Lack of information on the mode of inheritance of photoperiod sensitivity is a constraint to genetic improvement of tropically adapted vegetable runner bean. This study was conducted to determine the inheritance of short-day photoperiod in runner beans. Seven single crosses between female parent; White Emergo (long day imported variety) and seven short day local landraces (Kin 1, Kin 2, Kin 3, Nyeri, Dwarf1, Dwarf 2 and Dwarf 3) were developed in Kenya. The parents, F1, F2 and backcrosses were

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... valuated at Kabete (warm conditions) and Ol Joro-Orok (cool condtions) under the natural short-day length of 12hrs. From the results, short day parents flowered earlier (within 40-48 days) and formed more racemes (on average 10 racemes/plant) and pods (at least 25pods/plant) than long day parent (White Emergo) at both locations. The populations in the seven cross combinations flowered earlier at the warmer location (Kabete) than cooler one (Ol Joro Orok). In all crosses, F1 and F2 means of days to flowering and number of racemes were within parental range while the backcrosses' showed means that were close to their recurrent parents. The additive-dominance model [m+a+d] was found to adequately explain the genetic influence on studied traits with additive gene effects accounting for about 90% of the genetic action. The predominance of additive gene action indicates that improvement of this crop for short day adaptation can be easily achieved by trait integration through hybridization followed by pure selection methods such as single seed descent, bulk breeding or pedigree.