Chapter 29 Control Systems in Plants

Unlike animals, plants possess no nervous system and thus their co-ordination is achieved almost entirely by hormones. Also plant hormones almost affect some aspect of growth which may lead to movements of plant parts, although these are relatively slow responses compared to those of animals.

29.1 Plant Responses - Plant movements are performed by growth responses 29.1.1 Tropisms - a directional growth movement made by a part of a stationary plant in response to a unilateral stimulus such as water, gravity or light. When the plant part grows towards the stimulus: positive When the plant part grows away from the stimulus: negative

- The growth movement is caused by different rates of growth on the two sides of the plant under the influence of the unilateral stimulus. The part bends to that side which grows more efficiently. Actively growing structures show tropisms more obviously, thus young seedlings are used to show tropisms. - According to the nature of the stimuli, tropisms are classified into (1) phototropism (light), (2) geotropism (gravity), (3) hydrotropism (water), (4) chemotropism (chemicals), (5) thigmotropism (touch), (6) aerotropism (air).

1. Hydrotropism - growth movement in response to water - roots are positively hydrotropic 2. Geotropism - growth movement in response to gravity - roots: positively geotropic shoots: negatively geotropic If a plant is subjected to two external stimuli such as water and gravity, to which stimulus will the plant respond? Which stimulus has greater influence? The plant will respond to water which has a greater influence.

3. Phototropism - growth movement in response to light - roots are negatively phototropic ; they grow away from light - shoots and leaves are positively photopropic ; they grow towards light

4. Chemotropism - growth response to chemicals, e.g. pollen tubes growing towards chemicals at the micropyle

4. Chemotropism - growth response to chemicals, e.g. pollen tubes growing towards chemicals at the micropyle 5. Thigmotropism - growth response to touch, e.g. tendrils of peas twine around support 6. Aerotropism - growth response to air, e.g. pollen tubes grow away from air

29.1.2 Taxes - a directional movement made by the whole plant (usually motile) in response to a unilateral stimulus - Examples: (1) Phototaxis Chlamydomonas (a motile unicellular alga) swims towards unilateral light for photosynthesis. They are scattered evenly in water in complete darkness. (2) Thermotaxis Green algae or bacteria swim towards regions of optimum temperature (3) Chemotaxis Antherozoids of mosses are attracted by chemicals secreted by the archegonia

29.1.3 Nasties - a non-directional movement made by part of a stationary plant in response to an external stimulus. The direction of movement is not determined by the direction of the stimulus. This response is reversible and repeatable. Examples: thigmonasty (Mimosa, Venus flytrap), photonasty, thermonasty, etc

29.2 Plant Hormones - Characteristics of plant hormones: 1. They are organic substances which regulate growth & other physiological functions of plants 2. They lack specificity 3. They act on parts of the plant other than the part that produced them.

29.2.1 Auxins 1. Auxins are produced in small amounts at the tips of growing shoots and roots. 2. Auxins diffuse from the tip to the region of elongation where they exert their effects: cell walls become more easily stretched & cells elongate. 3. Transport of auxins is one direction (polar), i.e. away from tips by diffusion or by phloem

From Figure: - Shoots show an increased growth rate as the concentration of auxins increases. Once auxins concentration reaches a maximum, the growth of the shoot will be inhibited.

The growth of a root is promoted by a lower concentration of auxins than the growth of a shoot. The concentration suitable for shoot growth has an inhibitory effect on root growth. The concentration that stimulates maximum root growth has no stimulating effect on shoot growth.

