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 between plants can only be demonstrated where this occurs. This means that no work has been possible with monocotyledons. It also means that interchangeability can only be studied between closely related plants.

Successful transfers of flowering between grafts of photoperiodically sensitive plants have been tabulated by Lang (1965) and more recently by Grayling (1988). A large number has been recorded and a selection of examples is given in Table 6.8. The majority of experiments have been carried out with herbaceous plants but graft transmission of flowering has also been recorded for woody species. In mango

A TRANSMISSIBLE FLORAL STIMULUS TABLE 6.8 Successful transfers of flowering by grafting.

Donor

Response type

Receptor

Response type

Intraspecific grafts

Glycine max cv Agate Chenopodium rubrum 60° 47' N Pisum sativum various genetic lines

G. max cv Biloxi SDP

Interspecific grafts

Gossypium hirsutum Nicotiana tabacum Delcrest

DNP DNP

G. davidsonii N. sylvestris

SDP LDP

Intergeneric grafts

Blitum virgatum Chenopodium polyspermum Cucumis sativus Centaurea cyanus

LDP Chenopodium rubrum SDP

SDP Blitum capitatum LDP

DNP Sicyos angulatus SDP

LDP Xanthium strumarium SDP

" Facultative photoperiodic response. Examples taken from Grayling (1988).

Response type Flowering donor

Receptor

Day neutral Nicotiana tabacum Trapezond and others

Short day

Long day

Nicotiana tabacum Maryland Mammoth

Nicotiana sylvestris-

Hyoscyamus niger

Day neutral Nicotiana tabacum Trapezond and others

Nicotiana tabacum Maryland Mammoth

Nicotiana sylvestris-

Hyoscyamus niger

Nicotiana tabacum Trapezond and others

Nicotiana tabacum Maryland Mammoth

> Nicotiana sylvestris

Hyoscyamus niger

Nicotiana tabacum Trapezond and others

Nicotiana tabacum Maryland Mammoth

> Nicotiana sylvestris

Hyoscyamus niger

FIG. 6.8. Diagrammatic representation of the successful graft transmission of flowering from donors to receptors in various members of the Solanaceae. After Lang, 1987.

(Mangifera indica), the cultivar Royal Special (which was capable of flowering) was a successful donor for defoliated scions of cultivars which were in the off-season (non-flowering) condition (Kulkarni, 1986). Successful intergeneric and interspecific grafts indicate that the stimulus is interchangeable between plants of different genera and species. Flowering has also been transferred between all major photoperiodic classes.

In the Solanaceae, LDP, SDP and DNP have been shown to cause flowering in receptor plants of all three photoperiodic categories and successful inter-generic grafts between Hyoscyamus and Nicotiana have been carried out (Fig. 6.8). In some, though not all combinations, a single leaf is sufficient as the donor (Lang, 1987). Acceleration of flowering was obtained in the DNP N. tabacum, Trapezond when plants were grafted to the SDP, Maryland Mammoth or to the LDP, N. sylvestris and maintained in photoperiods inductive for the donors; thus transmission of a floral stimulus occurs from photoperiodically-sensitive plants to DNP as well as vice versa (Lang et al., 1977). Work with the Crassulaceae (Table 6.9) has extended the range of photoperiodic categories in which interchangeability has been demonstrated to include dual daylength plants; for example, both the SLDP Echeveria harmsii and the LSDP, Bryophyllum daigremontianum, caused flowering in receptors of Kalanchoe bloss-feldiana (SDP) and flowering plants of the LSDP B. daigremontianum were successful donors of flowering for vegetative plants of the SLDP E. harmsii, maintained in either LD or SD. Other examples of transfer of flowering between plants of different photoperiodic classes are given in Table 6.8.

It is also evident that a flowering stimulus can be transferred from plants where induction is achieved by means other than exposure to favourable photoperiodic cycles (Table 6.10). Day-neutral plants, which achieve flowering through autonomous induction, can bring about flowering in both LD and SD receptor plants; even a single leaf of the DNP Glycine cv Agate was sufficient to induce flowering in the SD cv Biloxi (Heinze et al., 1942). Indirectly induced plants of green Perilla (induced by the arrival of a floral stimulus from flowering donors) cause flowering in red Perilla receptors (Zeevaart and Boyer, 1987). Perilla (green) can also be induced to flower by exposure to low temperature in LD; a single leaf from these plants can act as donors of flowering to receptor plants maintained in LD at high temperature (Deronne and Blondon, 1977). Similarly, the LDP Silene armeria can be induced in SD by either high or low temperature, or by the application of GA3; all can act as donors to cause flowering in vegetative receptors (Lang, 1987). The LSDP Bryophyllum daigremontianum induced in SD by the application of gibberellin was an effective donor of flowering to receptor plants of Echeveria harmsii in non-inductive SD; this was not due to transmission of gibberellin from the donor plant since GA treatment does not cause flowering in Echeveria (Zeevaart, 1982).

