In Chapter 2, we looked at the mechanisms that are involved in the timekeeping aspects of photoperiodism. Most of the evidence and discussion centred on SDP. This reflects the fact that, in terms of photoperiodic mechanisms, SDP have proved to be the most accessible and rewarding and have therefore received the most attention. Nevertheless the history of research into LDP extends as far back as for SDP, the first experiments on LDP (Klebs, 1910) being performed at about the same time as the first studies on SDP were being undertaken by Tournois (1912). In the initial systematic studies of Garner and Allard (1920), the subjects were Biloxi soybeans and Maryland Mammoth tobacco, both of which were SDP. However, in their follow-up studies they included a number of plants, such as radish, in which flowering occurred in long days but not in short days. Based on the results, they classified species as SDP or LDP and defined LDP as those plants which flower and fruit in a wide range of photoperiods above a critical daylength. This definition, although often attributed to Garner and Allard, appeared in a single-author paper by Garner (1933).
The preference for working with SDP rather than LDP was evident from early studies. When Borthwick and Parker began their investigations into the spectral responses of photoperiodic plants, they consulted with Allard who recommended using SDP. He said that LDP were much less precise in their daylength responses and therefore less suitable for Borthwick and Parker's purposes (Borthwick, 1972). However, where it was possible to compare the night-break responses in LDP with those of SDP, a similar action spectrum, with maximum effectiveness in the red, was obtained (Borthwick et al., 1948). Furthermore, under the appropriate experimental conditions, FR reversibility of the night-break could be demonstrated, confirming the action of phytochrome (Downs, 1956). It was already known (Knott, 1934) that the site of daylength perception in LDP is located in the leaves, as it is in SDP. Thus, it might be argued on the basis of early comparative data that separate consideration of
LDP is unnecessary inasmuch as the mechanisms involved are the same for both response types, but with the responses of LDP being mirror images of those in SDP. Alternatively it could be argued that the imprecise responses of LDP, in experimental protocols which reveal precision in SDP, indicate in themselves that the underlying mechanisms are not the same. It is evident that the situation is not entirely clear cut. There are undoubtedly elements of daylength perception in LDP which are directly comparable to components of the SDP response. On the other hand there are certain characteristics which are highly distinctive for LDP. We will now consider components of the daylength responses of LDP in more detail.
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