EXPERIMENTAL CONTROL OF FLORAL REVERSION IN ISOLATED SHOOT APICES OF THE LONG-DAY PLANT SILENE COELI-ROSA

The fate of shoot meristems of the long day (LD) plant Silene coeli-ro sa in culture was examined (complete, reverted or arrested flowers) to establish whether these different patterns were related to a particul ar stage of morphogenesis and the extent to which the fate of the patt ern was regulated by either added plant growth regulators (PGRs) or ch anging the carbohydrate source in the medium. In particular, the frequ ency of reversion was measured to test the stability of the determined state for each whorl. The plants were given various inductive treatme nts (4-7 LD, 7 LD + 1 to 3 SD) and the apices were explanted onto Mura shige-Skoog medium supplemented with 3% sucrose (controls) +/- IAA, +/ - kinetin, +/- GA(3) or onto the basal medium containing 1 or 3% sucro se, glucose or fructose or 7% sucrose. The apices were examined 12 wee ks later. When the data were pooled from all inductive treatments, IAA resulted in more reversions, GA(3) caused more arrested flowers while kinetin hardly affected the pattern of meristem fate compared with th e controls. However, each PGR treatment did not perturb the pattern of organ formation for those apices that formed either arrested or compl ete flowers. The time for determination (days) of the earlier formed w horls (determination times for the controls in brackets): sepals (2), stamens 1-5 (3) and petals (3), was shortened by about a day in all PG R treatments whereas the corresponding times for the later formed whor ls: stamens 6-10 (4) and carpels (4), were either lengthened to 5 days or unaffected. The response of the apices to the various sugars was s imply a reflection of concentration. Hence, more complete flowers form ed at 7 or 3% and more flowers were arrested at 1% regardless of the s ugar moiety. However, the frequency of reversion was similar on each o f the media. Pooling all data from all treatments enabled a statistica l analysis of the pattern of reversion and the pattern of arrest. Reve rsion was more common from apices which exhibited the later-formed who rls (stamens 6-10 and carpels) than from the earlier whorls. Moreover, the stronger the inductive treatment the less frequent was reversion. The most common stage of arrest was at the stamen 6-10 whorl and this was particularly so for the GA(3) treatment. The data indicated that reversion could occur from any whorl, which suggests that determinatio n of each whorl is independent of the next. This conclusion is underli ned by the more frequent occurrence of reversion the carpel whorl. How ever, the longer the inductive treatment the less likelihood of revers ion; this suggests that in Silene, the floral stimulus is required con tinuously to stabilise the determined state of each whorl and to ensur e smooth completion of floral morphogenesis.