Competence and Determination Are Two Stages in Floral Evocation

The term juvenility has different meanings for herbaceous and woody species. Whereas juvenile herbaceous meristems flower readily when grafted onto flowering adult plants (see Web Topic 24.3), juvenile woody meristems generally do not. What is the difference between the two?

Extensive studies in tobacco have demonstrated that floral evocation requires the apical bud to pass through two developmental stages (Figure 24.12) (McDaniel et al. 1992). One stage is the acquisition of competence. A bud is said to be competent if it is able to flower when given the appropriate developmental signal.

For example, if a vegetative shoot (scion) is grafted onto a flowering stock and the scion flowers immediately, it is demonstrably capable of responding to the level of floral stimulus present in the stock and is therefore competent. Failure of the scion to flower would indicate that the shoot apical meristem has not yet attained competence. Thus the juvenile meristems of herbaceous plants are competent to flower, but those of woody species are not.

The next stage that a competent vegetative bud goes through is determination. A bud is said to be determined if it progresses to the next developmental stage (flowering) even after being removed from its normal context. Thus a florally determined bud will produce flowers even if it is grafted onto a vegetative plant that is not producing any floral stimulus.

In a day-neutral tobacco, for example, plants typically flower after producing about 41 leaves or nodes. In an experiment to measure the floral determination of the axillary buds, flowering tobacco plants were decapitated just above the thirty-fourth leaf (from the bottom). Released from apical dominance, the axillary bud of the thirty-fourth leaf grew out, and after producing 7 more leaves (for a total of 41), it flowered (Figure 24.13A) (McDaniel 1996). However, if the thirty-fourth bud was excised from the plant and either rooted or grafted onto a stock without leaves near the base, it produced a complete set of leaves (41) before flowering. This result shows that the thirty-fourth bud was not yet florally determined.

Vegetative growth

Expressed:

The apical meristem undergoes morphogenesis.

FIGURE 24.12 A simplified model for floral evocation at the shoot apex in which the cells of the vegetative meristem acquire new developmental fates. To initiate floral development, the cells of the meristem must first become competent. A competent vegetative meristem is one that can

Flowers

Vegetative growth

FIGURE 24.12 A simplified model for floral evocation at the shoot apex in which the cells of the vegetative meristem acquire new developmental fates. To initiate floral development, the cells of the meristem must first become competent. A competent vegetative meristem is one that can

Expressed:

The apical meristem undergoes morphogenesis.

Flowers respond to a floral stimulus (induction) by becoming flo-rally determined (committed to producing a flower). The determined state is usually expressed, but this may require an additional signal. (After McDaniel et al. 1992.)

In another experiment, the donor plant was decapitated above the thirty-seventh leaf. This time the thirty-seventh axillary bud flowered after producing four leaves in all three situations (see Figure 24.13B). This result demonstrates that the terminal bud became florally determined after initiating 37 leaves.

Extensive grafting of shoot tips among tobacco varieties has established that the number of nodes a meristem produces before flowering is a function of two factors: (1) the strength of the floral stimulus from the leaves and (2) the competence of the meristem to respond to the signal (McDaniel et al. 1996).

In some cases the expression of flowering may be delayed or arrested even after the apex becomes determined, unless it receives a second developmental signal that stimulates expression (see Figure 24.12). For example, intact Lolium temulentum (darnel ryegrass) plants become committed to flowering after a single exposure to a long day. If the Lolium shoot apical meristem is excised 28 hours after the beginning of the long day and cultured in vitro, it will produce normal inflorescences in culture, but only if the hormone gibberellic acid (GA) is present in the medium. Because apices cultured from plants grown exclusively in short days never flower, even in the presence of

(A) Bud not determined

Decapitation here

(A) Bud not determined

Donor In situ Rooted Grafted

(B) Bud florally determined

Decapitation here

(B) Bud florally determined

FIGURE 24.13 Demonstration of the determined state of axillary buds in tobacco. A specific axillary bud of a flowering donor plant is forced to grow, either directly on the plant (in situ) by decapitation, or by rooting or grafting to the base of the plant. The new leaves and flowers produced by the axillary bud are indicated by shading. (A) Result when the bud is not determined. (B) Result when the bud is florally determined. (After McDaniel 1996.)

Donor In situ Rooted Grafted

FIGURE 24.13 Demonstration of the determined state of axillary buds in tobacco. A specific axillary bud of a flowering donor plant is forced to grow, either directly on the plant (in situ) by decapitation, or by rooting or grafting to the base of the plant. The new leaves and flowers produced by the axillary bud are indicated by shading. (A) Result when the bud is not determined. (B) Result when the bud is florally determined. (After McDaniel 1996.)

Donor In situ Rooted Grafted

Donor In situ Rooted Grafted

FIGURE 24.14 Effect of plant age on the number of long-day (LD) inductive cycles required for flowering in the long-day plant Lolium temulentum (darnel ryegrass). An inductive long-day cycle consisted of 8 hours of sunlight followed by 16 hours of low-intensity incandescent light. The older the plant is, the fewer photoinductive cycles are needed to produce flowering.

GA, we can conclude that long days are required for determination in Lolium, whereas GA is required for expression of the determined state.

In general, once a meristem has become competent, it exhibits an increasing tendency to flower with age (leaf number). For example, in plants controlled by day length, the number of short-day or long-day cycles necessary to achieve flowering is often fewer in older plants (Figure 24.14). As will be discussed later in the chapter, this increasing tendency to flower with age has its physiological basis in the greater capacity of the leaves to produce a floral stimulus.

Before discussing how plants perceive day length, however, we will lay the foundation by examining how organisms measure time in general. This topic is known as chronobiology, or the study of biological clocks. The best-understood biological clock is the circadian rhythm.

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  • Elanor
    What is competance in flower?
    2 years ago

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