Plant Physiology

Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu

Flowering

I. An overview
The process of flowering requires the vegetative meristem (buds) to change into a reproductive meristem. This process, which is termed evocation, involves the following sequence of events:

juvenile vegetative phase � adult vegetative phase � adult reproductive phase � flowering

A. Juveniles vs. adults
Juvenile plants cannot flower; they are capable of only vegetative growth. Thus, like in humans and other animals, the ability to reproduce marks the transition from the juvenile phase to adulthood. As we previously discussed (GA section), juvenile plants often differ in appearance from the adult. For example, leaves may change shape (i.e., ivy adult - simple leaves; juvenile - lobed leaves) or degree of compounding (i.e., beans: adult � compound leaf; juvenile � simple leaf). Juvenile tissues are produced first, near the base of the plant. The adult phase is usually stable and can be propagated from plant to plant.

B. Ripe to Flower.
The adult can flower and is said to be "ripe-to-respond (or flower)" or "competent". In other words, it has the potential to flower when the conditions are appropriate. This may be a mechanism to insure that there is a sufficient vegetative mass (i.e., leaves, roots) to support the reproductive output.

C. Juvenile to Adult Transformation.
The maturation of the juvenile into the adult may be mediated by a variety of factors including:

size (more important than age)
age (i.e., century plants, bamboo);
leaf number;
growth conditions (conditions that favor growth promote the transition to adult phase; poor conditions, such as water stress, lack of light, low temp, prolong the juvenile phase)
other factors.

Ultimately, one or more of these factors likely induce changes in (1) hormones (such as GA; for example, recall that GA application stimulates the adult phase in conifers but in ivy promotes juvenility), (2) nutrient levels (i.e., lack of a carbohydrate supply to the meristematic region), or (3) other chemicals, that in turn trigger the developmental switch to adulthood.

Thus we can modify our original scheme:

juvenile vegetative phase � transition factors (i.e., size, age) � induce hormonal or other changes � adult vegetative phase (competent) � adult reproductive phase � flowering

D. Adult Vegetative-to-Reproductive Transition
The transition from the vegetative to reproductive buds is usually triggered by an environmental signal, typically photoperiod or temperature. This signal synchronizes flowering to environmental events. Thus, this is a type of "timing mechanism" that plants use to coordinate actions with the season. If flowers are produced at the wrong time of the year the pollinator may not be available, or it may be too dry (or wet), or there may not be enough time before winter to allow time for successful seed set. Once the inductive signal has been received then the plant meristem is said to be "determined". In other words, it is now committed to flower. Thus, we can modify the diagram again:

juvenile vegetative phase � transition factors (i.e., size, age) � induce hormonal or other changes � adult vegetative phase (competent) � environmental signal (i.e., photoperiod, temperature) � adult reproductive phase (determined) � flowering expressed

In some plants, an environmental signal is not necessary to trigger the transition to the determined state. These plants move directly in the reproductive phase after becoming competent.

The two major signals for inducing flowering are light (photoperiod) and temperature (cold treatment)

II. Light, or more specifically, photoperiod and flowering

A. A brief history.
Tournois (1914) was one of the first to report the influence of photoperiod on hops and hemp. Garner's and Allard's classic studies showed that a tobacco mutant, Maryland Mammoth, which failed to flower under field conditions, did so in the greenhouse in the winter in response to photoperiod.

B. The flowering response to day length varies with the species:

Short day plants (SDP) - require one or more days with less than a certain amount of daylight. Or, the critical day length to induce flowering must be less than some maximum. These species usually flower in the spring or fall.
Long day Plants (LDP) - require one or more days with more than a minimum day length to flower. The critical day length must be longer than a minimum.
Day neutral plants (DNP) - ambivalent to day length
Plants exhibit a variety of intermediate responses and combinations. For example, there are long-short day plants. After an inductive long-day photoperiod, these plants require short days to flower. This is a good strategy to insure flowering in the late summer.
One inductive photoperiod may suffice to induce flowering (i.e., cocklebur, Japanese morning glory); or, flowering many require several days, with a cumulative effect.
Light may have a quantitative effect on flowering - in other words, SD may stimulate the percentage of plants that flower.

C. The night period is more important than the day.
Using cocklebur, a SDP, Bonner & Hamner (data provided in class) showed that it flowers if it received one critical photoperiod with less than 8 hours of light (or, > 16 h darkness). The proportion of light/dark is not important in flowering. A light break during the night interrupts the flowering response, but a dark period during the day has little effect on flowering. The timing of the night break is important (see data). Conclusion: long day plants can be called "short night plants" and short day plants can be called "long night plants".