TABLE 6.9 Successful transfers of flowering by grafting between different genera and photoperiodic response types in the Crassulaceae.

Donor

Response

Receptor

Response

(flowering)

type

type

Kalanchoe blossfeldiana

SDP

Sedum spp.

LDP

Sedum spp.

LDP

Kalanchoe blossfeldiana

SDP

Bryophyllum daigremontianum

LSDP

Kalanchoe blossfeldiana

SDP

Kalanchoe blossfeldiana

SDP

Bryophyllum daigremontianum

LSDP

Echeveria harmsii

SLDP

Kalanchoe blossfeldiana

SDP

Echeveria X pulv-oliver

LDP

Kalanchoë blossfeldiana

SDP

Bryophyllum daigremontianum

LSDP

Echeveria harmsii

SLDP

From Zeevaart (1978, 1982).

From Zeevaart (1978, 1982).

TABLE 6.10 Successful transfers of flowering by grafting with donor plants induced to flower by treatments other than exposure to appropriate photoperiods.

Inductive treatment

Donor

Receptor

Autonomous

Nicotiana tabacum

Nicotiana tabacum

Trapezond (DNP)

Maryland Mammoth

(SDP)

Nicotiana tabacum

Nicotiniana sylvestris

Trapezond (DNP)

(LDP)

Glycine max Agate (DNP)

Glycine max cv Biloxi

(SDP)

SD + GA3

Bryophyllum daigremontianum (LSDP) Echeveria harmsii (SLDP)

SD + high temperature

Silene armería

Silene armería (LDP)

SD + low temperature

Silene armería

Silene armería

SD + GA3

Silene armería

Silene armería

LD + low temperature

Perilla (green)

Perilla (green) (SDP)

Indirect, by grafting to

flowering donor

Perilla (green)

Perilla (red) (SDP)

For details see text.

For details see text.

Based on the results of many grafting experiments, it appears that the floral stimulus of plants in different photoperiodic classes is identical, or at least interchangeable. Moreover, plants which flower in response to triggers other than daylength produce a stimulus which is able to cause flowering in photoperiodically-responsive receptor plants. Thus, the differences between them appear to lie in the mechanisms leading to induction and production of the stimulus, rather than in the end product itself.

The many successful transfers of flowering between different species and genera (Tables 6.8, 6.9, Fig. 6.8) has sometimes led to the conclusion that there is a specific and universally effective flowering hormone, or florigen. However, it must be emphasised that neither the specificity for flowering nor the universality of florigen has been demonstrated. Interchangeability of the stimulus is limited by the need to establish a graft union between donor and receptor plant. This means that interchangeability can be shown only between plants of the same family. A reported inter-family transfer of flowering from Xanthium strumarium (SDP, Compositae) to Silene armeria (LDP, Caryophyllaceae) has not been repeated and was probably due to removal of the roots, which results in flowering of Silene in SD (Zeevaart, 1984). Nothing is known about interchangeability between species and genera in monocotyledons although, as in dicotyledons, their leaves produce a transmissible flowering stimulus as a result of photoperiodic induction. Much of this evidence comes from work with Lolium temulentum (Evans and King, 1985). In some cases, the floral stimulus does not appear to be interchangeable even between closely-related plants (Table 6.11). One of the most striking examples is the failure to transfer flowering between the LSDP Cestrum diurnum and C. noctumum. This does not appear to be due to the failure of graft union or transport because, when an intergraft of C. nocturnum was placed between two shoots of C. diurnum (one of which was defoliated to act as receptor), both shoots flowered, although defoliated shoots normally remain vegetative (Griesel, 1963). A further example of failure to obtain graft transmission in the Solanaceae is from the DNP Nicotiana tabacum cv Delcrest to the SD cultivar Maryland Mammoth; however, Delcrest was able to cause flowering in N. sylvestris and other day-neutral

170 6. PHYSIOLOGY OF PHOTOPERIODIC FLORAL INDUCTION

TABLE 6.11 Examples of failure to transfer flowering by grafting.

