Abstract
Seven soybean varieties, five from low latitude regions and two from Japan, were treated by various photoperiod ranging from 12 to 14 h. Number of days to flowering (NDF) as well as number of stem nodes (NSN) was increased exponentially with the increase of photoperiod. Thus, NDF and NSN at 12h photoperiod (N12) and increasing rate (IR) of NDF and NSN due to photoperiod were estimated by means of exponential regression. No substantial difference was found in IR between Japanese and low latitude adapted varieties. On the contrary, Japanese varieties exhibited smaller N12 than low latitude adapted varieties. Five soybean varieties, three from low latitude regions and two from Japan, were sown at the interval of 20 days from May 21 to August 9. NDF and NSN decreased as the plants were sown later. Daily natural photoperiod was averaged during the period from emergence to flowering for obtaining "mean photoperiod" for each variety and for each sowing date. NDF and NSN increased exponentially with the increase of the mean photoperiod. NDF and NSN under the mean photoperiod of 14 h (Nl4) and IR were estimated for each of the varieties. As in the case of the previous experiment, no substantial difference was found in IR between low latitude adapted varieties and Japanese ones, whereas low latitude adapted varieties exhibited larger N14 than Japanese varieties. The significance of IR and N12 or N14 in photoperiodic response was discussed in relation to the adaptability of soybean varieties. The larger N12 or N14 of low latltude adapted varletles observed in the present experiment indicated that adequate vegetative growth could be ensured in those varieties even under short photoperiod in low latitude regions.
