Discovery Of Photoperiodism

Henfrey suggested as long ago as 1852 that the natural distribution of plants was due, at least partly, to latitudinal variations in summer daylength (Henfrey, 1852). However, the first experiments in which the daily duration of light was controlled were carried out by Kjellman in the Arctic Circle (see Naylor, 1961). In these experiments plant development was faster in longer light periods but a clear distinction was not made between photoperiodic and photosynthetic effects. Experiments to extend the daily light period were made possible by the invention of the incandescent electric light in the latter part of the nineteenth century (Bailey, 1893; Rane, 1894). Many of these attempts at electrohorticulture were successful in causing the acceleration of flowering in a number of summer annuals. Although the illumination levels used in these experiments were relatively low, photosynthetic effects were not rigidly excluded and the importance of the photoperiod was not recognised.

That the duration, rather than the quantity of light, in the daily cycle is a major factor in plant development was proposed independently by Julien Tournois and Hans Klebs at the beginning of the twentieth century (Tournois, 1912, 1914; Klebs, 1913). Tournois, in his studies with the SDP Humulus and Cannabis, found that the plants flowered precociously in winter under glass. He eliminated temperature, humidity, and seed origin as causal factors and began his critical experiments on daylength in 1912. He established that plants given only 6 h of light each day flowered most rapidly, even though they grew more slowly. Initially, Tournois believed that the determining factor was the reduction in light quantity, but in his last paper on sexuality in Humulus, he showed that this had only a small effect. He concluded that the precocious flowering was due to the short periods of daily illumination and furthermore concluded that it was 'not so much caused by the shortening of the days but by the lengthening of the nights'. Although Tournois planned further experiments he was killed in action as a soldier shortly after the publication of his last paper.

At about the same time Klebs was carrying out carefully controlled experiments on flowering in Sempervivum funkii, an LDP. He succeeded in inducing the rosettes to flower in the middle of winter by giving a few days of continuous illumination from incandescent lamps. Non-irradiated rosettes always remained vegetative. He concluded that 'in nature, flowering is probably determined by the fact that from the equinox (21 March) the length of the day increases . . . when it reaches a certain length flowering is initiated. Light probably acts as a catalytic rather than a nutritive factor'. Klebs thus recognised that flowering could be accelerated by long days. It was Garner and Allard (1920, 1923), however, who first saw clearly that flowering and many other responses in plants could be accelerated either by long days (LD) or short days (SD), depending on the plant. They introduced the terms photoperiod and phot ope riodism and classified plants into the photoperiodic groups we know today.

They were led to their discoveries by observations on two species of plants being used in breeding programmes at the time. In certain varieties of Glycine max, particularly the late maturing strain Biloxi, flowering tended to occur at the same time, independent of planting date (Table 1.1). Secondly, the Maryland Mammoth variety of Nicotiana tabacum grew to a prodigious size out of doors in summer in Washington DC but failed to flower. However, plants growing in pots under glass flowered while still quite small in winter and early spring. The tobacco result, in particular, suggested a seasonal factor and after eliminating temperature and light intensity as causal factors, Garner and Allard concluded (apparently with extreme diffidence) that the only remaining seasonal phenomenon was the relative length of the day and night. To investigate this they transferred Glycine max cv Peking and Nicotiana tabacum cv Maryland Mammoth to a darkened, ventilated hut for part of the daily light period, in order to limit the exposure to light to 7 h, during the summer and compared the response with the plants grown in the open. Plants which received the shortened day flowered promptly, while those exposed to natural summer day-lengths remained vegetative. They proposed that the answer to the tobacco and soyabean problems was the same; the varieties in question would only flower if the duration of the daily light period was sufficiently short.

After their initial experiments on flowering in Nicotiana and Glycine, Garner and Allard extended their observations to a wide range of species and responses. They established that daylength influences many aspects of plant behaviour, including flowering, tuberisation and dormancy and considered its possible effects on plant distribution and crop yield. Further, their experiments with plants suggested to Garner and Allard that the time of bird migration might also be controlled by daylength, which was later confirmed. The ability of organisms to measure time, often with a high

TABLE 1.1 Time of flowering of Glycine max cv Biloxi planted at different times and grown under natural conditions in Washington DC.

Date of germination

Date of anthesis (first flower)

Number of days to flowering

2 May

4 September

125

2 June

4 September

94

16 June

11 September

92

30 June

15 September

77

15 July

22 September

69

2 August

29 September

58

16 August

16 October

61

Data of Garner and Allard (1920), from Hillman (1969a).

degree of accuracy, is now fully accepted and photoperiodism is established as a major factor in the seasonal regulation of plant and animal behaviour, although it is only one of many examples of biological time measurement.

