Coordination in plants

Coordination is a process of linking up the process in the body of an organism such that the various process occurs in an organized manner.

Plant responses to the environment are controlled by chemicals or hormones. Responses in plants occur in different ways including

1. Tropisms
2. Tactic movements
3. Nastic responses
4. Flowering responding to day length

A. Tropisms

Tropic movements are growth responses towards directional stimuli.

Types of tropisms

Tropisms are classified based on the type of stimulant.

1. Phototropism (light)
2. Hydrotropism (water)
3. Geotropism (gravity)
4. Thigmotropism (touch)
5. Chemotropism (chemical)

Phototropism

The growth response towards the light from one direction. Shoots grow towards unidirectional light and are said to be positively phototropic. Most roots do not respond to light.

Experiment to demonstrate effect of unidirectional light on the growth of shoot

1. Select two potted seedlings of bean of equal age and size and water them.
2. Place one of the seedlings in a box painted black on each side. The box should have a hole on one side to allow in light.
3. Subject the second seedling to the same conditions as the first one except that the pot is mounted on a slowly rotating clinostat.
4. Leave the experiment to stand for 3 days

Note that each seedling should be growing straight at the beginning of the experiment.

 

Observation

The seedlings on the clinostat grew straight while the other bent toward the light

NB. Slowly rotating clinostat ensures uniform exposure of the shoot from all directions.

The significance of phototropism, allow the shoot to grow towards and receive light for photosynthesis.

Importance of phototropism

Positive phototropism causes the stems of plants to grow towards a light source causing the leaves of the plant to be pointing towards the light source; this allows the leaves to absorb more light which maximizes photosynthesis.

B. Taxes

Taxes:-  is a movement of an entire cell or organism ( i.e. locomotion) in response to and directed by an external stimulus.  Tactic responses can be described as positive or negative and can be further classified according to the nature of the stimulus

Types of tactic responses

There are many types of tactic movements including

• Phototaxis (stimulation by light)
• aerotaxis (stimulation by oxygen)
• anemotaxis (by wind)
• barotaxis (by pressure)
• chemotaxis (by chemicals)
• durotaxis (by stiffness)
• electrotaxis or galvanotaxis (by electric current)
• gravitaxis (by gravity)
• hydrotaxis (by moisture)

C. Nastic responses

Nasties:-  is a non-directional movement of part of a plant is a response to an external stimulus. 

For example, “ sensitive plant”  Mimosa pudica is sensitive to touch as well as a variety of other stimuli.  It exhibits normal sleep movement but responds very rapidly to shocks (seismonasty) such as a sharp blow, injury, or sudden change in temperature or light intensity.  If leaflets at the tip are shocked, they fold upwards in seconds.  If the stimulus is strong or sustained, successive pairs of leaflets fold up and stimulus eventually passes through the whole leaf, resulting in the petiole drooping.  The stimulus will pass in the reverse direction if the stem is stimulated.

D. Kinesis

Kinesis:-  This is a type of locomotory response.  Since this is virtually confined to the animal kingdom it will be discussed with animal behavior plant growth substances.  

Plant hormones

There are chemicals substances that control co-ordination in plant growth

(a) Auxins

Auxin, any of a group of plant hormones that regulate or modify the growth of plants, especially root formation, bud growth, and fruit and leaf drop. They include indole acetic acid (IAA)

Auxins are produced at the shoot and root tips and concentrate on the side opposite the stimuli. In stem high concentration of auxins stimulates growth while a high concentration of auxins in roots inhibits growth as shown in the graph below

Role of auxins in plants

• Stimulates cell elongation
• Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture
• Stimulates differentiation of phloem and xylem
• Stimulates root initiation on stem cuttings and lateral root development in tissue culture
• Mediates the tropistic response of bending in response to gravity and light
• The auxin supply from the apical bud suppresses the growth of lateral buds
• Delays leaf senescence
• Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission
• Can induce fruit setting and growth in some plants
• Involved in assimilate movement toward auxin possibly by an effect on phloem transport
• Delays fruit ripening
• Promotes flowering in Bromeliads
• Stimulates growth of flower parts
• Promotes (via ethylene production) femaleness in dioecious flowers
• Stimulates the production of ethylene at high concentrations

(b)  Gibberellins

This is another class of growth promoters produced particularly in young epical leaves (possibly in chloroplasts), buds, seeds, and root tips.  They fall into a group of chemical compounds called terpenes.

Role of gibberellins in plants

1. Promotes stem and root elongation thus, genetically dwarf varieties of peas and maize are restored to normal growth and dwarf beans can be converted into runners by application of gibberellins.
2. Mobilize enzymes that release nutrient reserves in grass seeds.
3. they stimulate the growth of side branches from axillary buds
4. they break dormancy and promotes germination of seeds
5. promotes bolting and flowering in long-day plants
6. promotes the growth of lateral buds
7. promotes the development of seedless fruits (parthenocarpy)
8. delays senescence

(c) Cytokinins

This is a class of growth substances that promote cell division.  They do so, however in the presence of auxins.  Gibberellins may also play a role, as in the cambium.

