Long Juvenile Soybean Flowering Responses under Very Short Photoperiods

Long juvenile (LJ) soybean [Glycine max (L.) Merr.] lines exhibit delayed flowering under short photoperiods. Two loci, J and E6, have been reported to control this response. The objectives of this work were to compare phenology of E6E6 and e6e6 isolines across a range of photoperiods, to determine the genetic control of the LJ trait during the introgression of e6 into an early maturity background, and to compare E6E6 and jj lines in short photoperiods. Flowering time of 'Paran' (E6E6) and its LJ isoline 'Paranagoiana' (e6e6) was observed in photoperiods of 3 to 16 h. Both lines flowered late (about 100 d) in a 16 h photoperiod. Parana flowered in less than 30 d in 6, 8, 10, and 12 h photoperiods. Paranagoiana responded to decreases in photoperiod from 12 to 4 h with earlier flowering, although always flowered a minimum of 5 d later than Parana. The LJ trait in Paranagoiana was introgressed into OT94-47 with selection for late flowering under 12 h photoperiods. From the initial cross to the third backcross, F(2) populations exhibited a 15: 1 early: late flowering ratio in 12 h photoperiods. A similar 15: 1 ratio was observed in a cross between OT94-47 and LJ PI 159925. In comparisons, under 3 to 12 h photoperiods, Paranagoiana and PI 159925 flowered similarly and later than Parana. It appears that the LJ lines may have a 5 d juvenile period but flowering delays greater than 5 d are likely due to photoperiod responses to very short photoperiods (>= 5 h).