The possibility that phytochrome is involved in the promotion of flowering by far-red light was investigated. The addition of far-red (FR) to a day extension with red (R) light promotes inflorescence initiation in Lolium. A 2-hour interruption with darkness also promoted flowering compared with the uninterrupted red light control; apex length was further increased by a 10-minute FR irradiation given before the 2-hour dark interruption and was decreased by 10-minutes of R light given in the middle: both FR promotion and R inhibition were reversed by R and FR respectively. Apex length increased approximately linearly with increasing duration of dark interruption up to at least 21/2 hours. When varying ratios of R :FR light were substituted for a 2-hour dark period, apex length was increasingly depressed as the % R was increased above 25% ; no difference between 25% R/75% FR and 100% FR could be detected. Apex length was inversely linearly related to the calculated [Pfr]/[P] ratios above about 40% Pfr.
FR promoted flowering when given during a 5-hour interruption of a day extension with R light but, between 0.25 and 0.90 J m 2 s -1, there was no effect of intensity of FR; at 0.11 g m-2s -1 apexlength was shorter than at 0.25 J m -2 s -1 but longer than in darkness. When the duration of FR (from the beginning of a dark interruption of a day extension with R) was varied, apex length increased with increasing duration of FR up to 11/4 to 2 hours but further increasing the duration of FR did not promote flowering more.
The results implicate phytochrome in the promotion of flowering by FR light. It has been demonstrated that a low [Pfr]/[P] ratio (less than present in 25% R/ 75% FR) is needed over a relatively long period of time: this explains why a relatively high proportion of FR light must be added to R for several hours in order to give maximum promotion of flowering. It is concluded that, in Lolium, the increased flowering response to FR light is brought about by a reduction of [Pfr]/[P] ratio at the appropriate time, although the possibility that another effect of far-red is also involved has not been rigorously excluded. 1969) the m e c h a n i s m b y which far-red light promotes flowering is far from being understood. F r o m time to time several explanations have