D. Receptor.

1. Location.
The receptor is located in the leaves. Evidence: (a) defoliated plants are insensitive to photoperiod; (b) plants with a single leaf in the inductive photoperiod will bloom (e.g., Perilla; Chailakhyan, 1961); (c) the receptor doesn't seem to reside in meristem since treating the meristem with inductive photoperiod doesn't initiate flowering (Chailakhyan).

2. Nature of the receptor.
Since light is involved, it suggests that a pigment plays a role in photoperiodism. Phytochrome is a likely candidate because: (a) the light break shows red/far red sensitivity; (b) the action spectrum for the light break in cocklebur is consistent with phytochrome (data provided in class). In fact, there is evidence for the participation of two forms of phytochrome.

E. Transducing mechanism

A diffusible substance plays a role. This is clear since the leaves are the receptors, but the meristem is converted to reproductive stage. Something must be translocated from the reception site (leaves) to the action site (meristem). Grafting experiments provide further evidence (see data).
Rate of transport consistent with phloem movement.
The diffusible substance is probably similar in most plants. If a SDP is grafted to a LDP and they are placed in a short-day photoperiod (that can induce flowering in the SDP), they will both flower, even though the LDP is not in its inductive photoperiod.
Florigen - name given to the proposed flowering "hormone". There is little direct chemical evidence for its existence. Most evidence is from physiological experiments as described above. An extract of induced cocklebur has weak floral-inducing ability.
GA may be involved (recall our hormone discussion?). In brief, exogenous application of GA can substitute for photoperiod, especially in LDP's
Ethylene, IAA or cytokinin are associated with flowering in some species. One problem with many of these studies is that the concentration of hormone used was much larger than the amounts that are naturally-occurring and could stimulate abnormal processes. Other substances may be involved in flowering, too.
Anthesin is a hypothetical hormone proposed by Chailakhyan that may stimulate flowering in SDP when associated with GA. This existence of this substance is dubious.

III. Timing Mechanisms in plants - a small, but necessary tangent - you�ll soon see how it relates

A. The plant way of life revisited.
Recall that a stationary organism like a plant must be able to accurately predict when the environment is going to change. Since plants can�t move they must respond to these changes by relatively slow growth responses/movements that take time. Thus, plants must have an accurate sense of time to distinguish daily and seasonal changes and respond to them.

B. Examples of timing mechanisms in plants:
Timing mechanisms include: (1) Flowering - recall that plants must flower at the appropriate time of the season; (2) Bud break and seed germination - timing mechanism to determine when spring has arrived; develop too soon and they risk freeze injury, develop too late and they may not have enough time to complete the life cycle.

C. Requirements for a biological clock.
The clock must be:

accurate (keep good time or be reset each day);
insensitive to capricious events in the environment (those that are not predictable such as temperature, wind speed, precipitation); and
have a transducing mechanism to couple the clock to a physiological response.

D. Hourglass (or cumulative) Timers.
Egg timers and water clocks are good examples of hourglass timers. These clocks measure time by monitoring the interval of time required for a certain event (i.e., sand to run out, or water to drip) to occur. They measure a single interval and then need to be reset. Hourglass timers in plants include:

ripeness-to-flower. Essentially the life of the plant can be considered to be an hourglass timer that isn�t reset;
induction/reversion phenomena due to phytochrome (i.e., seed germination); and
seed germination/bud break - many seeds and buds measure the amount of inhibitor present and develop when the level drops below some critical value.

E. Oscillating or rhythmic timers
These measure time intervals between regular oscillations, such as the sweeps of a pendulum. Many events in plants show rhythmic oscillations such as the opening/closing of flowers, leaf movements (nyctinasty), and even growth rates.

Anatomy of an oscillation
Period refers to the time between repeating points of the cycle, or in other words, the time it takes to complete one cycle. Period is symbolized by tau. Most biological rhythms have a period of ca. 24 hours though they vary from 21-27. Hence, they are called circadian, because they last approximately (circa) one day (diem). Other types of rhythms include lunar, annual, ultradian (less than a day). Amplitude - intensity of oscillations, or in other words, the difference between the peaks and troughs.
Oscillating timers.
These are free running - in other words they don�t have to be restarted at each period. And they will continue under constant conditions though the response will eventually damp out. This is evidence that there is an endogenous oscillator; an hourglass timer would not keep running without an external stimulus to reset it. This is analogous to a spring-loaded watch.

