Increased density facilitates plant acclimation to drought stress in the emergent macrophyte Polygonum hydropiper

Wetland plants frequently experience drought events due to large water level fluctuations; drought might present a greater limitation to plant survival than flooding in many wetlands. High plant density can increase the relative humidity, which might facilitate the acclimation of plants to drought stress. However, studies on the effects of plant density on acclimation to drought are scarce. This study aimed to elucidate the influence of density on plant response to drought stress by investigating morphological and physiological characteristics of Polygonum hydropiper L. var. flaccidum, one of the dominant species in the Dongting Lake wetland, China. Experimental greenhouse treatments, including three density (16, 144, and 400 plants m−2) and three water level (0, −20, and −40 cm relative to the soil surface) treatments, were performed in a factorial design. Soil water content increased with increasing water level and plant density. In the 0- and −20 cm water level treatments, biomass accumulation per plant decreased significantly with increasing density. However, in the −40 cm treatment, biomass accumulation did not change among the three plant densities. Higher density and water level led to lower leaf mass fraction. Both electrolyte leakage and malondialdehyde (MDA) content increased along with decrease in water level. Moreover, they showed no change with plant density in the 0- and −20 cm water level treatments, but decreased with increased density in the −40 cm treatment. Soluble sugar content increased with decreasing water level and increasing density, whereas starch content decreased significantly with decreasing water level. These data indicate that a higher density facilitates the acclimation of P. hydropiper to drought stress. Moreover, our results might reveal the important role of density in the restoration of P. hydropiper communities.