Assimilate Partitioning

Patterns of growth and development need to adjust to the varying demands of different seasons. When alterations in the daylength bring about large developmental changes such as the onset of flowering, dormancy or the formation of storage organs there are certain to be modifications to the distribution of resources within the plant. Where they are a secondary event and a consequence of the developmental change, rather than a direct response to daylength, the changes will take several days to...

Fractional Induction

Fractional induction is the summation of inductive cycles despite the intercalation of non-inductive cycles. Plants reported to show fractional induction include the LDP sugar beet, Hyoscyamus niger, Plantago lanceolata, Lolium temulentum and the SDP Bidens tripartita, Glycine max cv 'Biloxi', Chenopodium amaranticolor, Impatiens balsamina, Caryopteris clandonensis (Lang, 1965, 1980 Vince-Prue, 1975) and Lemna paucicostata (Oota, 1984). In Hyoscyamus, summation occurred when 1 LD was alternated...

Reversion To Vegetative Growth

The ability of a plant to exhibit floral reversion may involve factors unrelated to photoperiodism. The most basic factor concerns the nature of the flower development itself. Heslop-Harrison (1963) postulated that flower development represents an inevitable series of events, which are initiated by the appropriate stimulus, such as daylength. In his relay hypothesis, each stage and its associated gene products triggers the progression to the next until a fully differentiated flower is formed....

Attempts To Extract Floral Inhibitors

The idea of a specific floral inhibitor(s) was proposed almost as early as that of a floral promoter (Lang, 1980 Jacobs, 1980). The identification of such substances, or antiflorigens has, however, met with no more success than the identification of florigens. One problem in evaluating the physiological significance of flower-inhibiting chemicals is the notorious lack of specificity common to inhibitory effects. The inhibition of flowering by a particular chemical is not strong evidence for its...

Light After an Inductive Dark Period

In some early experiments, there were indications that light conditions following an inductive dark period were important for flowering in SDP. Thus the existence of a postinductive light reaction has sometimes been postulated as part of the photoperiodic mechanism. If such a reaction occurs, it can only easily be demonstrated in single-cycle plants since in multicycle plants any postinductive effect of light is confounded with its effect in the subsequent cycle (except for the last cycle in...

What Is Perceived

The factor which governs classification as a SDP or LDP is whether flowering occurs (or is accelerated) only when the daylength exceeds (LDP) or only when it is less than (SDP) a certain critical duration in each 24 h cycle (Fig. 1.4). Thus plants are able to measure the duration of light and or darkness in each 24 h period. In order to be able to do this, photoperiodically sensitive plants must possess both a clock (to measure time) and a photoreceptor (to discriminate between light and...

Requirement for Light

Although flowering in SDP requires exposure to a sufficient duration of darkness, the light period is not without effect. Perhaps the most important attribute of the photo-period is that a preceding exposure to light is necessary for a subsequent dark period to be inductive. The requirement for light before an inductive dark period was first demonstrated by Karl Hamner (1940). After several cycles of 3 min light 3 h dark, Xanthium plants completely failed to flower in response to a single 12 h...

Effects Of Daylength

Dormancy in woody plants was among the earliest phenomena shown by Garner and Allard to be under photoperiodic control. It is important to recognise, however, that daylength influences several other aspects of vegetative growth and that these may, in part, be confounded with effects on dormancy. For example, internodes are generally longer in LD in both herbaceous and woody plants and leaves are often larger (see Chapter 13). Thus care must be taken to consider those attributes that are...

Production of a Transmissible Inhibitor

Several types of physiological experiment suggest that one or more transmissible inhibitors may be produced in the leaves of some plants when they are in non-inductive cycles. A few plants have been found to flower when the leaves are removed examples are known for both LDP (Hyoscyamus Lang, 1980) and SDP (strawberry Guttridge, 1985). In Hyoscyamus, the presence of even one mature leaf in non-inductive cycles restores the inhibition. It is evident that, in these plants, the inhibition of...

