Disruption by Temperature of Floral Evocation and Cell-Cycling in the Shoot Apical Meristem of Helipterum roseum

Abstract

Flowering is inhibited in plants of Helipterum roseum grown under constant 25°C temperature conditions with a 12 h photoperiod and irradiance of 250 W m-2, but not at a constant temperature of 20°C. Floral inhibition was investigated by transferring plants between the two temperature con- ditions at different times to determine the morphological stage of inhibition, and by investigating cell-cycling at the shoot apex at the two temperatures. Floral initiation in Helipterum roseum was inhibited if the temperature increase from 20 to 25°C occurred at the doming of the apical meristem, and was delayed when the increase occurred at the initiation of involucral bracts. Steady-state cell-cycling was observed in the shoot meristem at 20°C and the cell-cycle duration was estimated at the morphological stages of large vegetative meristem, doming of the meristem and initiation of the involucral bracts. The length of the cell-cycle at these stages was 64 h, 41 h and 47 h respectively. Steady-state cell-cycling was not observed in shoot apical meristems at 25°C, and the meristem did not undergo the floral transition. It is concluded that the stage of commitment to flower is the initiation of involucral bracts, and that floral initiation is inhibited at 25°C by the loss of steady-state cell-cycling at the shoot apex.