|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; firstname.lastname@example.org|
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:
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:
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".
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
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:
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:
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.
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.
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:
D. Photoperiodism summary.
V. Temperature and flowering
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?).
Cold, actual temperature varies from -5 to 15 C.
D. Transducing mechanism
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