Nectar Production in White Mustard Is Associated with Carbon–Water Physiology Under Contrasting Water Regimes

Climate change-driven water limitation is an increasing constraint in agricultural systems. We investigated nectar traits in two white mustard (Brassica alba L.) cultivars under contrasting soil-water regimes in a two-year field experiment, examining links among water regimes, plant physiology, and nectar production. Drought significantly decreased nectar mass by 40% but increased sugar concentration by 26.6% relative to the control treatment. Nectar mass nevertheless showed substantial inter-annual variability. Within individual water-regime treatments, nectar mass maintained a stable association with transpiration across all regimes, whereas its link with photosynthesis was treatment-specific, becoming significant exclusively under drought. In contrast, sugar concentration showed no direct relationships with instantaneous gas exchange parameters within water regimes, demonstrating that overall correlations were driven by collective treatment responses rather than direct physiological relationships. Overall, nectar production in B. alba is linked to the interplay between carbon assimilation and hydraulic regulation, with distinct physiological parameters associated with nectar mass and concentration. Our findings indicate that water regime effects on nectar production are largely associated with plant physiological responses, particularly limitations in photosynthetic carbon gain under drought, with potential consequences for pollinator visitation and crop yield requiring direct testing under future climate scenarios.