Speed Breeding of Soybean by Using 22 h Photoperiod Increases Photochemical Efficiency of Pods and Produces Six Generations Per Year

Fast generation cycling of plants has the potential to overcome the bottleneck of traditional breeding programmes, which often require several years to achieve the desired outcomes. Recent speed breeding methodologies have reduced generation times in both short- and long-day species by optimizing environmental conditions. However, protocols for short-day plants impose a constant short-day photoperiod throughout the entire life cycle, even though plants could benefit from extended light exposure. Here, we report a speed breeding scheme for soybean (Glycine max) based on a long-day photoperiod of 22 h (LD-22 h) applied upon flowering initiation (stage R1) using light-emitting diodes (LEDs) with a cool white (6000 K) and red light (660 nm) spectrum at 550 μmol/(m²s) photosynthetic photon flux at canopy level. We also outline an immature seed germination technique for early harvested green seeds collected from speed-breeding plants that markedly increased the germination rate. Combining these methods allowed our soybean speed breeding system to acquire a 92% germination rate from 58-day-old seeds, enabling six generations y⁻¹ compared to typically only 1–3 using standard approaches. The impact of long photoperiods on soybean leaf and pod photochemical efficiency was examined. Although photosynthetic capacity (Vc_max, J_max, and A_max) was significantly lower in leaves grown under LD-22 h photoperiod, seed production was unaffected, while PSII operating efficiency (F_q′/F_m′) in pods was markedly higher under LD-22 h compared to the SD-10 h photoperiod. Implementing our post-flowering long photoperiod conditions followed by an enhanced germination technique could facilitate rapid breeding for soybeans and be adapted for use with other photoperiod-sensitive short-day crops.