Where Is Daylength Perceived

It was recognised quite early that the main site for the perception of daylength is the leaf, even though the observed responses usually take place elsewhere in the plant. This was first demonstrated by Knott (1934) for the LDP spinach, where exposing only the leaves to long photoperiods resulted in the initiation of floral primordia at the shoot apex, while plants remained vegetative when the apical bud alone was treated. Some of the earliest work on the site of perception was concerned with...

The Tobacco Thin CellLayer System TCL

Explants consisting of a few of the outermost cell layers of the stem have been used by several groups to study the action of various substances on development. When taken from stems of flowering tobacco plants, these TCLs produced roots, shoots or flowers with no apparent intermediate callus (Tranh Than Van et al., 1974). Four types of organ-inducing media (distinguished by auxinrcytokinin concentrations) have been defined, namely root, vegetative shoot, and flower media and a transition...

On the Content of Endogenous Growth Substances

APPENDIX II content of endogenous growth substances 367 Chenopodium rubrum' Cucumis sativus Beit Alpha Matsunomidori Higan-fushinari Dahlia hybrida Spinacia oleraceak 368 APPENDIX II content of endogenous growth substances Lolium temulentum LDP C Apex leaf Spinacea From Table 2 in Vince-Prue 1985, with additional information as indicated. Oden and Heide (1989) ''Henson and Wareing (1977c) Kopcewicz et al. (1979) Hansen et al. (1988) 'van Staden (1981) 'Mach ckov et al. (1993) Lozhnikova et al....

Orcadian Rhythms

The most detailed approach to the timing of FR has been to examine the response over the whole circadian cycle. In Lolium, 4 h of FR was added at various times during a 40 h period of R, following an 8 h day in sunlight (Vince-Prue, 1975). The response to the added FR varied in the form of a circadian rhythm which peaked at about 8-10 and 35 h from the beginning of the photoperiod. A similar rhythm was found in Hordeum when plants grown in SD were transferred to 72 h of white fluorescent light...

Interference with Translocation of the Floral Stimulus

Although flower initiation can be brought about by exposing one leaf, or part of a leaf to appropriate photoperiods, this frequently occurs only when some, or all of the other leaves are removed. To induce transfer of the floral stimulus from one branch to another of a two-branched plant, or from donor to receptor in grafting experiments, it is often necessary to de-foliate the receptor branch or plants. Several lines of evidence suggest that the non-induced leaves may, at least partly, act by...

THE PfrRequiring Reaction

One problem that is repeatedly encountered in studies of photoreceptor action in SDP and often complicates their interpretation is an additional component of the overall flowering response for which Pfr is required see Fig. 4.1 . The promoting effect of Pfr on the magnitude of flowering in SDP was first shown for Pharbitis. In young seedlings with expanded cotyledons but no foliage leaves , flowering was depressed when a short exposure to FR was given at the end of the day before transferring...

Tubers And Corms

The formation of storage organs involves a series of morphogenetic events. Stem tubers, such as in the potato, arise by a swelling of a cluster of internodes, often at the 318 12. VEGETATIVE STORAGE AND PROPAGATION TABLE 12.1 Influence of photoperiod on the formation of storage organs. Apios tuberosa groundnut Begonia evansiana B. socotrana B. tuberhybrida cv Camelliaflora B. tuberhybrida cv Multiflora Dahlia hybrida Dioscorea divaricata D. alata yam Helianthus tuberosus artichoke Oxalis sp....

Cucurbitaceae

The control of sex expression has been studied in considerable detail in a number of cucurbits, where the floral primordium can develop into a pistillate, staminate or hermaphrodite flower depending on genetic constitution and environment. At least seven genes are known to be involved in cucumber sex expression Malepszy and Niemirowicz-Szczytt, 1991 . In several members of the Cucurbitaceae, including most cultivars of cucumber, SD and low temperatures enhance femaleness however, in the...

Overt Circadian Rhythms And The Photoperiodic Clock

The difficulties inherent in studying the operation of the photoperiodic clock are fairly obvious. It is necessary to give complex treatments over one or several cycles and observe effects on flowering often several weeks later. The induction mechanism operates in the leaves but translation into a flowering response requires the information to be relayed to the apical meristems. Thus several events must take place after the photoperiodic clock has led to a switch between induction and...

