Water-carbon trade-offs in contrasting Lupinus angustifolius cultivars under combined deficit irrigation, heat stress, and elevated CO2

• Mirabor sustained yield via conservative water use; Primabella did not. • Yield stability under combined stress in eCO 2 linked more to WUE than photosynthetic capacity. • e CO 2 improved WUE, but biomass and yield responses varied by cultivar. • Grain protein and carbohydrate remained stable under eCO 2 and stress. • Grain alkaloids remained stable in tolerant Mirabor but increased sharply in sensitive Primabella. To address climate change and nutritional security, Lupinus angustifolius was used as a model high-protein legume to quantify physiological, biomass, and yield responses to abiotic stress under future climate scenarios. Heat-tolerant ‘Mirabor’ and heat-sensitive ‘Primabella’ were grown under ambient and elevated CO₂ and exposed to single (deficit irrigation) and combined (heat + deficit irrigation) stress. Physiological performance was assessed using gas exchange and chlorophyll fluorescence, while yield components and grain nutritional composition, including quinolizidine alkaloids, were determined at harvest. A key objective was to investigate the role of heat stress tolerance in lupin under combined heat and deficit irrigation stress. Our results indicate that the heat-tolerant cultivar showed a degree of physiological cross-tolerance, maintaining higher water use efficiency under combined stress, especially under e CO 2 levels compared to the heat-sensitive cultivar (WUE i of 0.205 and 0.146 μmol mmol -1 , respectively). Furthermore, the cultivars appeared to employ distinct coping strategies: the heat-tolerant cultivar prioritized water conservation at the expense of some physiological traits (e. g. A n -32.6 %), while the heat-sensitive cultivar better maintained photosynthetic activity (A n -12.4 %) but at a higher water cost. Our findings suggest that under combined heat and deficit irrigation stress, conserving water is a more advantageous strategy for sustaining biomass and yield traits. This work underscores the importance of integrative phenotyping for identifying climate-resilient legume cultivars to support sustainable plant protein production in future environments.