Summary
Summary Module 5.4: Plant responses and hormones
- Module
- Module 5.4: Plant responses
- Institution
- OCR
OCR A level biology A Key concepts of plant responses and the effect of hormones.
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PLANT RESPONSES - Elongation, no abscission, apical dom, stims ethene- ↓ conc promote root growth, too ↑ conc inhibit
Types of plant response - Elongation of apical shoot – auxin effects plasticity of
Abiotic stress cell wall (stretches more easily), bind to receptor
- Seasonal changes – light/temp changes gene express sites, ↓ pH, optimum enzymes (mature = no aux)
- Lack of light – photoperiodism (phytochrome) - Apical dominance, ↑ conc inhibit lateral shoot grow,
- Abscission – ↓ auxin, ethene, enzyme weaken leaf further down ↓ aux = lateral shoots grow, (apical
stalk at abscission zone, fatty acids build = seal removed, lateral shoots grow as no dominance)
- Heat – open stomata to cool
- Cold – solutes ↓ freezing point, mem not disrupted
- Water availability – close to conserve, ABA produced
by roots, guard cell mem receptors, ↓ tugor
Responses to herbivores
- Physical – thorns, stings, inedible tissue, hairs
- Chemical = through transport tissue
> Tannins – 50% dry weight, bitter, toxic to insects
> Alkaloids – bitter, poison, spread in soil, prevent
seed germination (caffeine, nicotine)
> Pheromones = in same species, absorbed by
other branches, produce callose, between roots
> VOCs – diffuse in air to others + attract predators
to pests, respond to saliva chemicals (cabbages)
- Fold in response to touch
> Mimosa pudica moves at visible speed – contains
toxic alkaloids, prickles, dislodges pests
Gibberellins
Tropisms - Internode stem elongation (meristem between node)
- Growth responses to stimuli in 1 direction - Seed germination + fertilisation, absorb water,
- Nastic response = non directional response gibberellins break down food source, ATP for building
- Germinating seeds used – easy, cheap, respond materials to grow, genes on for amylase + protease
quickly, easy to observe + measure - Dwarf = no gibberellin, less waste + damage
- Chemotropism = chemicals, thigmotropism = touch - Evidence
> Seeds lacking gene do not germinate
Geotropism = gravity > Gib inhibitors applied to seeds, no germinate
- Shoots = -ve geotropic (away)
- Roots = +ve geotropic (auxin Commercial hormone usage
inhibit elongation) Control of ripening
- Clinostat to measure effects - Ethene ↑ resp, climatic fruits ripen after harvested
- Picked when fully formed, less damage when
Phototropism (light) transported, ripened in controlled conditions
- Redistribution of auxin across root / shoot - Prevents wastage + longer time available
- ↑ conc dark side, elongation
- Originally though auxin destroyed, disproven by exp Rooting powder
- Surrounding light (straight) unilateral (bends) - Higher chance of roots forming for propagation
- +ve phototropism = shoots towards light (max photo) - Help product clone plants on large scale
- -ve phototropism = roots grow away (back to soil)
- Dark = rapid growth (reach light), no comp, gibberel Hormonal weedkillers
- Weeds compete with crops
Plant hormones - Synthetic auxins ↑ growth rate unsustainably
Ethene = fruit ripening (+ cytokinin), abscission - Simple, cheap, low toxicity
Abscisic acid ABA = seed dormancy, stomatal closing
Other uses
Auxins 1) Auxins produce seedless fruit
- Growth stimulants redistribute to shaded side 2) Ethene promotes fruit dropping
- Made at top of roots + shoots (apical meristems) 3) Cytokinin prevent aging of ripe fruit
- Lateral bud meris (side shoots), lateral meris (widen) 4) Gibberellins delay ripening to improve size
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