Shoot Meristem Activity during Floral Transition in Glycine max (L.) Merr.

Abstract

The soybean (Glycine max [L.] Merr.) is a quantitative short-day (SD) plant requiring two inductive cycles for floral initiation, which occurs first in the most undifferentiated meristem in an axil of a main stem leaf. Floral initiation at the main stem apex, however, requires additional SD inductive cycles. Under continuous SD the transition to flowering in the main stem apex is completed after 8 SD cycles. Differentiation and organogenesis of the first flower in the terminal raceme is apparent after 10 SD cycles. The changes in apical size and geometry, nuclear DNA, and rate of leaf initiation were followed daily during this 10-d period and compared with apices from plants kept under noninductive long days (LD). At emergence all plants had initiated three trifoliolate leaf primordia and during the vegetative stage of development maintained a plastochron of 2.0 d/leaf. The plastochron was shortened to 1.0 d/leaf in SD plants on day 7, just prior to the end of the transition. Apical size and geometry remained unchanged until after 6 SD cycles when height of the dome decreased and there was less elongation of the rib meristem. Earlier events included significantly lower amounts of nuclear DNA in cells of SD apices after 1 and 3 SD cycles. Later, the amount of nuclear DNA increased in cells of SD apices beginning after 5 SD and peaking after 6 SD before decreasing back to control levels. Shifts in increasing proportions of the population of nuclei from the 4C to 2C condition occurred after 1 SD and 3 SD. As in other species, both of these shifts are apparently essential components for the floral transition at the shoot apex in soybean. The first shift, or "mitotic" stimulus, signals that the process of the floral transition has begun, while the second shift, or "floral" stimulus, is required for completion of the process.