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To predict time to flowering in field-grown soybean [Glycine max (L.) Merrill], it is essential to know how floral initiation and development are affected by changing daylengths. There is some evidenc...e that both the rate of change in daylength and the direction of that change affect flowering. The purpose of this research was to establish how increasing and decreasing daylengths of various magnitudes influenced progress toward floral initiation (R0) and progress in floral development (R0 to R1). Seeds of 'Johnston' (MG VIII) and 'Clark' (MG IV) were sown every two weeks in a greenhouse at 39° N latitude in which temperature was controlled at 25℃ day and night. The time from the end of juvenility to floral initiation (R0) and the floral development phase (time from R0 to first open flower, R1) were recorded for each cultivar. R0 was determined by apical dissection at 45×magnification. Most of the variation in the time to floral initiation in 'Johnston' could be accounted for by a constant value when daylengths were below≈13h and a linear increase when mean daylengths exceeded 13h. The time to R0 in increasing daylengths was not different from the time to R0 in decreasing daylengths when the magnitudes of the daylengths during the floral initiation phase were similiar. The length of the floral development phase in 'Johnston' was longer when mean daylengths exceeded 13h, suggesting that the floral development phase was also sensitive to photoperiod. However, the end of sensitivity to photoperiod coincided with R0 when mean daylengths were≤13h, suggesting that photoperiod during the floral development phase did not influence development equally over the time to R1. There were small but significant increases in time to R0 and in the length of floral development in 'Clark' as daylength increased, but no effect of direction of change in daylength was apparent in either phase. Photoperiod sensitivity during floral development was not established unequivocally. At least part of the period between R0 and R1 was not sensitive to photoperiod. Time to flowering could be explained without invoking an effect of change in direction of daylength.続きを見る
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