Roles of cytokinins

1. Promotes cell division
2. Promotes morphogenesis
3. Promotes the development of lateral roots
4. Delays senescence
5. Promotes stomata opening

(d) Abscisic acid (ABA)

Abscisic acid is a major inhibitor in plants and is antagonistic to all the three classes of growth promoters. 

Role of abscisic acid

1. Maintains leaf dormancy
2. Causes stomata closure
3. Causes seed dormancy
4. Promotes fall of leaves and fruits

(e) Ethene

1. Promotes fruit ripening by promotes the conversion of starch to soluble sugars and triggers a sudden and dramatic increase in the respiration rate which leads to ripening.
2. Promote wound healing.
3. Promotes etiolation of in plants
4. Promotes leaf and fruit fall
5. In plants whose stems grow underwater, it causes the formation of aerenchyma tissue parenchyma tissue with large air-filled spaces between the cells: it helps to make the plant buoyant.

Phytochrome and effects of light on plant development

Phytochromes are a class of photoreceptors in plants, bacteria, and fungi used to detect light.

In plants, the phytochrome molecules are found in small amounts, in the tips of the growing shoot. 

The compound consists of a pigment portion attached to a protein and exits in two forms inter-convertible forms.  One form Pfr or P730 absorbs far-red light and the other Pr or P660 absorbs red light.  Absorption of light by one form converts it rapidly and reversibly to the other form.

In natural sunlight Pr (inactive) is converted into Pfr (active), and Pfr into Pr.  However, the former predominates because sunlight contains more red than far-red light and in any case, less energy is needed to convert Pr into Pfr than vice versa.  So Pfr tends to accumulate during daylight hours, whilst at night is converted slowly back into Pr.

Besides flowering, the phytochrome system regulates the germination of seed (photoblasty), the synthesis of chlorophyll, the elongation of seedlings, the size, shape, and number and movement of leaves and the timing of flowering in adult plants.

The control of flowering photoperiodism & flowering

Flowering and many other responses shown by both plants and animals are regulated by day length, i.e. the duration of the photo-period.   The general term for this phenomenon is photoperiodism. 

In order to respond to day length, the plant must be able to: measure” the duration of the light period or dark period or both.  It is now known that the critical factor is the duration of the dark period, in other words, the time that elapses between two consecutive light periods.  Experiments have indicated that photoperiodic control flowering is achieved through the phytochrome system.

On the basis of their flowering responses to the photoperiods, flowering plants can be divided into three groups.

• Short day plants e.g. Chrysanthemums, poinsettias, and orchids.  Only flower if the period of uninterrupted darkness is more than a certain length each day.  Short day plants can be induced to flower by lights period that is longer than the critical lengths.  Conversely, they can be prevented from flowering by nights that are shorter than the critical length.  On the basis of the phytochrome system, short-day plants flowering is promoted by the absence of Pfr, which is converted into Pr during the long nights.

• Long day plants:-  e.g. petunias, spinach, radishes, and lettuce, on flower if the period of uninterrupted darkness is less than a certain critical length each day.  Long day plants can be induced to flower by nights that are longer than the critical length.  These plants will flower due to the accumulation of Pfr resulting from long exposure to light.

• Day-neutral plants:-  e.g. geranium, tomato, cucumber, and snapdragon are indifferent to day length and will flower irrespective of the relative durations of light and dark which they receive each day.  If follows, therefore, that flowering in these plants will occur of whether Pfr or Pr are present.

A short day and day-neutral plants, on the other hand, tend to live nearer the equator where days and nights are about the same length all the year, but in the temperature zone long day tend to flower in summer and short-day plants flower in the autumn.

As regards to the phytochrome system, the photoperiodic stimulus is detected by leaves.  From the leaves, the message is transmitted to the buds, some of which respond by changing into flower buds.  The latter, instead of giving rise to vegetative structures such as side branches and leaves, develop into flowers.  In other words, on receipt of the message, a potentially vegetative apex is turned into a floral apex.  The message itself takes the form of a chemical substance and this hypothetical flowering hormone has been named florigen.

Florigen is transmitted to the buds that may respond by changing into flower buds. 

Note: Unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. Therefore, it is the length of darkness that is critical.

 

The significance of photoperiod in plants

• Enables plants to flower insect pollinators are active in summer and autumn.
• Long day plants require more exposure to lightin order to initiate flowering and can be induced to flower by flashes of light at night
• Short day plants can be induced to flower covering and eliminating nature light or move the plants into the dark chamber for specific periods of time.
• Short day and day neutral plants on the other hand, tend to live nearer the equator where days and nights are about the same length all the year, but in the temperature zone long day tend to flower in summer and short day plants flower in the autumn.

Vernalization and flowering

This is the promotion of flowering by a period of low temperature.

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Plant responses (tropism, auxins, taxis and nastic movement, photoperiodism)

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Dr. Bbosa Science