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 flowering in non-inductive cycles could not result from an interference with transport of a floral stimulus, nor could the inhibitory effect be due to an antagonistic process in the leaves which prevents stimulus production.

More direct evidence for a transmissible inhibitor of flowering comes from strawberry, where an inhibitory effect is transmitted from a mother plant in LD to a daughter joined to it by a stolon. Mature leaves were found to be most inhibitory and transmission of the LD inhibition was increased by treatments which would be expected to increase the flow of carbohydrates from donor to receptor; for example, by restricting the number of daylength hours given to the daughter plants, or reducing the light intensity (Guttridge, 1985).

The strongest evidence for the existence of transmissible inhibitors of flowering comes from grafting experiments in the Solanaceae (Lang, 1987). These are summarised in Fig. 6.9. In grafts between the LDP Nicotiana sylvestris and the day-neutral tobacco Trapezond, flowering was inhibited in Trapezond when the grafts were maintained in SD. The growth habit of the grafted Trapezond was also modified; the internodes were much shorter and became thickened, approaching to some extent the rosetted habit of the LD partner when maintained in SD. Similar results were obtained when a SD tobacco cultivar was used as the receptor plant and the LDP, Hyoscyamus niger was used as the donor in SD. A single leaf of the LD partner was sufficient to cause these effects. In contrast, flowering in the day-neutral tobacco was not affected by grafting to the SDP, Maryland Mammoth in long days. In similar grafts between Maryland Mammoth (as donor) and Trapezond, Hyoscyamus or Nicotiana sylvestris, no consistent inhibitory effects on flowering were obtained. Thus, whereas the LDP N. sylvestris and H. niger appear to produce a transmissible inhibitor of flowering in non-inductive photoperiods, Maryland Mammoth does not, or at most produces it in much lower quantities. The putative antiflorigen is apparently inter-

Response type Receptor

Non-flowering donor

Day neutral Nicotiana tabacum

Long day Nicotiana sylvestris

Short day Nicotiana tabacum Maryland Mammoth

Trapezond and others

Hyoscyamus niger,

Long day Nicotiana sylvestris

Short day Nicotiana tabacum Maryland Mammoth

Trapezond and others

Nicotiana tabacum Maryland Mammoth

Nicotiana sylvestris

Nicotiana tabacum Maryland Mammoth

Nicotiana sylvestris

Hyoscyamus niger

FIG. 6.9. Diagrammatic representation of experiments demonstrating the existence of graft-transmissible floral inhibitors in some members of the Solanaceae. Solid lines indicate that flowering was inhibited in receptors capable of flowering, by donors maintained in non-inductive photoperiods. Dashed lines indicate that the donor had no inhibitory effect on flowering in the receptor. After Lang 1987.

changeable between species and photoperiodic categories. For example, the inhibitor from the LDP Hyoscyamus affects flowering in both the SDP Nicotiana tabacum Maryland Mammoth and the day-neutral tobacco, Trapezond. A simple stoichiometric relationship appears to exist between the flowering promoter and inhibitor in these Solanaceae; when a scion of Maryland Mammoth and one of N. sylvestris were grafted to Maryland Mammoth, flowering in the receptor became earlier as the number of induced leaves on the Maryland Mamoth donor was increased and later as the number of non-induced N. sylvestris leaves was increased (Table 6.15).

Grafting experiments have also been carried out in the LDP Pisum sativum and these have been coupled with a detailed genetic analysis. It has been shown that the Sn gene controls the production of a graft transmissible inhibitor which begins within 4 h of the beginning of darkness (Reid and Murfet, 1977). Graft-transmissible promoters of flowering also appear to be produced, however, since leaves of wild-type shoots can supply a substance necessary for flowering in mutant scions (Taylor and Murfet, 1994). It is thought that this floral stimulus is transported with assimilates within the phloem, which would bring its movement under the influence of the photoperiod gene system {Sn Dne Ppd) which controls the production of the inhibitor. It is suggested that the delayed flowering in late photoperiodic genotypes under SD conditions arises because the inhibitor directs an increase in the basipetal flow of assimilate, leading to transport of the floral stimulus away from its site of action in the apical bud (see also Chapter 9). Thus, in this plant, the inhibitory action of leaves in non-inductive daylengths appears to depend on assimilate movement, even though a transmissible inhibitor is produced. It is interesting that the floral promoter has a specific role in floral initiation in pea, while the floral inhibitor has generalised effects on both reproductive and vegetative characters.

TABLE 6.15 Flowering of indicator shoots of Nicotiana tabacum Maryland Mammoth in double grafts with a Maryland Mammoth and a N. sylvestris scion.

MM indicator shoot

MM indicator shoot

TABLE 6.15 Flowering of indicator shoots of Nicotiana tabacum Maryland Mammoth in double grafts with a Maryland Mammoth and a N. sylvestris scion.

