THE REGULATION OF FLOWERING IN LONG-DAY PLANTS
Under natural conditions long-day plants flower with long days and short nights and short-day plants flower with short days and long nights. It is well known that one of the most important factors which controls the flowering response to a succession of light/dark cycles is the length of the uninterrupted dark period; this was demonstrated by the observation that a long night becomes ineffective, or at least is less effective, if a relatively short light break is given near the middle. This is true of both long-day and short-day plants and of other responses to day length such as the onset of dormancy. The action spectrum for this night-break was found to be similar to that for other phytochrome mediated responses such as germination, with maximum effectiveness in the red part of the spectrum near 660 nm, and loss of the red effect when followed by far-red light at 730 nm (4). The same action spectrum was found in long-day plants, where a night-break promotes flowering, and in short-day plants, where it inhibits flowering (5). It has generally been concluded from such experiments that the Pfr of phytochrome is required for flowering in long-day plants and inhibits flowering in short-day plants. It is assumed that during long dark periods a level of Pfr is established, by thermal reversion of Pfr to Pr, which no longer prevents flowering in short-day plants and is too low to promote flowering in long-day plants. In short-day plants phytochrome reversion appears to be coupled into a time-measuring reaction of some kind; when this is completed hormone synthesis begins in the leaves and continues throughout the remainder of the night (6). Long-day plants have, on the whole, proves less amenable to study than short-day plants but it is generally accepted that a similar sequence of events controls their flowering; the conversion of Pfr to Pr checks the reactions leading to hormone production, or may lead to its destruction or inactivation. The hormone appears to be the same in long and short-day plants but the conditions leading to its formation differ.
The original determinations of action spectra, which were made with rather long light periods and short night breaks, showed that a brief exposure to red light promoted flowering and far-red reversed the action