Rapid generation advancement in soybean: approaches, constraints, and the stage-dependent photoperiod window

Long generation intervals constrain realized genetic gain in soybean ( Glycine max (L.) Merr.) breeding. Speed-breeding protocols developed for long-day and day-neutral crops use an extended photoperiod throughout the entire life cycle. They are not directly applicable to soybean, because the E1 family represses the florigen genes GmFT2a and GmFT5a under long days and delays flowering. Rapid generation advancement in soybean therefore depends on a stage-dependent photoperiod window. Before flowering, the photoperiod (10–12 h in published stage-dependent protocols) must remain below the genotype-specific critical photoperiod, and an extended photoperiod can be introduced only after flowering (R1 stage), to accelerate seed maturation. Optimized continuous short-day systems are reported to allow up to five generations per year, and six are reported by one study that combined a post-flowering 22-h photoperiod with germination of immature seed. These figures come from different studies, genotypes and facilities. To our knowledge, no study has yet compared cycle length under a post-flowering long day with a corresponding short-day control in the same genotypes. A post-flowering 22-h photoperiod also reduced leaf photosynthetic capacity, and applying the same early harvest date to all plants may change allele frequencies in segregating populations. This review summarizes soybean speed-breeding protocols, their infrastructure requirements, and their physiological and genetic limitations. It also evaluates the integration of these protocols with marker-assisted selection, genomic selection and genome editing. Early seed harvest and high-density planting can be transferred between crops, but the photoperiod regime must be tailored to soybean. We conclude that the evidence supports soybean speed breeding as a method for accelerating generation turnover, and that phenotypic selection in the chamber needs to be validated in the field. Published controlled-environment soybean protocols do not report changes in allele frequency during accelerated advancement or genetic correlations between traits measured in the chamber and in the field. Both are needed to show whether rapid cycling translates into genetic gain in the field.