LAUSR

Abstract Faba bean is a cool-season, indeterminate grain legume grown in Mediterranean and temperate environments where yield loss associated with abiotic and biotic stress is common. Yield responses depend on the timing of stress relative to the species-specific vulnerable stages in the crop lifecycle. To determine the critical period for yield determination in faba bean, we used sequential 14-d shading periods in locally-adapted varieties grown in five environments with yield of unshaded controls from 2.3 to 6.8 t ha−1. Yield of shaded treatments diverged from the controls around 450 °Cd before flowering, reached the largest difference around 100 °Cd after flowering, and converged with controls towards 700–800 °Cd after flowering; the most critical stage aligned with pod emergence. Seed number accounted for most of the variation in yield response to shading. Shading increased seed size when stress severely reduced seed number. Pod number accounted for most of the variation in seed number for shading before flowering, and both pod number and seeds per pod contributed to the reduction in seed number in crops shaded after flowering. The defined critical period is a useful reference for breeding and agronomic solutions to improve yield under stress.