B5.1.5: Plant responses - OCR A Biology A level A* student notes
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Module 5: Communication, homeostasis and energy
5.1.5: Plant responses
- Though often regarded as passive objects, plants are dynamic systems that not only
photosynthesise (--> prod food) but also respond to their environment in a variety of ways
The types of plant responses
Plant responses to abiotic stress
- Examples of stresses: changes in day length, cold, heat, lack of water, excess water, high winds,
changes in salinity
Type of abiotic Plant response Explanation
stress
Change in Sensitivity to Photoperiod: no. of daylight hours
photoperiod no. of daylight Photoperiodism: organism’s response to seasonal changes in photoperiod
hours (or ‘dark [__ plants are sensitive to a LACK of light in their environment. This sensitivity results from
period’) phytochrome - light-sensitive pigment (type of photoreceptor, protein) which exists in 2 form
(Pr + Pfr), each absorbing a different type of light → ratio of Pr : Pfr varies depending on
light levels and therefore photoperiod
Examples of plant response sensitive to photoperiod: time of flowering, time tubers are
formed in preparation for overwintering, leaf formation rate, seed germination
Winter in temperate Leaf loss in The role of hormones in leaf loss in deciduous plants
climates deciduous Plants growing in temperate climates (eg. the UK’s climate - cool, wet winters + warm, wet
plants summers) experience great seasonal changes in environment during the year, notably change
in photoperiod and change in temp.
Light and temp affect rate of photosynthesis.
There reaches a point in winter when there is a greater demand for glucose (in maintaining
leaves with respiration, prod chemicals from chlorophyll to protect leaves against freezing)
than there is supply of glucose, because rate of photosynthesis has decreased.
Leaf loss also occurs in winter in temperate climates when absorption of water from frozen
soils is difficult → reduces water loss → retain water to survive
TMT deciduous trees lose all of their leaves in winter and remain dormant until spring, when
photoperiod lengthens and temps rise again. -- ABSCISSION*
Also helpful/increases likelihood of survival because a tree w/ leaves is more likely to be
damaged or blown over by winter gales.
Hot and dry climate Leaf loss in The role of hormones in leaf loss in deciduous plants
deciduous
plants Deciduous plants lose their leaves* when it is very hot and dry → reduces water l;oss
Increasing dark Leaf Leaf abscission = leaf fall
,period abscission* Abscission: natural detachment of parts of a plant, typically dead leaves + ripe fruit
Increasing dark photoperiod = decreasing light levels
[__ leads to decreasing auxin concentrations
Leaf responds by prod ethene, which initiates gene switching that results in prod of new
enzymes in the plant’s abscission zone. This zone is the region at the base of the leaf stalk
made up of the separation layer and the protection layer - both sensitive to ethene, layer of
parenchyma cells with thin walls (weak and easily broken)
New enzymes prod digest and weaken cell walls in the separation layer (outer)
Vascular bundles are sealed off and fatty material is deposited in cells on the stem side of the
separation layer → forms protective scar when leaf falls → prevents entry of pathogens,
waterproof
Cells deep in separation layer respond to hormonal stimuli by retaining water → swelling →
increases strain on already weakened separation layer
Further abiotic factors (eg. low temps, strong autumn winds) provide final strain, causing the
leaf to separate from the plant
Auxins are also involved in leaf loss:
- Are prod by young leaves
- Inhibit leaf loss by making the leaf stalks insensitive to ethene
- Leaf gets older → conc of auxin decreases → allows leaf loss to occur in response to
ethene
Decrease in Mechanisms Cell freezes → membranes disrupted → cell death
temperature that protect
cells in Plant cell cytoplasm and sap in vacuoles cont solutes which lower freezing point → reduces
freezing likelihood of freezing
conditions
Some plants prod sugars, polysaccharides, amino acids, proteins which act as antifreeze →
prevent cytoplasm from freezing or protects plant cell from damage if it does freeze
[__ most plant species that prod chemicals which make them frost resistant only do so during
the winter months (decrease in temp and photoperiod → suppression and activation of
different genes → synthesis. Thus a sustained incr in temp and photoperiod in spring reverse
these changes)
Changes in temp Stomatal The role of hormones in stomatal closure
and water control Open stomata → incr water evap from cells in leaves during transpiration → cools plant
availability (open/close) Close stomata → decr water evap from cells in leaves during transpiration → conserves wate
Largely controlled by ABA - hormone which activates changes in ionic concentration in
guard cells, reducing water potential and therefore turgor → guard cells close stomatal pore
ABA is produced by leaf cells in response to decreased soil water potential (when soil dries
, out) - response to abiotic stress. Root cells also secrete ABA → translocated to leaf cells →
binds to receptors on guard cell plasma membrane -- provides early warning of water stress
1 - ABA binds to receptors on the guard cell plasma membrane
2 - A complex series of events is set in motion which results in the opening of calcium
channels, causing Ca2+ ions to enter the guard cell. The pH of the cytoplasm is also raised
(less acidic)
3 - These events cause K+ ions, and also NO3 − and Cl− ions, to leave the cell.
4 - As a result, the water potential of the cell increases, and water is moved by osmosis into
surrounding cells
5 - The resulting loss of turgor causes the stomata (stomatal pore) to close.
- Herbivore: animal that eats plants
- Herbivory: process by which herbivores eat plants. Equivalent of predation, which is a term used
in relation to carnivores.
- Plants cannot escape herbivores so they have evolved a wide range of defences to prevent attack
(discourage herbivores from eating them), or minimise damage
Physical defences in response to herbivory
- Examples of common physical defences: thorns, barbs, spikes, spiny leaves, fibrous and inedible
tissue, hairy leaves, stings
Chemical defences in response to herbivory
Chemical Role in response to herbivory
Tannins Type of phenol prod by many plants. Can compose up to 50% of dry weight of leaves. Stored in plant cell vacuoles
Have a v bitter taste → discourages herbivores from eating them
Bind to digestive enzymes prod in insect saliva and inactive them → toxic to insects - breakdown of tannins produces
toxic chemicals in the insect (can be fatal)
Therefore reduces herbivore populations in a plant’s environment
Alkaloids Large group of v bitter tasting, nitrogenous compounds found in many plants. Many act as drugs - affect metabolism
of animal:
Eg. caffeine (occurs in many plant species; cocoa beans, tea leaves + coffee beans being the best known) - toxic to
fungi and insects so functions as a chemical defence, caffeine prod by coffee seedlings spreads through soil and
prevents germination of seeds of other plants (protects against both herbivores and plant competition)
Eg. nicotine - potent neurotoxin prod in roots of tobacco plants (and also found in tomatoes, aubergines etc) →
transported to leaves → stored in vacuoles in leaf cells → released when leaf is eaten → can be fatal to insects
[__ neurotoxin: chemical that interferes with the ability of neurones to conduct nerve impulses.
Eg. capsaicin - found in chilli peppers, produces the characteristic burning sensation that many humans actually
like but which will deter herbivores
Other examples: morphine, cocaine
Terpenoids Large group of compounds prod by plants that often act as toxins to insects and fungi that may attack
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