Growth and Development in Plants and Animals
Senior 4 · Biology
Growth and Development in Plants and Animals
Lesson 3 of 5
25 min
Put a hard green banana in a closed bag with a ripe one, and within a day or two the green one turns yellow and soft. Keep another green banana alone on an open table and it takes much longer. The ripe fruit gives off a gas, ethylene, and the gas switches on ripening in the fruit next to it. Ethylene is one of five main groups of plant hormones: chemical messengers that a plant makes in tiny amounts and that control how it grows, develops and responds. In S1 and S3 you met auxin and tropisms; this lesson covers all five groups and the experiments that revealed them.
Definition
Plant hormone (phytohormone, plant growth regulator): an organic substance made in small amounts in one part of a plant that affects growth or development in the same or another part, often far from where it was made.
Apical dominance: the inhibition of lateral (side) buds by the growing shoot tip.
Abscission: the shedding of leaves, flowers or fruit at a special layer of cells at the base of the stalk. Senescence: ageing.
Synergism: two hormones together have a greater effect than either alone. Antagonism: two hormones have opposite effects (for example gibberellin and abscisic acid in germination).
| Hormone | Mainly made in | Main effects | Uses by people |
|---|---|---|---|
| Auxins (IAA, indoleacetic acid) | Shoot tips, young leaves | Cell elongation in shoots (auxin makes cells pump out , the acid loosens the cell wall, and the cell stretches as it takes up water); phototropism and gravitropism; apical dominance; root formation on cuttings; fruit growth; delays abscission | Rooting powders for cuttings; synthetic auxins such as 2,4-D as selective weedkillers that kill broad-leaved weeds in maize, sorghum and rice; seedless fruit |
| Gibberellins (GA) | Young leaves, seeds | Stem elongation (longer internodes); breaking seed and bud dormancy; amylase production in germinating cereal grains; flowering in some plants | Malting barley and sorghum; larger seedless grapes; dwarf crop varieties are dwarf because they make or respond to less GA |
| Cytokinins | Root tips | Cell division (with auxin); growth of lateral buds, opposing apical dominance; delay leaf senescence | Plant tissue culture (micropropagation); keeping cut flowers and leafy vegetables fresh |
| Abscisic acid (ABA) | Leaves (especially when short of water), seeds | Closes stomata in drought; keeps seeds and buds dormant; slows growth under stress | Research on drought tolerance |
| Ethylene (ethene, a gas) | Ripening fruit, ageing leaves, nodes | Fruit ripening (bananas, avocados, mangoes, tomatoes); abscission of leaves and fruit; senescence | Ripening rooms for fruit picked green, which travels better; chemicals that release ethylene |
Hormones rarely act alone. In tissue culture, a high ratio of auxin to cytokinin makes a lump of cells grow roots, a low ratio makes it grow shoots, and equal amounts keep it dividing as an undifferentiated mass. In seeds, gibberellin and abscisic acid are antagonists: the balance between them decides whether the seed germinates.
In 1880 Charles Darwin and his son Francis showed that grass seedlings bend towards light only if the tip of the coleoptile (the sheath around the young shoot) is uncovered: the tip senses the light but the bending happens lower down, so "something" travels down from the tip. In 1928 the Dutch botanist Frits Went showed that the "something" is a chemical that can be collected in agar (Figure 1A). Later work showed that, in light from one side, auxin moves to the shaded side of the shoot, so the shaded cells elongate more and the shoot bends towards the light.
Figure 1. A: Went's experiment showed that a chemical from the tip causes growth. B: the shoot tip, through auxin, keeps lateral buds dormant.
Worked example 1: measuring auxin with a bioassay
Agar blocks that had held 1, 2 or 3 coleoptile tips produced curvatures of 5°, 10° and 15° (illustrative). A block soaked in a plant extract produced 12°. (a) What does the pattern show? (b) Express the extract's auxin in "tip equivalents".
Worked example 2: testing apical dominance
Bean plants were treated as follows and the length of their highest lateral bud measured after 14 days (illustrative): A intact, 2 mm; B shoot tip removed, 28 mm; C tip removed, lanolin paste containing auxin on the cut, 3 mm; D tip removed, plain lanolin paste on the cut, 27 mm. (a) What do A and B show? (b) Why was D needed? (c) Conclude.
Worked example 3: designing a ripening test
Plan an investigation to show that ripe bananas speed up the ripening of green ones, and state the expected results.
CrashCourse: "Plant Cells & Hormones" (Crash Course Botany #3). Watch for the five main hormone groups, starting with ethylene, and what each one does to growth.
Warning
Common mistakes
Tip
Memory hook: "All Good Crops Avoid Excess": Auxin (elongation, tropisms), Gibberellin (stems, germination), Cytokinin (cell division), ABA (dormancy, drought), Ethylene (ripening, falling leaves).
Practice: plant hormones
Key points
Sources and credits
Quick check
Match each hormone to one of its effects.
Tap one on the left, then its partner on the right.
