Plant Responses

• Seed absorbs water, which stimulates gibberellin production. Enzymes breakdown food stores - ATP production.
• Gibberellins switch on genes for amylase and protease.
• ABA interferes with gibberellin, determining germination.
• Experimental evidence: mutant seeds with no gibberellin don't germinate, but with external application it occurs; application of gibberellin biosynthesis inhibitors means seeds stay dormant.

Auxins: 

• Stimulate growth of main apical shoot: auxins synthesised in meristem cells; auxins bind to receptor sites, forming vacuoles and lowering pH to increase flexibility; large central vacuoles created due to increased absorption of water; enzymes destroy auxin so no more elongation.
• Suppresses lateral shoot growth (apical dominance): auxin inhibits lower shoot growth; apical shoot removed means lateral shoot growth due to absence.
• Auxin in root tips stimulates growth: apical shoot removed reduces root growth, but too much inhibits.

Gibberellins

• Affect internode length (regions between leaves)
• Without them, stems are shorter so there is less waste and they are less vulnerable to weather and harvesting.

Synergism - complementary hormones work together to give greater response.

Antagonism - opposite effects balance to determine response.

Leaf loss and abscission

• Breaks dormancy of buds, timing of flowering and tubers formed for winter.
• Phytochrome (Pr and Pfr) - different types of light and ratio depends on levels

• Lack of light lowers auxin levels, producing ethene. Abscission zones are ethene-sensitive.
• Enzymes digest outer layer of the zone and vascular bundles sealed off. Fatty material on stem side forms a scar to protect from pathogens.
• Cells retain water, putting stress on outer layer, causes leaf to break off.

Preventing freezing

• Cytoplasm and sap have low freezing points. Act as antifreeze.
• Gene suppression/activiation on fall in temperature or day length.

Stomatal control

• Opens for cooling and closes for water conservation. 
• ABA stomatal closure e.g. low soil water levels - lowers potential of guard cells so turgor.

Physical - thorns, barbs, spikes, stings. Fibrous/inedible tissue, spiny or hairy leaves.

Chemical:

• Tannins - bitter taste and toxic to insects because inactivate digestive enzymes.
• Alkaloids - nitrogenous compounds that affect metabolism, sometimes poisons. Caffeine is toxic to fungi and insects and prevents germination of seeds. Nicotine is stored in vacuoles.
• Terpenoids - form essential oils and is a toxin.

Pheromones - chemical which affects others of the same species. E.g. insects attack maple trees so other leaves produce callose; cabbages produce chemical to attract parasitic wasp which kills and deters cabbage white butterflies.

Folding - frightens larger herbivores, dislodges. Recovery by potassium movement into cells causing water to move in. E.g. mimosa pudica potassium into flexor cells and out of extensors.

Phototropism practicals:

• Unilateral light with plant shoot - bends towards the light because auxin moves laterally across the shoot (positive phototropism).
• Light and tip removed - no response because detect stimulus or produce messenger.
• Light and lightproof cover - no response because light stimulus needs detection.
• Light and mica barrier - movement down shaded side causes bending towards light.
• Light and mica on shaded side - no response because no movement.
• Light and gelatin block between tip and shoot - movement of chemical bends towards light as gelatin allows passing of chemicals.
• In the dark - grows directly upwards to photosynthesise due to gibberellins.

Geotropism practicals:

• Seeds in petri dish stuck to the wall - 90 degree rotation, geotropism within two hrs.
• Clinostat (rotating drum) rotating slowly and in the dark - stimulus applied, root straight.

Ripening:

• Climacteric fruit, e.g. bananas, avacadoes- ethene continues ripening after harvest. One ripe can spread effect.
• Non-climacteric fruit, e.g. oranges, strawberries - low ethene. Ethene gas reduces transportation damage.

Hormone rooting powder and micropropogation:

• Auxin on cut shoots stimulates root production + rooting powers increases root formation (cuttings).
• Micropropogation produces 1000s from few cells - differentiation controlled.

Weedkillers - synthetic auxins increases growth. Simple, cheap, low toxicity, selective.

Other uses: gibberellins prevent aging and ripening; auxins for seedless fruit; ethene promotes fruit dropping e.g. cotton and walnuts; cytokinins prevent aging and micropropogation controls tissue development.