LED-Driven Rapid Generation in Soybean: Photoperiod and Spectral Effects

This study evaluated LED-supported speed-breeding conditions for two registered soybean cultivars (‘Sonya’ and ‘Victoria’; approximately Maturity Groups III–IV) by testing four constant photoperiod regimes (10, 14, 18, and 22 h light) combined with four LED spectral treatments differing in high red (HR; 640–660 nm), deep blue (DB; 450–460 nm), far-red (FR; 720–730 nm), cool white (CW; 6500 K; 400–700 nm) and warm white (WW; 3000 K; 400–700 nm) components. Photoperiod treatments were implemented as sequential runs in a controlled growth-chamber (28 ± 2 °C; ≥50% RH; 400–600 µmol·m−2·s−1 PPFD), and performance was compared with a field trial in Antalya, Türkiye. Developmental timing (R1, R6), plant height, stem dry weight, pod number, seed oil concentration, fatty acid composition, and seed germination were analyzed using mixed-effects and generalized linear mixed models. Photoperiod exerted dominant control over developmental timing. The 10 h photoperiod regime produced the fastest progression (R1 ≈ 24.5 DAS; R6 ≈ 50 DAS), whereas 18 h markedly delayed R1 and R6 (with one LED treatment within 18 h failing to reach R6), and 22 h did not reach R6. LED spectral treatment did not affect R1 or R6 within photoperiods, but influenced plant height, stem dry weight, total seed oil, and germination. Field-grown plants produced substantially higher biomass and pod number than growth chamber-grown plants, consistent with greater branching and pod set under open-field, direct-sown conditions. Total seed oil concentration differed by environment and cultivar with no environment × cultivar interaction, while centered log-ratio PERMANOVA indicated largely stable fatty acid proportional composition across environments and LED treatments. Germination analyses (restricted to 10 h and 14 h due to insufficient seed production) showed significant effects of LED treatment and harvest timing, with the highest germination under LED–3. Overall, our results indicate that a constant 10 h photoperiod, combined with immature seed harvest from approximately R6 + 20 onward, provides an effective strategy for accelerating soybean generation turnover while maintaining high germination capacity. LED spectral design can also be used to modulate plant architecture and seed quality without substantially altering developmental timing.