Accession‐Specific Responses to Submergence Stress in Lotus japonicus : Implications for Legume Climate Resilience

ABSTRACT Flooding is a major abiotic stress that limits legume productivity and ecological resilience. Identifying variation in submergence tolerance among legume accessions is critical for developing climate‐resilient cultivars. This study investigated phenotypic and physiological responses to complete submergence in nine Lotus japonicus accessions, selected from a range of latitudes, subjected to 10 days of submergence followed by recovery. Growth traits (leaf area, dry biomass), chlorophyll fluorescence (Fv/Fm) and reproductive timing (flowering onset and cumulative flower production) were assessed relative to non‐flooded controls. We hypothesised that developmental traits would vary in response to complete submergence. Submergence induced significant mean reductions in biomass (−36.9% ± 3.9%) and photosystem II efficiency (−27.4% ± 2.6%), though all accessions fully recovered photosynthetic function within 10 days post‐flood. Surprisingly, the accessions displayed two contrasting adaptive strategies: six exhibited growth arrest consistent with a low‐oxygen quiescence response, while three maintained shoot elongation under water, indicative of an escape strategy. The negative correlation between change in leaf area during submergence and early recovery growth suggested a physiological trade‐off. Accessions also displayed different responses in terms of flowering time with mean delays of between 0.5 and 21 days. Flooding also reduced cumulative flower production by 24.8% ± 3.9%. Notably, there was a strong correlation between latitude and flowering time delay caused by submergence stress. These results reveal substantial intra‐specific variation in submergence responses among L. japonicus accessions and demonstrate the utility of this species as a model for dissecting stress‐tolerance mechanisms in Fabaceae . The findings could inform breeding strategies for improved flood resilience in forage and grain legumes.