Photoperiod control of yield and sink capacity in Bambara groundnut (Vigna subterranea) genotypes

In Bambara groundnut [Vigna subterranea (L) Verdc], long photoperiods can cause a decline in pod and seed number. While negative photoperiod effects on pod yield have been reported, positive effects and the existence of genotypes less sensitive for photoperiod have not been reported. Ten geographically diverse genotypes were evaluated over 3 years under short and long photoperiod for yield‐related traits (pod number, pod weight, seed number, seed weight, and 100 seed weight), pod growth habit (geocarpic or ageocarpic pods), peduncle elongation and soil penetration, and pod set/abortion. Anatomical sections of gynophores and embryonic pod development of a highly photoperiod sensitive genotype “Ankpa‐4” from Nigeria were examined. A strong photoperiodic effect on yield component traits (pod number, pod weight, seed number, seed weight, and 100 seed weight) was observed and with considerable variation between genotypes. Three classes of photoperiod effect on pod yield were identified, (a) qualitative short‐day types; (b) quantitative short‐day types; and (c) quantitative long‐day types. In long photoperiods, above‐ground vegetative biomass and the length of lateral branches in some genotypes increased by at least twofold. Morphological and anatomical characterization of gynophores and developing pods of the most sensitive line shows continuous geocarpic growth, but with healthy embryonic pods failing to enlarge after soil penetration. Results from the yield patterns of the three phenotypic classes confirm that pods and seeds are high priority sinks, and long photoperiod alters the balance in assimilate distribution between competing yield forming processes and vegetative sinks. By exploiting the genetic variation characterized here for photoperiod requirement for pod filling, the geographical range of this crop could be extended beyond current growing regions. In the near future, we anticipate that Bambara groundnut will become a significant contributor to global food, nutritional, and environmental security once these photoperiod issues are resolved. A strong photoperiodic effect is observed on the yield‐forming processes in Bambara groundnut. Manipulation of photoperiod increases yield in newly identified quantitative long‐day genotypes. Manipulating photoperiod requirement will enable breeding to expand the geographical range of this drought tolerant legume crop.