Differential responses of the floating-leaved aquatic plant Nymphoides peltata to gradual versus rapid increases in water levels

We compared the growth responses of the floating-leaved species Nymphoides peltata to gradual and rapid rising water levels under two nutrient concentrations (1 g and 12 g of slow released fertilizer (N-P-K: 16-8-12) per container filled with 8 kg washed sand), and predicted the population expansion after these floods. The results showed that the capacity for petiole elongation was dependent on leaf age, and only leaves that were no more than five days old had the capability to reach the water surface when the water level increased rapidly from 50 cm to 300 cm. Plants subjected to a gradual rising water level tracked the increase in water depth whose petioles elongated at 3.96 ± 1.70 cm per day and 4.80 ± 0.16 cm per day under low and high nutrient concentrations respectively throughout the experiment period. When water levels were rapidly raised, leaf petioles elongated rapidly at 25.48 ± 1.51 cm per day and 26.64 ± 2.24 cm per day under low and high nutrient concentrations respectively during the first ten days. Plants under a constant water level maintained highest mean leaf recruitment (mean 3.0 ± 0.33 leaves and 24.4 ± 5.87 leaves every ten days under low and high nutrient concentrations, respectively). Therefore, more young leaves existed in the canopy ensuring that when the water level increases, young leaves can rapidly emerge after submergence. Gradual water level rise did not significantly affect biomass and ramet production (4.75 ± 1.41 g and 5.50 ± 1.22 ramets in low nutrient; 48.49 ± 21.45 g and 35.67 ± 11.78 ramets in high nutrient), but rapid water level rise negatively affected ramet production in both nutrient concentrations (3.00 ± 1.26 ramets and 11.25 ± 4.19 ramets in low and high nutrients, respectively). The results indicated that continual leaf recruitment and rapid petiole elongation were both important ways in which N. peltata adapted to increasing water levels. Extreme flooding may be a disturbance factor that affects plant growth and the population expansion of N. peltata, while small gradual water level rise should not harm this species.