Optimizing Daily Light Integral in Seedling Stage Accelerates Heading and Flowering in Wheat Under LED Lighting

Conventional wheat breeding in the field is limited to two generations per year and is susceptible to environmental fluctuations. Speed-breeding in a plant factory offers a solution; however, recommendations for lighting strategies remain limited. To identify the optimal daily light integral (DLI) for the seedling stage, we tested three light intensities (300, 500, and 700 μmol m^-2 s^-1) and four photoperiods (10, 14, 18, and 22 h d^-1), resulting in DLIs ranging from 10.8 to 55.4 mol m^-2 d^-1. Our results indicated that an optimal DLI of 39.6 mol m^-2 d^-1 was associated with the highest seedling index (0.26) and root-to-shoot ratio (0.42), as well as enhanced photosystem performance. Beyond this DLI, these parameters and shoot biomass plateaued as the DLI increased. Moreover, treating seedlings with this optimal DLI of 39.6 mol m^-2 d^-1 (using 500 μmol m^-2 s^-1 light intensity and a 22 h d^-1 photoperiod) resulted in heading and flowering 5.9 and 7.5 days earlier after transplanting, respectively, than those under the lowest DLI (10.8 mol·m^-2·d^-1). This study established a lighting strategy to produce high-quality seedlings and accelerate heading and flowering, thereby offering a valuable physiological framework for advancing speed-breeding systems in wheat.