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

## TL;DR

This study develops a protocol to shorten rice growth cycles in plant factories by optimizing light, planting density, and phytohormone use.

## Contribution

The study introduces an integrated protocol combining LED light regimes, planting density, and GA3 to accelerate rice breeding in controlled environments.

## Key findings

- High planting densities (>50 plants/tray) prolonged growth cycles and reduced yield.
- GA3 treatment at 60 ppm increased immature seed germination by 31%.
- Optimized light regimes and planting density shortened generation time to 54 and 70 days for Nipponbare and Wufeng B.

## Abstract

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.