This study aimed to investigate the effect of potassium (K) on the waterlogging tolerance of rapeseed (Brassica napus L.) and elucidate the physiological regulatory mechanisms involving K in the plant's response to waterlogging stress, to provide a theoretical basis for effective waterlogging management in rapeseed production.

A sand culture pot experiment was conducted with two water treatments: well-watered (CK) and waterlogging (WL), and two potassium levels: 1.0 mmol K₂SO₄·L^-1 (HK) and 0.1 mmol K₂SO₄·L^-1 (LK). Seven-day waterlogging stress was imposed at the seedling stage. Differences in growth, photosynthetic characteristics, antioxidant enzyme activities, and endogenous hormone levels under different K levels were compared.

Rapeseed growth was significantly affected by waterlogging and K level. Biomass, root-shoot ratio, and net photosynthetic rate consistently exhibited the hierarchical pattern: HK_CK＞HK_WL＞LK_CK＞LK_WL. Waterlogging elevated superoxide dismutase (SOD) and catalase (CAT) activity in leaves, while the activities of both enzymes were suppressed in LK treatments (LK_CK and LK_WL) compared to HK treatments (HK_CK and HK_WL). Consequently, the MDA content was significantly higher in the LK treatments than in the HK treatment. Waterlogging and K deficiency profoundly altered the endogenous hormone profiles in rapeseed. Leaf abscisic acid (ABA) cotent progressively increased across treatments: HK_CK＜HK_WL＜LK_CK＜LK_WL. Both waterlogging and potassium deficiency significantly increased the leaf jasmonic acid (JA) content after 7 d of waterlogging stress, peaking at LK_WL treatment. Waterlogging and K deficiency induced salicylic acid (SA) accumulation, with the highest SA content observed in LK_WL treatment. During the recovery stage, both waterlogging and potassium deficiency induced the decrease of indole-3-acetic acid (IAA) content in root and leaf, with the most pronounced depletion occurring in LK treatments (LK_CK and LK_WL).

Waterlogging stress inhibited rapeseed growth and severely restricted root development. Root growth and K⁺ uptake were significantly promoted, and leaf photosynthetic capacity as well as SOD and CAT activities were enhanced by elevated K levels, thereby improving waterlogging tolerance. Waterlogging stress triggered the accumulation of ABA, JA, and SA in leaves. The stress responses induced by ABA and JA accumulation were significantly alleviated by improved K levels and promoted the accumulation of IAA in roots and leaves after stress removal, facilitating the recovery growth of rapeseed.