AGE-DEPENDENT COMPETENCE FOR PHOTOPERIODIC FLORAL INDUCTION OF THE STEM APICAL MERISTEMS IN BICOLORED CONEFLOWERS

The effect of photoperiodic induction by long days (LD), the number of which increased on stem apices of long-day rosette plants Rudbeckia b icolor Nutt of various ages was studied. Stem apices of the seedlings already had the competence for photoperiodic induction of flowering; t his competence increased with plant age. The area of the stem apices a nd the number of apical cells increased with age and with the duration of photoperiodic induction by LD. The mitotic index was the highest i n the seedling apices; it decreased as the plants grew under short day (SD) conditions, and increased under LD conditions. Hence, neither th e threshold stem apex size, the number and size of apical cells, nor a ny other structural feature determine a stem apex's competence for pho toperiodic induction by LD in coneflowers.