Donor

Sedum spectabile (LDP)

Cestrum diurnum (LSDP) Cestrum nocturnum Ipomoea repens (SDP)

/. batatas early blooming cv Kleinia articulata K. repens (LSDP) Nicotiana tabacum

Maryland Mammoth Silene cucubalis Silene armeria (LDP) Aster savatieri (LDP) Coreopsis lanceolata (LDP)

Receptor

Bryophyllum daigremontianum (LSDP) C. nocturnum (LDP) C. diurnum

I. batatas non-flowering cv

I. batatas non-flowering cv K. articulata (SDP) K. articulata (SDP) N. tabacum

Maryland Mammoth (SDP) S. cucubalis (LDP) S. gallica (LDP) Xanthium strumarium (SDP) Xanthium strumarium (SDP)

Comments

Reciprocal graft effective. S. spectabile is effective donor for Brassica crenatum

Flowering of a non-flowering cv of I. batatas achieved by graft with other early flowering cvs

Reciprocal graft succcessful No known success for this combination

Examples taken from Grayling (1988).

tobacco cultivars were successful donors of flowering to Maryland Mammoth (Lang, 1987). Within the family Chenopodiaceae, transmission from the SDP Chenopodium to the LDP Blitum occurred without defoliation of the receptor shoot whereas, to obtain the reciprocal transfer, it was necessary both to remove mature leaves from the Chenopodium receptor and to remove flowers from the Blitum donor (Jacques and Leroux, 1979). Another case involves members of the Compositae. The SDP Kleinia articulata caused flowering in the LSDP K. repens but not vice versa (Kulkarni and Schwabe, 1984). There are, of course, a number of possible explanations for the failure to obtain graft transmission of flowering, other than differences in the identity of the stimulus. Assimilate movement from donor to receptor may not have been assured (in Impatiens balsamina, the flowering effect was not transmitted between branches when the receptor shoot was defoliated, unless the apical bud was also removed (Sawhney et al„ 1978); tissue union may not have been established; there may have been inhibitory effects within the receptor plant; the amount of stimulus produced by the donor leaves may differ between species, as could the sensitivity of the apical meristems of the receptor to the arriving stimulus (donor plants of Pharbitis nil required 4 SD in order to induce flowering in receptors of Ipomaea batatas, whereas Pharbitis itself requires only a single SD (Takeno, 1991)).

An important observation with reference to the specificity for flowering of the putative floral stimulus comes from grafting experiments in which tuber-forming plants have been used as receptors. The potato, Solanum andigena, forms tubers only when the leaves are exposed to SD; however, tuberisation also occurred when receptor plants were grafted to flowering donors of either the LDP Nicotiana sylvestris or the SDP N. tabacum Maryland Mammoth (Martin et al., 1982). Non-flowering plants did not cause tuberisation. Although these experiments do not prove that the stimuli for flowering and tuberisation are interchangeable, it is evident that exposing leaves to photoperiodic cycles that are inductive for flowering also leads to the production of a transmissible stimulus which can evoke photoperiodically-controlled tuberisation at the apical meristems of shoots in plants of another species. Neither species of tobacco is, itself, capable of forming tubers. A similar situation has been reported (Nitsch, 1965) for grafts between Helianthus annuus (sunflower) and H. tuberosus (artichoke). At least some of the events in the induced leaf appear to be common to both the tuber-forming and flowering pathways as a gene expressed in the leaves of potato plants that are beginning to initiate tubers is also expressed in tobacco plants during flowering (Jackson et al., 1993). However, since this occurs fairly late, it may only be associated with a secondary event such as the increased export of substances to the newly developing sinks. Nevertheless, it remains possible that the specificity of the response to the arrival of a stimulus from induced leaves lies in the target cells rather than in the identity of the substance itself. This important question requires further investigation.

Overall, a considerable body of physiological evidence points to the existence of transmissible flowering stimulus which appears to be common to (or, at least, interchangeable between) plants of all photoperiodic classes. From such studies, the idea of a florigen, i.e. a specific floral-forming hormone, has developed. Nevertheless graft transfer of flowering is only possible between closely related plants where a graft union is possible; thus there is no evidence for a universally effective florigen, as has sometimes been suggested. Attempts to extract and identify a photoperiodic floral stimulus are discussed in Chapter 8.

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