Responses of riparian plant roots to acute combined nitrogen and phosphorus stress: changes in root morphology and antioxidant system

Riparian plants are vulnerable to acute combined nitrogen (N) and phosphorus (P) stress induced by water quality fluctuations. Here, we carried out pot experiments on Lythrum salicaria and Phragmites communis. Three N concentration levels (0 mg L-1, 0.8 mg L-1, and 20 mg L-1) and three P concentration levels (0 mg L-1, 0.03 mg L-1, and 1 mg L-1) were set up according to the actual water environment to investigate the effects of acute combined N and P stress on root morphology, photosynthesis function, and root antioxidant system. Our results revealed that the N-free treatment (0 mg N L-1) and P-free treatment (0 mg P L-1) formed deficiency stress on L. salicaria and P. communis, inhibiting root morphological growth, photosynthesis, and antioxidant capacity. Additionally, the N-free treatment improved the root potential to acquire nutrients with increases of specific root length and root length density. L. salicaria and P. communis had differences in tolerance to the high-N treatment (20 mg N L-1) and high-P treatment (1 mg P L-1). Specifically, the high-N treatment and high-P treatment caused the inhibition of root morphological and physiological traits in L. salicaria. Conversely, the high-N treatment or high-P treatment had positive effects on the root growth of P. communis. Integrated assessments of biomarker values showed that the simultaneous existence of two nutrient stresses resulted in pile-up effects and species-specific responses. These results will contribute to providing suggestions on the selection of appropriate plant species in future riparian zone construction.