No. of leaves

No. of leaves

Days to first

No. of leaves

on MM donor

on NS donor

flower bud

to flower bud

10

0

29

27

10

2

47

34

10

5

89

41

10

10

122

46

5

0

37

29

5

2

75

42

5

5

113

44

5

10

136

46

2

0

82

41

2

2

127

47

2

5

136

49

2

10

145

54

Grafts consisted of a shoot of the LDP Nicotiana sylvestris (NS) and one of the SDP Maryland Mammoth tobacco (MM) both grafted onto a MM receptor. The amount of induced and non-induced tissue was varied by altering the numbers of leaves on both the induced (MM) and non-induced (NS) shoots. From Lang (1980).

Grafts consisted of a shoot of the LDP Nicotiana sylvestris (NS) and one of the SDP Maryland Mammoth tobacco (MM) both grafted onto a MM receptor. The amount of induced and non-induced tissue was varied by altering the numbers of leaves on both the induced (MM) and non-induced (NS) shoots. From Lang (1980).

In SDP, grafting experiments which indicate the production of a transmissible inhibitor have only been reported in detail for Coleus (Jacobs, 1980). A single leaf of the SDP Coleus frederici was grafted into the third internode of a C. frederici stock plant with two leaves retained; flowering was recorded at the node below the grafted leaf. A leaf taken from a flowering plant was an effective donor of flowering to plants maintained in LD, whereas one taken from a vegetative plant (in LD) was found to delay flowering compared with plants without a grafted donor leaf. There is also some evidence for the production of a transmissible inhibitor in SD tobacco. When grafted to the day-neutral cultivar, Trapezond, there was litle evidence for the production of an inhibitor by Maryland Mammoth in LD; however, a marked inhibition of flowering was obtained in grafts with N. sylvestris (Lang, 1980). As with a flower-promoting stimulus, therefore, differences between success and failure to transmit an inhibitory effect may be due to differences in the sensitivity of the receptor plants.

Transmissible inhibitors also appear to participate in the control of flowering in plants which respond to a single inductive cycle. For example, in Lolium, 10 cm of the sixth (uppermost) leaf was sufficient for the induction of flowering in the absence of the other leaves, but flowering failed if the remaining leaves were exposed to SD (Evans, 1960); these leaves were inserted below the LD leaf and did not affect the movements of assimilates from it. The number of plants which initiated flowers depended on the length of time that the SD leaves remained on the plant during induction of the uppermost leaf by a single LD (Fig. 6.10). It was assumed that the inhibition of flowering resulted from action of a transmissible inhibitor at the apex. Like other inhibitory effects (and unlike induction) it is not cumulative since the threshold number of LD for flowering decreased, rather than increased as the plants continued to grow in SD (Fig. 6.3). An interesting experimental observation was that

16 00

8,00

Time of removal of leaves

16.00

16 00

8,00

Time of removal of leaves

16.00

Light □ Sunlight treatment D Low intensity illumination (20 W m } with tungsten-filament lamps ■ Darkness

FIG. 6.10. Flower promotion by a LD leaf and flower inhibition by SD leaves in Lolium temulentum as influenced by the time that the leaves were allowed to remain on the plant. The uppermost leaf was given a single inductive exposure of 24 h light; the other leaves were either removed at the beginning of the inductive LD or remained in SD. At the time indicated, either the LD leaf or the SD leaves were removed. After Vince-Prue, 1975 (data of Evans, 1960).

the production of the putative inhibitor in SD leaves was inhibited by anaerobic condition (nitrogen gas) but not the production of the floral stimulus in LD (Evans, 1962b).

It is evident that leaves in non-inductive cycle antagonise, to a greater or lesser extent, the effect of leaves in favourable cycles. While this may, in part, be due to an alteration in the pattern of assimilate translocation affecting the movement of the floral stimulus from induced leaves to the apex, there is evidence in both LDP and SDP that processes antagonistic to flowering may occur in leaves exposed to non-inductive cycles. In some cases, the results indicate the production of a transmissible inhibitor which presumably acts at the apex. In other cases, it has been suggested that it is the inductive process in the leaves which is affected. A characteristic of the inhibitory effect is that, in contrast to the promoting effect of favourable cycles, it is not cumulative and several unfavourable cycles usually have no greater effect than a single one (Fig. 6.1). However, a cumulative inhibitory effect of LD has been reported in the SDP Hibiscus cannabinus (Ren et al., 1982).

Even in those plants where an inhibitory effect of unfavourable cycles is most clearly seen, a flower-promoting effect has also been demonstrated. Although Hyo-scyamus flowers readily in any daylength when defoliated, it will flower more quickly with one leaf in LD (Lang, 1965). Similarly, Lolium initiated floral primordia when the inhibitory effect of SD was removed by maintaining the leaves in nitrogen during the long dark period, but the characteristic development of the spike required the inductive effect of LD (Evans, 1962b).

Was this article helpful?

0 0
30 Day Low Carb Diet Ketosis Plan

30 Day Low Carb Diet Ketosis Plan

An Open Letter To Anyone Who Wants To Lose Up To 20 Pounds In 30 Days The 'Low Carb' Way. 30-Day Low Carb Diet 'Ketosis Plan' has already helped scores of people lose their excess pounds and inches faster and easier than they ever thought possible. Why not find out what 30-Day Low Carb Diet 'Ketosis Plan' can do for you by trying it out for yourself.

Get My Free Ebook


Post a comment