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The grain legume Lathyrus sativus L. (grass pea) is of high economic importance for food and feed in Asian and African developing countries and is part of cultural heritage of marginal areas in the Mediterranean region. Its outstanding robustness under adverse environmental conditions, when compared with other legume species, raises particular interest under the current climate change scenario. The aim of this work was to study the range, extent and mechanisms underlying this species’ reported tolerance to water stress. To achieve this, a worldwide representative collection of 194 grass pea accessions was subjected to specific well-watered, water deficit and waterlogging conditions. These conditions were defined previously to elicit differential physiologic responses among the collection on 21 days old plants as continuously kept at 95% field capacity, at 5 days without irrigation reaching 55% field capacity, and at 14 days of partial submergence, respectively. The assessment encompassed a comprehensive set of 20 traits, including gas exchange, chlorophyll a fluorescence, leaf photosynthetic pigments, leaf water status and biomass partitioning. This detailed phenotypic analysis demonstrated that leaf relative water content and gas-exchange related traits are particularly useful to differentiate accessions’ response to water treatments. We discovered that accessions with light and large seeds mostly of Mediterranean and East European origins produce plants with high total dry biomass, a trait that is mostly genetically determined and less affected by the water treatment. On the other hand, it is mainly dark and small seeded accessions that show advantageous physiological responses to water deficit and/or waterlogging, according to pigment contents, root to shoot ratio and water use efficiency traits. This integrative screening also allowed the identification of accessions displaying contrasting levels of tolerance and susceptibility to each stress (five tolerant and five susceptible to each stress), plus one multi-tolerant (PI283546) and one multi-susceptible (PTLS1001) accession. A comprehensive and deep understanding of grass pea’s water stress tolerance mechanisms is the first step for a more efficient exploitation of this grain legume as a model crop for multiple abiotic stress resistance studies and opens doors for further progresses on crop breeding efforts towards securing food and feed supplies.