SOYBEAN FLOWERING IN RESPONSE TO THE LONG-JUVENILE TRAIT

A long-juvenile trait that delays flowering has been identified in soybean [Glycine max (L.) Merr.l. This trait may be especially important in extending the range of adaptation of soybean germplasm to lower latitudes and to new management schemes with shifted sowing dates. These studies were undertaken to characterize more fully the physiological consequences of the long-juvenile trait. Isogenic pairs were developed and subjected to three experimental tests: (i) sowing date response, (ii) extended photoperiod response, and (iii) reciprocal grafts. Analysis of the flowering response under a range of sowing dates showed that the main effect of the long-juvenile trait was to retard the overall development of the plants toward flowering. Even though the long-juvenile trait tended to stimulate flowering at longer photoperiods than the normal isoline, the slower development associated with the long-juvenile trait delayed the mean flowering date by 7 to 19 d for the 15 isogenic pairs studied. Experiments under a 15-h photoperiod confirmed that development rate towards flowering of the long-juvenile isolines declined less than that of normal lines. The threshold illuminance for responding to the extended photoperiod was also significantly greater in the long-juvenile isolines compared with the normal lines. Reciprocal grafts showed that the relative number of long-juvenile leaves on a plant was associated with the delay in flowering. Since the long-juvenile leaves altered flowering in the normal part of the grafted plant, expression of the long-juvenile trait is apparently associated with a translocatable material.