Interruption by light of a non-inductive long dark period in LDP can, under the right conditions lead to floral promotion. However, only a few species of LDP can be induced by a single light break of under 30 min. In this respect they differ significantly from SDP. The initial action spectra for induction in LDP were based on the night-break response of barley and Hyoscyamus (Borthwick et al., 1948; Parker et al., 1950). These were obtained by growing the plants on marginally inductive photoperiods of 11.5 or 12 h and induction did not occur when night-breaks were given in combination with 8 h short days. Even with marginal daylengths repeated cycles of treatment were needed to obtain a reasonable flowering response. Evans (1964a), working with the Ceres strain of Lolium temulentum which required only one long day for induction, was unable to show inflorescence initiation in a single plant out of 550 which were exposed to a 2 h night-break in the middle of a 16 h dark period. However, 5 min R given 10 h after the end of the main light period did cause some induction when the photoperiod was extended with 4 or 6 h tungsten filament (TF) light which was, in itself non-inductive. Under conditions which allow induction of flowering by night-breaks, 1-2 h is usually sufficient to induce flowering but may not saturate the response (Hughes and Cockshull, 1969; Vince, 1965). The responses of LDP to a night-break are frequently semiquantitative in nature (see Fig. 1.7). Reversibility of the night-break response is also problematical; only partial FR reversal of the promotion of flowering by R could be demonstrated in barley and Hyoscyamus (Downs, 1956; Lourtioux, 1961) and to the knowledge of the authors no action spectrum for night-break reversal in LDP has been published. From the limited reversibility studies carried out it can be concluded that phytochrome is the photoreceptor for the night-break response in most LDP, although blue light night-breaks are the most effective for Sinapis and maybe other cruciferous plants (Hanke et al., 1969).

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