A Protocol to Shorten Rice Growth Cycle in Plant Factories: An Integrated Study of Light, Planting Density and...

Speed breeding represents a pivotal technology for enhancing crop breeding efficiency. This study systematically examined the regulation of LED light environments, planting density, and gibberellic acid (GA₃) on rice growth cycle progression in plant factories, establishing an integrated speed breeding protocol. The experimental design comprised three components: (1) coupling seedling age (9-25 days, variety-dependent) with LED environments and planting densities (25-100 plants/tray); (2) combining light intensity gradients (450 and 900 μmol·m^-2·s^-1) with photoperiod control; (3) applying GA₃ gradients (0-120 ppm) to enhance immature seed germination. Results indicated that high planting densities (>50 plants/tray) prolonged the growth cycle and decreased yield, whereas 25 plants/tray optimally balanced growth cycle shortening and yield maximization. Under short-day induction, Nipponbare (Nip) and Wufeng B (WFB) reached heading at 39 and 58 days after sowing (DAS), respectively. Stage-specific light responses were observed: 450 μmol·m^-2·s^-1 during the basic vegetative phase (BVP) promoted morphological development, whereas 900 μmol·m^-2·s^-1 during the photoperiod-sensitive phase (PSP) accelerated tillering and panicle differentiation. GA₃ treatment (60 ppm) enhanced the germination rate of immature seeds by 31%. The optimized lightregimes comprised natural light + 900 μmol·m^-2·s^-1 (NL-900) and 450 μmol·m^-2·s^-1 + 900 μmol·m^-2·s^-1 (450-900), combined with density control (25 plants/tray) and GA₃-mediated immature seed utilization, shortened the generation time to 54 days and 70 days for Nip and WFB, respectively. This integrated protocol establishes an efficient strategy for rice speed breeding in plant factories.