Characterization of Flowering Response to Photoperiod in Diverse Soybean Genotypes 1

Photoperiodic response of soybean [ Glycine max (L.) Merr.] a major factor determining the latitude at which soybean cultivars are grown. Soybean cultivars adapted to temperate North America are often unproductive when grown under short‐day conditions because flowering and reproductive growth are initiated before adequate vegetative growth has occurred. To compare initiation of flowering in response to photoperiod in soybean, a total of 12 soybean genotypes were evaluated under constant photoperiodurations ranging from 11 to 20 h per 24 h. The soybean lines included PI 274454 and PI 159925 which, as a result of previous field tests, had been identified as having delayed flowering under short‐day conditions. In addition, a number of cultivars from a spectrum of maturity classifications were tested. In constant photoperiods of less than 14 h, PI 274454, PI 159925, and lines derived from crosses with PI 159925 flowered significantly later than any of the other genotypes tested including the normally late maturing cultivars ‘Biloxi’ and ‘Jupiter’. The regression of days to first flower on photoperiod duration was used to characterize the photoperiod response of each genotype. The soybean lines were placed in three response categories: those with no response, those with only linear response, and those with both significant linear and quadratic response. The genotypes characterized as late flowering under short‐day conditions showed only linear response. Cultivars such as ‘Maple Arrow’, ‘Williams’, ‘Hill’, ‘Tracy’, and ‘Forrest’, typical of soybeans grown in much of temperate North America, have both linear and quadratic response.