Daphne Vince-Prue

Some General Principles

This chapter discusses some general principles related to photoperiodic response in plants. The classification of plants according to their photoperiodic responses has usually been made on the basis of flowering. With respect to flower initiation, the responses to day length are of three basic types: short-day plants (SDP), which only flower, or flower most rapidly, with fewer than a certain number of hours of light in each 24-hour period; long-day plants (LDP), which only flower, or flower most rapidly, with more than a certain number of hours of light in each 24-hour period; and daylight indifferent or day-neutral plants (DNP), which flower at the same time irrespective of the photoperiodic conditions. Plants that are responsive to day length may be further subdivided into obligate (or qualitative) types, where a particular day length is essential for flowering, and facultative (or quantitative) types, where a particular day length accelerates but is not essential for flowering. It is not always easy to discriminate between these two subcategories because a photoperiodic requirement may be more pronounced under some conditions than others. For example, a plant can have an obligate photoperiodic requirement at one temperature but only a facultative response at another.

Vegetative Storage and Propagation

This chapter focuses on vegetative storage and propagation in plants. One of the features of many higher plants is their capability of vegetative (that is, nonsexual) means for propagating or perpetuating themselves. In many cases, this takes place through the formation of resting structures that have both a storage and reproductive function. Vegetative storage organs arise by lateral swellings of a number of different tissues including stems (tubers, corms), roots (tuberous roots or root-tubers) and leaves (bulbs). Physiologically, however, they all have a similar function as perennating organs and are regions into which storage materials are mobilized. Their development is usually accompanied by the cessation of active growth followed by senescence and death of the remainder of the plant. Once formed, storage organs enter into a state of dormancy during which they are more resistant to unfavorable environments such as water stress and high or low temperatures. In some cases, plants propagate vegetatively without passing through a dormant storage phase. A good example of this in a commercially important species is strawberry, where daughter plants are produced on runners, which are formed by extreme elongation of internodes in what is otherwise a rosette plant.

Daylength Perception in Long-Day Plants

This chapter describes the day length perception in long-day plants. Interruption by light of a non-inductive long dark period in long-day plants (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, and 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 these were obtained by growing the plants on marginally inductive photoperiods of 11.5 or 12 hours and induction did not occur when night-breaks were given in combination with 8-hour short days. Even with marginal day lengths, repeated cycles of treatment were needed to obtain a reasonable flowering response. Experiment 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 hour night-break in the middle of a 16 hour dark period. However, 5 minute R given 10 hours after the end of the main light period did cause some induction when the photoperiod was extended 4 or 6 hour tungsten filament light, which was, in itself, non-inductive.

The Nature and Identity of Photoperiodic Signals

There is a spatial separation between the generation of a stimulus in leaves exposed to a particular day length and the action of that stimulus to evoke flowering at the shoot apex. The perceptive mechanism for many of the vegetative responses of plants to photoperiod, such as tuber formation and the onset of dormancy, has also been shown to be located in the leaves. Thus, in addition to any stimuli specifically associated with flowering, substances that can bring about a variety of vegetative responses at widely differing sites in the plant must also be produced in the leaves. It is, therefore, not surprising to find that many chemical differences can be detected in leaves exposed to different day lengths. However, in view of the physiological evidence from grafting experiments for the interchangeability of the floral stimulus in plants of different photoperiodic classes, it is perhaps more surprising to find that many different substances and treatments can bring about flowering. Many physiological experiments have shown that the stimulus exported from a photoperiodically induced leaf differs in some as yet unidentified way from that exported from a non-induced leaf.

Genetic Approaches to Photoperiodism

This chapter focuses on genetic approaches to photoperiodism. The application of physiological and molecular genetics to the photoperiodic control of flowering is helping to clarify some of the long-standing questions in plant physiology. In particular, Arabidopsis mutants offer the prospect of physically isolating genes that code for components of the photoperiodic mechanism. The late flowering mutants co and gi are good candidates for plants that are modified in their photoperiodic induction processes, and some of the early flowering mutants are also probably impaired in photoperiod perception or transduction of the day length signal. Photoreceptor mutants have shown a probable role for phytochrome A in sensing long days in the long-day plants (LDP), Arabidopsis , in agreement with physiological experiments, which have identified a requirement for FR light at certain times in the photoperiod for the promotion of flowering in light-dominant LDP. As yet there are no short-day plants (SDPs), either mutants or transgenic in which phytochrome A expression is prevented, for clarifying the role of phytochrome A in SD-sensing species. The role of light-stable phytochromes appears similar in LDP and SDP and these phytochromes modulate the response to day length; phytochrome B in the LDP Arabidopsis and a phytochrome B-like light-stable phytochrome in the SDP Sorghum .