IMPACT OF PHOTOPERIODISM

It is difficult to quantify the benefits of basic studies on photoperiodism, but there is little doubt that modern agricultural and horticultural practice has been profoundly influenced by the appreciation of its biological importance. The initial studies of Garner and Allard over a 10-year period were estimated to have cost approximately $10 000, but brought benefits to farmers, horticulturists and plant breeders of billions of dollars (Sage, 1992). Since the time of Garner and Allard, testing plants for their photoperiodic requirements has become standard practice in breeding programmes, and a major reason for crop failures can now be avoided. Plant breeders can manipulate daylength to obtain multiple generations of plants per year and obtain seed from plants that fail to reproduce out of doors because the days are too long, by growing them in shortened days in the greenhouse. Also, by using daylength to synchronise flowering in plants that normally flower at different times, new crosses between varieties are possible.

Photoperiodism has had a particular impact on horticulture. Daylength manipulation by the use of blackouts or supplementary lighting to promote or enhance flowering has been used for a wide range of ornamental species. In the case of the major horticultural crop, chrysanthemum, daylength sensitive cultivars of this SDP have been selected so that plants can be maintained in a vegetative state or brought to flowering as required by the grower through varying daylength in combination with other environmental conditions. Production of poinsettia, the most valuable ornamental crop in the USA, is also dependent on daylength management. Timing of production for a specific market, e.g. Christmas, Easter or Mother's Day, is very important in floriculture as there is a price premium at these times and the price is dramatically lower for crops produced too early or late. Daylength regulation in combination with appropriate temperature regimes make this possible for a range of crops such as begonias, Christmas cactus, chrysanthemums and poinsettias.

Almost all of the considerable benefits flowing from photoperiodism research have so far been to do with cultural practices and breeding. These have been achieved by using an understanding of the basic physiological processes and genetic variation in the response to daylength. As understanding proceeds from the whole plant to the molecular and genetic level the initial benefits will be related to the ability to use molecular markers for daylength sensitivity in breeding programmes and the direct manipulation of dosage of key genes by breeding and genetic manipulation. In the longer term, the introduction or removal of daylength sensitivity to particular species or cultivars will be feasible. One can predict that it will be possible to couple new processes, e.g. synthesis of high-value secondary products, to particular daylength regimes or to alter the response type of a particular species e.g., convert LDP to SDP or vice versa. It is against this background of new potential opportunities that this book has been written.

AIMS OF THIS BOOK

Photoperiodism is a complex and pervasive phenomenon, which has been the subject of scientific investigation for more than three quarters of a century. There is a huge volume of literature pertaining to all aspects of the subject. Experiments have been carried out on a range of processes in a range of species using a range of techniques and approaches. An account of photoperiodism research up until 1975 was published in the first edition of this book. This revised edition aims to update the work in the light of the 20 years of research that have subsequently been performed. As the new powerful technologies of molecular genetics are brought to bear on photoperiodism, it becomes particularly important to place new work in the context of the considerable amount of physiological information which already exists on the subject. Given the volume of literature that has been published on photoperiodism, it is no longer possible for a book of this sort to be encyclopaedic. However, while we cannot include everything that has been published, we have tried to ensure that a self-sufficient account of all the important subjects is included and the key literature references are there for those who want to follow topics up in detail. One thing that became increasingly clear to the authors as the book progressed is that despite the apparently diverse phenomena which are under photoperiodic control, a number of clear themes based on common underlying mechanisms emerge.

• Photoperiodism involves the coupling of the capacity for daylength perception to a number of target processes. The most important and best studied target is flowering, but vegetative reproduction and dormancy are also major processes which can be under photoperiodic control.

• It is increasingly clear that photoperiodic timekeeping is based on a circadian oscillator or oscillators, and these are coupled to rhythms in light sensitivity to form daylength detection mechanisms.

• From studies of a range of phenomena such as flowering, tuberisation, stem extension and dormancy we consistently find evidence for two distinct and apparently rather different daylength perception mechanisms. The first of these is a dark dominant mechanism, where the response is determined primarily by the length of the dark period and conditions in the light period are relatively unimportant. The second is a light dominant mechanism, where the primary response is to light given in the photoperiod and there is a requirement for particular spectral properties, specifically a strong effect of FR and a sufficiently high irradiance, to elicit a positive response. In some cases, dark dominant and light dominant mechanisms may coexist within the same species.

• Phytochrome, the main photoreceptor for daylength perception in higher plants, is now known to consist of a family of molecules, with different physiological roles. There is now strong evidence that one of these, phytochrome A, plays a central role in light-dominant responses, being responsible for the FR requirement. In contrast, dark dominant responses are not enhanced by FR and phytochrome A appears not to be involved.

We point out these themes to those who may wish to limit their reading to specific chapters and also hope that these recurring themes suggest themselves to readers of the whole book.

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Part I

Photoperiodic Control of Flower Initiation

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  • duenna
    Who discovered photoperiodism?
    2 years ago

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