Grafting Studies Have Provided Evidence for a Transmissible Floral Stimulus

The production in photoperiodically induced leaves of a biochemical signal that is transported to a distant target tissue (the shoot apex) where it stimulates a response (flowering) satisfies an important criterion for a hormonal effect. In the 1930s, Mikhail Chailakhyan, working in Russia, postulated the existence of a universal flowering hormone, which he named florigen.

The evidence in support of florigen comes mainly from early grafting experiments in which noninduced receptor plants were stimulated to flower by being grafted onto a leaf or shoot from photoperiodically induced donor plants. For example, in the SDP Perilla crispa, a member of the mint family, grafting a leaf from a plant grown under inductive short days onto a plant grown under noninductive long days causes the latter to flower (Figure 24.28). Moreover, the floral stimulus seems to be the same in plants with different photoperiodic requirements. Thus, grafting an induced leaf from the LDP Nicotiana sylvestris, grown under long days, onto the SDP Maryland Mammoth tobacco caused the latter to flower under noninductive (long day) conditions.

The leaves of DNPs have also been shown to produce a graft-transmissible floral stimulus (Table 24.2). For example, grafting a single leaf of a day-neutral variety of soy-

FIGURE 24.28 Demonstration by grafting of a leaf-generated floral stimulus in the SDP Perilla. (Left) Grafting an induced leaf from a plant grown under short days onto a noninduced shoot causes the axillary shoots to produce flowers. The donor leaf has been trimmed to facilitate grafting, and the upper leaves have been removed from the stock to promote phloem translocation from the scion to the receptor shoots. (Right) Grafting a noninduced leaf from a plant grown under LDs results in the formation of vegetative branches only. (Photo courtesy of J. A. D. Zeevaart.)

Induced graft donor

Uninduced graft donor

Induced graft donor

Uninduced graft donor bean, Agate, onto the short-day variety, Biloxi, caused flowering in Biloxi even when the latter was maintained in non-inductive long days. Similarly, a leaf from a day-neutral variety of tobacco (Nicotiana tabacum, cv. Trapezond) grafted onto the LDP Nicotiana sylvestris induced the latter to flower under noninductive short days.

In a few cases, flowering has been induced by grafts between different genera. The SDP Xanthium strumarium flowered under long-day conditions when shoots of flowering Calendula officinalis were grafted onto a vegetative Xanthium stock. Similarly, grafting a shoot from the LDP Petunia hybrida onto a stock of the cold-requiring biennial Hyoscya-mus niger (henbane) caused the latter to flower under long days, even though it was nonvernalized (Figure 24.29).

In Perilla (see Figure 24.28), the movement of the floral stimulus from a donor leaf to the stock across the graft union

FIGURE 24.29 Successful transfer of the floral stimulus between different genera: The scion (right branch) is the LDP Petunia hybrida, and the stock is nonvernalized Hyoscyamus niger (henbane). The graft combination was maintained under LDs. (Photo courtesy of J. A. D. Zeevaart.)

TABLE 24.2

Transmissible factors regulate flowering.

TABLE 24.2

Transmissible factors regulate flowering.

Donor plants maintained under flower-inducing conditions

Photoperiod type°'b

Vegetative receptor plant induced to flower

Photoperiod type°'b

Helianthus annus

DNP in LD

H. tuberosus

SDP in LD

Nicotiana tabacum Delcrest

DNP in SD

N. sylvestris

LDP in SD

Nicotiana sylvestris

LDP in LD

N. tabacum

SDP in LD

Maryland Mammoth

Nicotiana tabacum

SDP in SD

N. sylvestris

LDP in SD

Maryland Mammoth

Note:The successful transfer of a flowering induction signal by grafting between plants of different photoperiodic response groups shows the existence of a transmissible floral hormone that is effective. aLDPs = Long-day plants; SDPs = Short- day plants; DNPs = Day-neutral plants. bLD, long days; SD, short days.

Note:The successful transfer of a flowering induction signal by grafting between plants of different photoperiodic response groups shows the existence of a transmissible floral hormone that is effective. aLDPs = Long-day plants; SDPs = Short- day plants; DNPs = Day-neutral plants. bLD, long days; SD, short days.

correlated closely with the translocation of 14C-labeled assimilates from the donor, and this movement was dependent on the establishment of vascular continuity across the graft union (Zeevaart 1976). These results confirmed earlier girdling studies showing that the floral stimulus is translocated along with photoassimilates in the phloem.

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Responses

  • marcel
    Can flower stimuls transfer?
    2 years ago

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