- more auxins on shaded side MECHANISM OF PHOTOTROPISM OF SHOOTS AND ROOTS - more auxins on shaded side MECHANISM OF GEOTROPISM OF SHOOTS AND ROOTS - more auxins on lower side by gravity

Other effects of auxins: 1. Stimulate cell division, e.g. calluses formation 2. Help to maintain the structure of cell walls, thus inhibits abscission 3. Inhibit growth in high concentrations in apical dominance 4. Stimulate fruit development without fertilization (parthenocarpy)

Effect of gibberellins on plant growth 1. Reverse of genetic dwarfism, e.g. maize 2. Promote cell elongation, e.g. length of internodes 3. Break dormancy of buds & seeds --> see past paper on seed germination** 4. Stimulate fruit development 5. Remove the need for cold in vernalization 6. Affect flowering

29.2.3 Cytokinins 1. Promote cell division 2. Delay leaf senescence 3. Stimulate bud development 4. Break dormancy 29.2.4 Abscissic Acid - not required in syllabus 29.2.5 Ethene - not required in syllabus

Commercial Applications of Synthetic Growth Regulators Auxins as weedkillers; help to increase fruit yields; produce seedless fruits (parthenocarpy) Gibberellins keep leaves of crops, vegetables from yellowing when picked; with cytokinins to initiate germination Apple sectioned to show effects of auxin production

29.3 Control of Flowering - affected by daylength & temperature 29.3.1 The Phytochrome System Phytochrome is a photoreceptor for absorbing light. It is a blue-green pigment existing in two forms: Phytochrome 660 (P660) which absorbs red light Phytochrome 730 (P730) which absorbs far-red light

A short exposure to the appropriate light wavelength causes the conversion of one form into the other. These conversions may also be brought about by daylight and darkness (with slow conversion).

Phytochrome comprises a protein & a pigment. It is distributed in the plant in minute quantities, being more concentrated in growing tips. The actions of these two forms of phytochrome are usually antagonistic:

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits……

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits…… Stimulate flowering in long-day plants Inhibits ……

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits…… Stimulate flowering in long-day plants Inhibits …… Inhibits flowering in short-day plants Stimulate…..

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits…… Stimulate flowering in long-day plants Inhibits …… Inhibits flowering in short-day plants Stimulate….. Elongation of internode is inhibited ….promoted

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits…… Stimulate flowering in long-day plants Inhibits …… Inhibits flowering in short-day plants Stimulate….. Elongation of internode is inhibited ….promoted Induces increase in leaf area Prevents …..

TABLE 29.4 Summary of the effects of red light and far-red light Red light effect Far-red light effect P660  P730 P730  P660 Stimulate germination of some seeds, e.g. lettuce Inhibit germination of some seed, e.g. lettuce Induces formation of anthocyanins Inhibits…… Stimulate flowering in long-day plants Inhibits …… Inhibits flowering in short-day plants Stimulate….. Elongation of internode is inhibited ….promoted Induces increase in leaf area Prevents ….. Causes epicotyl hook to unbend Maintains epicotyl hook bent

29.3.2 Photoperiodism - affects flowering 1. Long-day plants: plants which flower when the period of daylight exceeds a critical minimum length 2. Short-day plants: plants which flower when the period of daylight is shorter than a critical maximum length  3. Day neutral plants: plants which flower regardless of the length of daylight

The length of the dark period is crucial in determining flowering: Short-day plants require a long dark period while long-day plants require a short dark period. Interrupting a long dark period with red light is as effective as daylight in stopping short-day plants flowering. Far-red light has no effect and short-day plants flower as if the dark period had been continuous.

These & other experiments suggest that phytochrome is the photoreceptor detecting different light wavelengths and ultimately determining whether or not a plant flowers. The light stimulus is detected by the leaves to induce flowering, with a hormone called florigen to travel from the leaf to the apex. Mechanism:

Vernalization Many plants grow vegetatively but fail to flower if they are not exposed to a period of low temperature. This phenomenon is called vernalization. A hormone called vernalin is responsible. Importance: 1.To ensure flowering at specific times of the year to suit the occurring of the particular pollinating agent, e.g. insects 2. To ensure the species produces flowers at the same time for cross pollination. Since photoperiod is consistent regardless of climatic conditions, thus a more reliable means of achieving synchronized flowering. Application: Growers can control temperature and lengths of light periods and produce the plant continuously throughout the year.