Just like we need to coordinate our clocks with the season (i.e., "spring ahead or fall back"), plants must be able to set their oscillating timer to correspond to environmental changes. This is called entrainment and the signal that synchronizes the rhythm is the zeitgeber. Light is the zeitgeber. Both blue and red light are important. The red light sensitive species show red/far-red reversibility suggesting that phytochrome is the receptor for the response.

The nucleus is the site of an endogenous, oscillating timer (see data).

Now back to our regularly scheduled program.......

IV. Transducing mechanism for photoperiodism

A. Photoperiodism as an hourglass timer.
According to this hypothesis, plants measure the ratio of Pfr/Pr. An LDP would flower when the ratio is high (i.e., more Pfr) and but SDP would flower when the ratio is low (more Pr). Since Pfr is labile and is broken down at night or reverts back to Pr - the longer the night, the lower the phytochrome (Pfr) content. Thus, phytochrome is like the sand in an egg timer; the relative amount of Pfr remaining at the end of the night would be an indication of the day length. To "reset" an egg timer, you simply turn it over. Similarly, the flowering timer would be reset during the day when Pfr levels are re-established.

Problems with this hypothesis: (a) the half-life of Pfr breakdown is too short; and (b) if phytochrome degradation is involved, the process should be temperature sensitive (i.e., Q10 should increase), but it is not.

B. Photoperiodism as an oscillating timer.
A more recent idea, and one that has greater support, is that flowering is controlled by an oscillating timer.

Evidence: (1) Chenopodium rubrum (SDP) seedlings maintained in continuous light will flower if exposed to single dark period. The length of the dark period shows a rhythmic effect on flowering; (2) Duckweed (Lemna) studies also show an oscillating timer to exist.

C. Photoperiodic Control of Plant Growth.
Other aspects of plant growth and development are affected by photoperiod including:

seed germination (some like long days, others short days);
stem growth (promoted by long days;
root and storage organ formation (induced by short days in potato, dahlia and radish, long days in onion);
vegetative reproduction (long days - strawberry runners, Bryophyllum plantlets);
sexual reproduction (flower induction and development);
autumn response; and
flowering.

D. Photoperiodism summary.

plants are affected by photoperiod in many ways;
there is a wide diversity of responses of flowering to photoperiodism (i.e., LDP, SDP); Plants must be ripe-to-flower before they will respond;
dark period is important;
an hourglass timer may be involved;
an oscillating timer is surely involved;
a flowering hormone is involved;
flowering can be induced by exogenous hormone application.

V. Temperature and flowering

A. Overview.
Many plants require a cold treatment to induce flowering. This is termed vernalization and is a "smart" way to time when winter is over (type of hourglass timer). Vernalization is common in biennials and winter annuals (such as winter wheat). The effect can be qualitative or quantitative. Vernalization usually works in concert with photoperiod - in other words, vernalization is required to make the plants sensitive to photoperiod. Thus, this acts as a "fail-safe" system to insure flowering at the appropriate time of year (after winter!). The term was coined by Lysenko (remember him - inheritance of acquired characteristics?).

B. Signal.
Cold, actual temperature varies from -5 to 15 C.

C. Receptor.

Some seeds can be vernalized (data for rye). However, they must be hydrated (dry, unimbibed seeds are insensitive). Biennials not responsive as seeds.
The meristem perceives cold treatment - grafting experiments and tissue culture experiments with rye.
Some plants (henbane) need to reach a certain size to be responsive to cold treatment, whereas others (rye) can be treated as seeds.

D. Transducing mechanism

Plants can "remember" the signal. In other words, cold-treated plants will grow vegetatively for quite awhile before flowering. This suggests that the induced state must be permanent, or at the least be relatively stable, in many species.
A hormone doesn't seem to be absolutely required since the meristem is the source of the receptor and it could pass the permanent change on to future cells.
A chemical signal may be involved - the transmission of a signal through grafts has been noted with some. This hormone has been termed vernalin, but not yet isolated.
Plants can be de-vernalized if the cold treatment is followed by a heat. In rye, 30 C for 3-5 days will do the job. Rye can also be devernalized by drying and anaerobic conditions.
GA can substitute for cold in some cold-requiring species. But, GA may primarily affect bolting (stem elongation).
Vernalization of isolated embryos requires oxygen and carbohydrate - suggests an energy-dependent process.

Last updated: 01/07/2009 / � Copyright by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html