Helophytes adapt to water and N-enrichment stresses by adjusting and coordinating stoichiometry characteristics in main organs.

Exploring the adaptation strategies of plants under stressful environments from an ecological stoichiometry perspective is a critical but underexplored research topic, and multi-organ collaborative research for multi-species can provide a comprehensive understanding. In this study, helophytes were selected as the subjects, and water depth and water N-enrichment were set as the stressors. A simulation experiment including three water depths (drought stress, control and flooding stress) and four water N-enrichment levels (control, low, medium and high N-enrichment stresses) for six helophyte species was carried out. Overall, C concentrations in all plant organs remained stable under water (drought-flooding stress) and N-enrichment stress. N concentrations increased under both flooding and drought stresses, while P concentrations and the N:P ratio showed an increase and decrease under only flooding stress, respectively. N concentration and N:P ratio increased with water N-enrichment level. The interaction only promoted the accumulation of N concentrations in aboveground organs. Especially, several species also changed organ C concentrations to adapt to water stress and adjusted root N concentrations for the combined stresses of flooding or drought and high N. Leaf and stem were strongly synergistic in N element, and leaf and root were mainly synergistic in P element. Water N-enrichment determined organ element concentrations more than water depth, and species identity dictated organ C:N:P ratios. Our results reveal that the allocation and synergy of nutrients among organs are important adaptive strategies for plants in stressful environments. Meanwhile, increasing water N-enrichment can be an unignored stressor, and species identity should be paid attention as a countermeasure.