FLORAL EVOCATION AND THE CONTENT OF CARBOHYDRATES IN STEM APICES OF RUDBECKIA BICOLOR AT FLORAL TRANSITION IN COMPLETE DARKNESS

When long-day rosette plants of Rudbeckia bicolor Nutt. grown under a noninductive short-day regime were transferred to complete darkness an d kept continuously under these conditions, they formed stems and bega n the transition to flowering. Cytological observations helped disting uish the first stage of this transition, floral evocation. At the begi nning of evocation, on the first day in complete darkness, i.e., in th e absence of photosynthesis, the contents of glucose, sucrose, and fru ctose in the stem apices dramatically increased (glucose and sucrose b y seven times and fructose by four times) and then dropped to a minimu m after five days. Thereafter, the content of glucose increased, while the contents of sucrose and fructose remained at a constant level. Si multaneously, starch disappeared from the medullar and pith zones of t he stem apices. The results obtained indicate that the increased sugar content in stem apices is characteristic of floral evocation, indepen dent of photosynthesis and photoperiodic floral induction. Both basal tissues and the medullar and pith zones of the stem apices appear to b e a source of soluble sugars, as evidenced by rapid starch mobilizatio n in these tissues.