Genotypic control of root sodium uptake drives ion imbalance and stress sensitivity under salinity and waterlogging

The impacts of individual salinity ('ST') and waterlogging ('WL') on crops are well-documented; however, the synergistic effects of concurrent salinity and waterlogging ('SWL') have garnered less scientific scrutiny. Here, we investigated the individual and combined effects of 'SWL' on four soybean (Glycine max) genotypes: BU3, PK472, Williams 82 (W82) and DT2008. Phenotypic and growth-related analyses under all stresses, particularly 'SWL', revealed genotype-specific responses, with PK472, BU3, and W82 exhibiting significant sensitivity and biomass reduction. In contrast, DT2008 demonstrated exceptional resilience, maintaining superior growth and minimal phenotypic damage, even under 'SWL'. These contrasting responses identified DT2008 as the most tolerant genotype and PK472 as the most sensitive one, demanding their further investigations into underlying mechanisms. PK472 exhibited ionic disruptions, including pronounced Na⁺ hyperaccumulation, substantial declines in the contents of essential ions (i.e., K⁺, Ca²⁺, Mg²⁺, and Fe²⁺), interrupted ion homeostasis, excessive root-to-shoot Na⁺ translocation, and impaired essential nutrient transport under 'ST' and 'WL', which were intensified under 'SWL'. In contrast, DT2008 effectively restricted Na⁺ accumulation, while retaining and facilitating the translocation of essential ions, even under 'SWL'. PK472 also exhibited steep declines in photosynthetic efficiency and pigment contents under 'ST' and 'WL', accompanied by elevated hydrogen peroxide, malondialdehyde, and electrolyte leakage, which were exaggerated under 'SWL'. The moderate increase in antioxidant enzyme activities provided limited protection, highlighting PK472's heightened sensitivity to stresses. Conversely, DT2008 genotype demonstrated superior photosynthetic efficiency, increased pigment contents, enhanced antioxidant enzyme activities, and reduced oxidative damage under 'ST' and 'WL', and even under 'SWL'. Additionally, DT2008 maintained elevated levels of proline and free amino acids under all types of individual and combined stresses, ensuring superior osmoprotective capacity and water balance.