The Physiology of Photoperiodic Floral Induction

This chapter discusses the physiology of photoperiodic floral induction. Flowering involves a dramatic change in the pattern of differentiation at the shoot apex, or in the axillary buds close to the apex. The idea that this is under the control of a specific flower-forming substance dates back to Julius Sachs who, in the 1880s, concluded that leaves in the light produce small quantities of substances which direct assimilates into the formation of flowers. This idea was later opposed by other workers who thought that only sugars were involved in determining whether or not flowering would occur. The idea that the differentiation of floral organs is under the control of a hormone applies to day-neutral plants as well as those where flowering depends on exposure to appropriate day lengths but, in the latter, it is supposed that the hormone is synthesized only when the correct day length is given. It has also been proposed that substances inhibiting flowering may be involved; the appropriate day length would then lead to removal of an antiflorigen rather than (or in addition to) synthesis of a floral hormone.

Daylength Perception in Short-Day Plants

This chapter discusses the light-dependent components of the photoperiodic mechanism of flowering responses of short-day plants (SDP). Light acts to control the phase of the photoperiodic rhythm and it also interacts with a specific phase of that rhythm to inhibit flowering. Although these two actions of light have not clearly been differentiated in the majority of SDP, there is much circumstantial evidence for their existence. Additionally, it is well documented that removal of the Pfr form of phytochrome early in the night may inhibit flowering in many SDP. The photoreceptor thus has multiple actions in the photoperiodic control of flowering. This R/FR reversible reaction was one of the first to be shown to be under the control of phytochrome. It is interesting that the association of flowering control with phytochrome was first observed in a reaction which, at the time, appeared to be an artificial treatment with little relevance to natural conditions. It is now thought that the night-break reaction represents the action of light at the inducible phase of the photoperiodic rhythm and that the position of this phase in circadian time underlies the timekeeping mechanism.

Biochemical and Molecular Aspects of Photoperiodism

This chapter discusses the biochemical and molecular aspects of the processes of photoperiodic floral induction in the leaf and of floral evocation in the shoot apical meristems. In photoperiodism, the key input functions are the durations of light and darkness and the output functions include, the initiation of flowering, the onset of dormancy, and the initiation of vegetative storage organs. Photoperiodic induction and the consequent floral evocation of vegetative shoots are mediated through a series of biochemical and molecular changes in the leaves and at the growing apex. The biochemical origin of the biological rhythm(s) that forms the basis for photoperiodic timekeeping is not yet clear and neither is it certain how photoperiodic rhythms of light sensitivity relate to other rhythms of biochemical or physiological function that occur in the leaves. The molecular or metabolic basis of photoperiodic induction in the leaf is still unknown. The evidence suggests that differences between induced and non-induced leaves are probably both small and subtle, even though the induced state may be highly stable, as in Perilla . Careful studies on the apex indicate that changes in patterns of gene expression occur and may be the key step in commitment of the apex to flower.

Other Effects of Daylength

This chapter discusses several aspects of plant development that are modified by photoperiod. These include: seed germination, stem and leaf growth, assimilate partitioning, and secondary metabolism. Seed germination was the first light response shown to be controlled by phytochrome. Many seeds, such as Lactuca sativa , require only a brief exposure to R or white light in order to induce germination; others require repeated short exposures to light or a single prolonged exposure. There are also a few cases where germination has been found to be influenced by the daily duration of light and which are thought to be under photoperiodic control. The photoperiodic requirement for germination is not necessarily the same as that required for flowering; for example, germination is promoted by LD in some cultivars of rice that require SD for flowering. A rapid change in the partitioning of assimilates between different organs or between different storage or structural components would indicate a direct effect of photoperiod on the process of assimilate partitioning, and studies have found that the proportion of photosynthate partitioned into starch in the leaves of Digitaria decumbens.

SPECTRAL PROPERTIES, PHOTOTRANSFORMATION KINETICS AND PELLETABILITY OF PHYTOCHROME IN Pharbitis nil Chois*

Abstract— An examination has been made of the involvement of phytochrome in the circadian rhythm of flowering in Pharbitis nil Chois. The peak position of Pfr absorption changes with time after a red light pulse. The shortest absorption wavelength of Pfr occurs at the same time as flowering is inhibited by red light in dark grown, red light pretreated plants. Pelletable and supernatant phytochrome show a similar trend with lowest values found at the time of flower inhibition. Neither phototransformation kinetics nor intermediates of phytochrome which accumulate in white light show such a relationship to the circadian rhythm found in flowering of dark grown P. nil.