DETERMINATION OF FLORAL ORGAN TYPE IN CULTURED SILENE SHOOT APICES

Shoot apices of the long day plant, Silene coeli-rosa, were cultured o n a basal medium (+3% sucrose) in non-inductive short days (SD) follow ing their excision from plants which had been exposed to long day (LD) treatments in order to examine the period for determination of each f loral whorl. In response to the inductive LD treatments, the pattern o f whorl formation in vitro reflected their normal appearance in Silene : sepals, stamens 1-5, petals, stamens 6-10 and carpels, although the number of apices initiating each whorl was lower in vitro compared wit h apices in vivo. However, supplementing the medium with 7 instead of 3% sucrose corrected this deficiency and, for the first time. resulted in apices initiating floral whorls in SD. The interval between the sh ortest treatment to result in whorl initiation in vitro, 4 LD (which a lso resulted in 50% flowering in vivo), and the treatment which gave 5 0% initiation of the corresponding whorl in vitro, was taken to be the period for determination of that whorl. The determination times on th e 3% medium were: sepals (2 days), stamens 1-5 (3 days), petals (3 day s), stamens 6-10 (4 days) and carpels (4 days); all of these periods s hortened to about 1 day on the 7% medium. Tissue culture did not pertu rb the pattern of initiation of each whorl since apices excised and cu ltured from plants which had received 7 LD + 2 SD, exhibited each whor l over the same time scale as those of intact plants which received th e same treatment. The data are consistent with a sequential determinat ion and initiation of each whorl in the order that they appear normall y in Silene. Synchronisation of cell division, as represented by peaks of the mitotic index and G2/G1 ratios on day 8 (7 LD + 2 SD), did not occur in vitro but the mitotic index did not descend to zero, further emphasising that tissue culture did not perturb the Silene apex.