Plant Growth Regulators Chemical Messengers Hormones In plants

Plant Growth Regulators Chemical Messengers

Hormones In plants, many behavioural patterns and functions are controlled by hormones. These “chemical messages” influence many patterns of plant development. Plant hormones – are a natural substance (produced by the plant) that acts to control plant activities - chemical messengers.

Hormones Are produced in one part of a plant and then transported to other parts, where they initiate a response. They are stored in regions where the stimulus are and then released for transport through either phloem or mesophyll when the appropriate stimulus occurs.

Growth Regulators Plant growth regulators – include plant hormones (natural & synthetic), but also include nonnutrient chemicals not found naturally in plants that when applied to plants, influence their growth and development.

Growth Regulators 5 recognised groups of natural plant hormones and growth regulators. 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Abscisic acid

1. Auxins Found in leaves and stems. Causes cell enlargement or elongation – located in meristems and shoot tips (terminal & lateral buds). Auxins move mainly from apex (top) down. Lengthening of the internodes and influence the developing embryos in the seed.

When auxin is added to a stem, the stem will bend away from the auxin. It elongates faster on the extra auxin side.

Auxins Apical dominance – high levels of auxin in the stem just above lateral buds block their growth (blockage of growth of lateral buds by presence of terminal buds). If shoot tip is removed. The auxin level behind the lateral buds is reduced and the lateral buds begin to grow (the auxin which formed the blockage to keep lateral buds small is reduced so they can grow).

Auxins Photo (light) and geotropism (gravity) – involved in tropism responses – positive responses Flower Root initiation and development – used on cuttings to help stimulate root growth

Examples of Auxins Plant Growth Regulators Indobutyric acid (IBA) – synthetic e. g. Seradix 2, 4 -dichlorophenoxyacetic acid (2 -4 D) - synthetic Hormone Indoleactic occurring. acid (IAA) -naturally

Commercial uses of synthetic Auxin Adventitious Weed control Increasing fruit set Preventing Spray root initiation pre-harvest fruit drop thinning Inhibition of stem sprouting Micro-propagation.

2. Gibberellins Gibberellic Have Acid (GA) a regulatory function Are produced in the shoot apex primarily in the leaf primordial (leaf bud) and root system Stimulates stem growth dramatically

Gibberellins Stimulates cell division, cell elongation (or both) and controls enzyme secretions. E. g. dwarf cultivars can be treated with GA and grow to normal heights – indicates dwarf species lack normal levels of GA. Involved in overcoming dormancy in seeds and buds. GA translocates easily in the plant (able to move freely) in both directions – because produced in not only shoot apex but also in the root structure.

Gibberellins Used commercially in: Increasing fruit size of seedless grapes Stimulating seed germination & seedling growth Promoting male flowers in cucumbers for seed production. Overcoming cold requirements in some seed. Application of GA foregoes the cold requirements (some seed require to be frozen or placed in the refrigerator for a period of time before they will germinate).

3. Cytokinins Promotes cell division Found in all tissues with considerable cell division. E. g. embryos (seeds) and germinating seeds, young developing fruits n Roots supply cytokinins upward to the shoots. n Interact with auxins to influence differentiation of tissues (may be used to stimulate bud formation).

Cytokinins n As roots begin to grow actively in the spring, they produce large amounts of cytokinins that are transported to the shoot, where they cause the dormant buds to become active and expand. n Tissue culture uses cytokinins to induce shoot development. n Cytokinins may slow or prevent leaf senescence (leaf ageing or leaf fall).

4. Ethylene Gaseous hormone. Produced in the actively growing meristems of the plant, in ripening or ageing fruits, in ageing or dying flowers, in germinating seeds and in certain plant tissues as a response to bending, wounding or bruising. Ethylene as a gas, diffuses readily throughout the plant.

Ethylene May promote leaf senescing and abscission (leaf fall). Increases female flowers in cucumbers (economically - will increase fruit production). De-greening of oranges, lemons and grapefruit – ethylene gas breaks down chlorophyll and lets colours show through.

