomass in the 30-cm flooding treatment were lower in plants from the low-elevation habitat than those from the high-elevation habitat. These results indicated that P. hydropiper plants from the low-elevation habitat exhibit a smaller modular size and lower growth rate under complete submergence than plants from the high-elevation habitat. Previous studies have also found a reduced modular size in lakeside genotypes in comparison with landside genotypes of Ranunculus reptans L. (Lenssen et al., 2004). During submergence, a low growth rate and small modular size may result in lower respiratory losses and consequently, in high carbohydrate reserves (Lenssen et al., 2004). Consistent with this hypothesis, water-soluble sugar and starch contents were higher in P. hydropiper plants from the low-elevation habitat than in those from the high-elevation habitat. A high carbohydrate supply is related to flooding tolerance under complete submergence (Ram et al., 2002; Qin et al., 2013). Large carbohydrate reserves may enable P. hydropiper plants to tolerate deep and long-lasting flooding in low-elevation habitats. Therefore, differences in growth and carbohydrate reserves under complete submergence between plants from low- and high-elevation habitats are most likely a response to the flooding pressures of the habitat of origin.

Flooding can act as a strong selective pressure for key adaptations, potentially leading to ecotype differentiation or even speciation (Lenssen and de Kroon, 2005; Voesenek et al., 2006). Mollard et al. (2010) have shown that subtle topographical differences along a floodplain may promote different plant strategies among Paspalum dilatatum Poir. populations and drive within-species ecotype differentiation. Lenssen et al. (2004) reported that flooding can induce genetic differentiation in the clonal species R. reptans along a small-scale gradient. In the present study, P. hydropiper populations from habitats differing in flooding regimes exhibited different morphological and physiological responses to flooding, suggesting there is a potential for population differentiation along flooding gradients (Chen et al., 2009; Ferreira et al., 2009).

Our results indicated that flooding reduced growth and carbohydrate reserves of P. hydropiper plants originating from both high-elevation and low-elevation habitats. However, low-elevation and high-elevation populations of P. hydropiper respond differently to flooding disturbances. Under complete submergence, the low-elevation population showed a smaller modular size and lower growth rate in comparison with the high-elevation population. Soluble sugar and starch contents were higher in plants from the low-elevation habitat than in those from the high-elevation habitat independently of the water level. Therefore, P. hydropiper plants from the low-elevation habitat exhibit a lower growth rate and more conservative energy strategy to cope with flooding in comparison with plants from the high-elevation habitat. These different adaptive strategies to tolerate flooding among P. hydropiper populations may potentially drive ecotype differentiation within this species along flooding gradients.

X-SC and Y-FL wrote the manuscript and executed the technical assays and statistical analyses. X-SC and Y-HX designed the experiments and edited the manuscript. Y-HC, Z-YH, Z-MD, and FL contributed to data collection and interpretation. All authors reviewed the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.