Optimizing light environment enables speed breeding in forage legumes: physiological limits and generation time reduction in Medicago sativa and Medicago truncatula

Climate change and increasing global demand for animal products are intensifying the need to accelerate genetic improvement of forage crops. Speed breeding (SB) has emerged as a powerful tool to shorten generation cycles; however, its application in perennial and autotetraploid forage legumes remains limited, particularly regarding reproductive performance and physiological constraints. Here, we optimized photoperiod, light intensity, and light quality to accelerate the life cycle of Medicago sativa (alfalfa) and its diploid relative Medicago truncatula under controlled conditions. We evaluated flowering, fruiting, seed harvest time, seed set, and germination across independent and combined SB treatments, and assessed photosynthetic performance to identify potential physiological trade-offs. Blue- red light supplementation, moderate-to-high irradiance (450 {micro}mol m-2 s-1), and extended photoperiods significantly accelerated reproductive development in both species, although optimal combinations differed between the diploid and autotetraploid backgrounds. A combined SB regime (20/4 h photoperiod at 450 {micro}mol m-2 s-1) reduced time to harvest by 17% in M. sativa and 28% in M. truncatula, while maintaining viable seed production. Chlorophyll fluorescence analysis revealed a higher photosynthetic plasticity in alfalfa compared with M. truncatula, indicating species-specific physiological limits to SB intensification. Our results establish practical SB conditions for alfalfa and an agronomically relevant M. truncatula genotype, providing an enabling platform to accelerate breeding cycles and trait evaluation in forage legumes.