Does Florigen Exist

The idea of florigen, a substance which is generated by leaves and is supposed uniquely to evoke flowering at the shoot apex, underlies many considerations of the biochemistry of floral induction and evocation. Much of the experimental evidence in support of such a floral stimulus comes from physiological experiments on plants where flowering is controlled by exposure to appropriate daylengths but the idea has been extended to include plants where flowering is induced by other means. Thus, the...

Discovery Of Phytochrome And Its Function In Photoperiodism

The history of phytochrome research, particularly in its early years, was intimately intertwined with the history of photoperiodism. Inevitably this meant that concepts of phytochrome function in photoperiodism were established against an incomplete understanding of the complexity of the phytochrome system. Many of the early ideas still form the core of explanations of photoperiodic mechanisms as presented in teaching and research texts. For these reasons it is useful to consider the properties...

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...

Phloem Analyses

The long-distance transport of the floral stimulus is thought to take place in the phloem and one approach to the problem of the identity of the floral stimulus has been to analyse phloem contents. A commonly used technique is to collect EDTA-enhanced phloem exudates King and Zeevaart, 1984 from induced and non-induced leaves. In Perilla green , over 80 of the dry weight of the exudate consisted of carbohydrates but the relative composition did not change significantly with different...

Direct and Indirect Induction

The demonstration of a more or less permanent induced state in the leaves has been demonstrated in Perilla red . However, while the leaves which are exposed directly to SD become induced and can bring about flowering in a receptor plant, leaves taken from this flowering receptor are not themselves capable of inducing flowering. Thus, in Perilla red , leaves can only be induced by exposing them directly to inductive SD cycles Fig. 6.2 . The presence of buds seems unimportant for maintenance of...

Molecular changes

An induced leaf can be regarded as being in a quasi-stable metabolic state which enables it to direct vegetative apices to be transformed to floral structures. It has been suggested that this altered metabolic state must involve changes in the pattern of gene expression through altered transcription or translation. A simple test of this hypothesis would be to investigate the effects of transcription and translation inhibitors on floral induction. However, many of the problems in evaluating the...

The Nature of the Floral Stimulus

Grafting experiments indicate that the final product of photoperiodic induction is the same in plants of different photoperiodic classes and is interchangeable with a floral stimulus in day-neutral plants. That this stimulus might be common to or at least physiologically equivalent for many plants is indicated from the results of interspecific and intergeneric grafts. As the stimulus is only transported when a successful graft union with a functioning phloem has been established, exchange...

Role of gibberellins

Gibberellins and particularly GAi appear to play a central role in the daylength-mediated regulation of stem extension in both woody species and rosette LDP. As mentioned earlier, applied gibberellins can replace a LD treatment for stimulating stem extension without causing floral initiation. Also, applied gibberellins GA and GA2o can replace LD in overcoming the cessation of growth in response to SD in birch see Fig. 11.19 . As discussed in Chapter 7, key steps in the 13-hydroxylation...

Classification of Plants

Qualitative or Absolute SDP Amaranthus caudatus albiflorus caudatus caudatus tricolor Ambrosia elatior H Andropogon gayanus Begonia bowerae X cheimantha gt 21 C, 9 at 12 C X hiemalis at gt 24 C, 2 at LT Caryopteris incana for completion of floral development, initiation may be promoted by LD especially in low intensity light Chamaelaucium uncinatum at gt 24 16 C, 2 at lower temperature Chenopodium album rubrum e.g. ecotypes 50 10'N, 34 20'N Echeveria carnicolor...

Shortlongday plants SLDP

Alopecurus pratensis LD promotes flower development at gt 12 C, little effect of daylength at 6 C Dactylis glome rata strain si 43 Scabiosa succisa Symphandra hoffmanni L 6a. SLDP require or accelerated by vernalisation Apium graveolens requires SD during vernalisation 6b. SLDP vernalisation substitutes at least partly for the SD effect and, after low temperature, plants respond as LDP Campanula medium Coreopsis grandiflora Dactylis glomerata Echeveria harmsii setosa at 15-20 C Hordeum bulbosum...