5. Abscisic Acid (ABA) Widespread in plant body – moves readily through plant. ABA appears to be synthesized (made) by the leaves. Interacts with other hormones in the plant, counteracting the growth – promoting the effects of auxins & gibberellins.

Abscisic Acid Involved with leaf and fruit abscission (fall), onset of dormancy in seeds and onset of dormancy (rest period) in perennial flowers and shrubs. ABA is effective in inducing closure of stomata in leaves, indicating a role in the stress physiology in plants. E. g. increases in ABA following water, heat and high salinity stress to the plant.

Shoot and Root Development The balanced growth of a plant is the result of varying auxin and cytokinins ratios within the plant. Auxins are produced in actively growing aerial buds in significant concentrations. Cytokinins are found throughout the plant. High auxin / cytokinins ratios result in lateral bud break and subsequent shoot development. These ratios can be used to explain observed seasonal growth patterns: In spring, low auxin / cytokinins ratios result in vigorous shoot development from dormant buds. Actively growing terminal buds increase the auxin / cytokinins ratio, resulting in increased root growth. Increased root growth lowers the auxin / cytokinins ratio, inhibiting further shoot growth.

Fruit Set – Fertilisation. Successful fertilisation requires; 1. Pollen compatibility – successful germination of the pollen tube. 2. Correct environmental conditions – this affects the rate of pollen tube growth to the ovule. Zygote development leads to the formation of the fruit and usually seeds within the fruit. Plant hormones are involved with this fruit set. When the zygote fails to develop and no seed forms, the immature fruit usually drops. With some fruits e. g. grapes and citrus, the fruit does not fall, even through the seed fails to develop.

Fruit Set – Growth regulators In the fruits of some species, auxin application can replace the stimulation of the developing zygote; e. g. the use of Fullset in tomatoes. Auxin sprays are also used to increase the production of green peppers, egg plants and figs. After initial fruit set, natural cytokinins ratios can be disrupted by the application of NAA (a synthetic auxin) to cause fruit thinning. Other compounds can also be used as chemical thinning agents e. g. the insecticide Carbaryl is used to thin pip fruit and the fungicide Saprol can have a similar effect on stone fruit. Fruit thinning is a management technique to regulate fruit size. Research suggests that the ratio of leaves to developing fruit is important, and should be in the range of 20/40 leaves per fruit.

Growth inhibition Use of some growth hormones to restrict plant growth, e. g. Delay of flowering in male kiwifruit Dwarfing of ornamentals e. g. chrysanthemums and poinsettias. Inhibiting elongation in punnet grown annual plants e. g. tomatoes. Inhibit sprouting of stored potatoes. To maintain the quality of harvested onions. The prevention of lodging in cereal crops. Lodging is the result of a weak stem and a full seed head. The seed head bends over and reduces the yield when harvesting.

Questions 1. 2. Link up the plant growth substances with the plant growth processes it controls. Auxin Fruit ripening Gibberellin Growth towards the light Cytokinin Cell division Abscisic acid Breaking seed dormancy Ethene Direction of root growth Auxin has a different effect on roots, shoots and _______ buds. Low concentrations of auxin _______ roots, while higher concentrations _______ roots. The apical tip produces high concentrations of ______ which inhibits the ______ buds and stops them from growing. If you want a busy plant, you can remove the apical _______ and the lateral buds will start to grow.

3. Each plant ______ to an environmental ______ is controlled by a ______ that is produced where the stimulus is ______, then transported to where it has its ______. 4. Auxins are produced at the tip of _______ and control the response to _______. In root tips auxins control the growth response to _______. 5. _______ break seed and winter dormancy; promote germination and cell expansion. 6. Cytokinins promote cell _______. 7. Abscisic acid maintains _______ in seeds and ______ plants over winter. 8. _______ promotes _______ ripening and loss of leaves from deciduous trees.