The Coincidence Model Is Based on Oscillating Phases of Light Sensitivity

The involvement of a circadian oscillator in photoperi-odism poses an important question How does an oscillation with a 24-hour period measure a critical duration of darkness of, say, 8 to 9 hours, as in the SDP Xanthium Erwin Bunning proposed in 1936 that the control of flowering by photoperiodism is achieved by an oscillation of phases with different sensitivities to light. This proposal has evolved into a coincidence model (Bunning 1960), in which the circadian oscillator controls the timing...

Meristem Identity Genes Regulate Meristem Function

Meristem identity genes must be active for the primordia formed at the flanks of the apical meristem to become floral meristems. (Recall that an apical meristem that is forming floral meristems on its flanks is known as an inflorescence meristem.) For example, mutants of Antirrhinum (snapdragon) that have a defect in the meristem identity gene FLORICAULA develop an inflorescence that does not produce flowers. Instead of causing floral meristems to form in the axils of the bracts, the mutant...

Far Red Light Modifies Flowering in Some LDPs

Circadian rhythms have also been found in LDPs. A circa-dian rhythm in the promotion of flowering by far-red light has been observed in barley (Hordeum vulgare) and Ara-bidopsis (Deitzer 1984), as well as in darnel ryegrass (Lolium temulentum) (Figure 24.24). The response is proportional to the irradiance and duration of far-red light and is therefore a high-irradiance response (HIR). Like other HIRs, PHYA is the phytochrome that mediates the response to far-red light (see Chapter 17). In both...

The Control of Flowering

MOST PEOPLE LOOK FORWARD to the spring season and the profusion of flowers it brings. Many vacationers carefully time their travels to coincide with specific blooming seasons Citrus along Blossom Trail in southern California, tulips in Holland. In Washington, D.C., and throughout Japan, the cherry blossoms are received with spirited ceremonies. As spring progresses into summer, summer into fall, and fall into winter, wildflowers bloom at their appointed times. Although the strong correlation...

Plants Monitor Day Length by Measuring the Length of the Night

Under natural conditions, day and night lengths configure a 24-hour cycle of light and darkness. In principle, a plant could perceive a critical day length by measuring the duration of either light or darkness. Much experimental work in the early studies of photoperiodism was devoted to establishing which part of the light-dark cycle is the controlling factor in flowering. Results showed that flowering of SDPs is determined primarily by the duration of darkness (Figure 24.19A). It was possible...

Biochemical Signaling Involved In Flowering

In the preceding sections we examined the influence of environmental conditions (such as temperature and day length) versus that of autonomous factors (such as age) on flowering. Although floral evocation occurs at the apical meristems of the shoots, some of the events that result in floral evocation are triggered by biochemical signals arriving at the apex from other parts of the plant, especially from the leaves. Mutants have been isolated that are deficient in the floral stimulus (see Web...

Photoperiodism Monitoring Day Length

As we have seen, the circadian clock enables organisms to determine the time of day at which a particular molecular or biochemical event occurs. Photoperiodism, or the ability of an organism to detect day length, makes it possible for an event to occur at a particular time of year, thus allowing for a seasonal response. Circadian rhythms and pho-toperiodism have the common property of responding to cycles of light and darkness. Precisely at the equator, day length and night length are equal and...

Blue Light Photoreceptor Also Regulates Flowering

In some LDPs, such as Arabidopsis, blue light can promote flowering, suggesting the possible participation of a blue-light photoreceptor in the control of flowering. The role of blue light in flowering and its relationship to circadian rhythms have been investigated by use of the luciferase reporter gene construct mentioned in Web Topic 24.6. In continuous white light, the cyclic luminescence has a period of 24.7 hours, but in constant darkness the period lengthens to 30 to 36 hours. Either red...

Vernalization Results in Competence to Flower at the Shoot Apical Meristem

Plants differ considerably in the age at which they become sensitive to vernalization. Winter annuals, such as the winter forms of cereals (which are sown in the fall and flower in the following summer), respond to low temperature very early in their life cycle. They can be vernalized before germination if the seeds have imbibed water and become metabolically active. Other plants, including most biennials (which grow as rosettes during the first season after sowing and flower in the following...

