The Inhibition of Flowering by Non-Induced Cotyledons of <italic>Pharbitis nil</italic>

Abstract

Inhibitory effects on flowering of a non-induced cotyledon have been examined in Pharbitis nil seedlings. The photoperiodic induction of one cotyledon was accomplished by wrapping it in aluminium foil for 13 to 15 h while the seedling remained in the light. The presence of the other cotyledon in the light blocked this inductive stimulus. The timing of its inhibitory effect suggested that its action was to block the expression of the inductive stimulus, presumably at the shoot apex. By varying the area of the non-induced cotyledon parallel inhibitory effects were shown on export of stimulus and of 14C-labelled assimilate to the apex from the induced cotyledon. Thus, part of the inhibition was by interference with assimilate/stimulus co-transport in the phloem. However, an additional inhibitory effect was also evident and for this second component there was no relationship between assimilate and stimulus transport. This latter inhibition was generated by brief light interruptions of darkness given to one cotyledon only whilst the other was wrapped. The control treatment, removal of the unwrapped cotyledon, did not alter flowering compared to seedlings with intact, darkened cotyledons. Thus, these studies show that the brief night interruptions acted to trigger a photoperiodically sensitive inhibitor not to block induction. The implications of these findings are discussed in relation to models of time measurement in the photoperiodic control of flowering.