DIFFERENTIAL RNA-SYNTHESIS IN THE SHOOT APEX OF PHARBITIS-NIL DURING FLORAL EVOCATION

Early events in the evocation of the flower in Pharbitis nil Chois seedlings were investigated by following the incorporation of tritiated uridine into the shoot apex. The uridine was applied to the expanded cotyledons of seedlings at 8 hr into the inductive dark period. The shoot tips were fixed at 20 hr (a 12-hr labeling period). After the 12-hr labeling period there was considerable label throughout the shoot tips of both control plants (dark period interrupted with 5 min of red light at 8 hr) and plants induced to flower. Both RNase and acid hydrolysis removed the nonexchangeable label and the Azure B staining, thus leading to the conclusion that the uridine was incorporated into RNA. Induction in the cotyledons was followed quickly by an increased synthesis of RNA in the rib meristem region of the receptor bud at the time when the floral stimulus is assumed to be arriving. The increase in RNA synthesis is revealed by an increase in the rib meristem/central zone ratio of counts due to the incorporation of tritiated uridine. A comparison of counts in each of the two regions revealed that the change in ratio was due to an increase in the rib meristem and not due to a decrease in the central zone in induced shoot apices. The initial activation of the rib meristem probably occurred by 16 hr from the beginning of the dark period. Tendencies in the literature to disregard the role of the rib meristem in giving rise to part of the flower are discussed.