Indirect Induction Implies That the Floral Stimulus Is Self Propagating

In at least three cases Xanthium (SDP), Bryophyllum (SLDP), and Silene (LDP) the induced state appears to be self-propagating (Zeevaart 1976). That is, young leaves that develop on the receptor plant after it has been induced to flower by a donor leaf can themselves be used as donor leaves in subsequent grafting experiments, even though these leaves have never been subjected to an inductive pho-toperiod. This phenomenon is called indirect induction. It is characteristic of indirect induction...

Circadian Rhythms Exhibit Characteristic Features

Circadian rhythms arise from cyclic phenomena that are defined by three parameters 1. Period, the time between comparable points in the repeating cycle. Typically the period is measured as the time between consecutive maxima (peaks) or minima (troughs) (Figure 24.15A). 2. Phase2, any point in the cycle that is recognizable by its relationship to the rest of the cycle. The most obvious phase points are the peak and trough positions. 3. Amplitude, usually considered to be the distance between...

The ABC Model Explains the Determination of Floral Organ Identity

In 1991 the ABC model was proposed to explain how homeotic genes control organ identity. The ABC model postulates that organ identity in each whorl is determined by a unique combination of the three organ identity gene activities (see Figure 24.6) Activity of type A alone specifies sepals. Activities of both A and B are required for the formation of petals. Activities of B and C form stamens. Activity of C alone specifies carpels. The model further proposes that activities A and C mutually...

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...

Plants Can Be Classified by Their Photoperiodic Responses

Different Types Tobacco Plants

Numerous plant species flower during the long days of summer, and for many years plant physiologists believed that the correlation between long days and flowering was a consequence of the accumulation of photosynthetic products synthesized during long days. This hypothesis was shown to be incorrect by the work of Wightman Garner and Henry Allard, conducted in the 1920s at the U.S. Department of Agriculture laboratories in Beltsville, Maryland. They found that a mutant variety of tobacco,...

Phytochrome Is the Primary Photoreceptor in Photoperiodism

Photoperiodism Figure

Night-break experiments are well suited for studying the nature of the photoreceptors involved in the reception of light signals during the photoperiodic response. The inhibition of flowering in SDPs by night breaks was one of the first physiological processes shown to be under the control of phytochrome Figure 24.22 . In many SDPs, a night break becomes effective only when the supplied dose of light is sufficient to saturate the photoconversion of Pr phytochrome that absorbs red light to Pfr...

Gibberellins and Ethylene Can Induce Flowering in Some Plants

Among the naturally occurring growth hormones, gib-berellins GAs see Chapter 20 can have a strong influence on flowering see Web Topic 24.8 . Recent studies suggest that gibberellin promotes flowering in Arabidopsis by activating expression of the LFY gene Blazquez and Weigel 2000 . Exogenous gibberellin can evoke flowering when applied either to rosette LDPs like Arabidopsis, or to dual-day length plants such as Bryophyllum, when grown under short days Lang 1965 Zeevaart 1985 . In addition,...

The Circadian Clock Is Involved in Photoperiodic Timekeeping

Photoperiodism Examples

The decisive effect of night length on flowering indicates that measuring the passage of time in darkness is central to photoperiodic timekeeping. Most of the available evidence favors a mechanism based on a circadian rhythm Bunning 1960 . According to the clock hypothesis, pho-toperiodic timekeeping depends on an endogenous circa-dian oscillator of the type involved in the daily rhythms described in Chapter 17 in relation to phytochrome. The central oscillator is coupled to various...

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...

Evidence for Antiflorigen Has Been Found in Some LDPs

Grafting studies have implicated transmissible inhibitors in flowering regulation as well. Such inhibitors have been called antiflorigen, but like florigen antiflorigen may consist of multiple compounds. For example, grafting an unin-duced leafy shoot from the LDP Nicotiana sylvestris onto the day-neutral tobacco cultivar Trapezond suppressed flowering in the day-neutral plant under short days but not long-day conditions Figure 24.31 . On the other hand, when an uninduced donor from the SDP...

Three Types of Homeotic Genes Control Floral Organ Identity

Floral Homeotic Genes

Five different genes are known to specify floral organ identity in Arabidopsis APETALA1 API , APETALA2 AP2 , APETALA3 AP3 , PISTILLATA PI , and AGAMOUS AG Bowman et al. 1989 Weigel and Meyerowitz 1994 . The organ identity genes initially were identified through mutations that dramatically alter the structure and thus the identity of the floral organs produced in two adjacent whorls Figure 24.5 . For example, plants with the ap2 mutation lack sepals and petals see Figure 24.5B . Plants bearing...