B16 - Plant responses

Question 1

What is tropism?

Answer 1

Plants show directional growth in response to environmental cues such as light (phototropism), gravity (geotropism) and water (hydrotrophism)

Question 2

What are the roles of different hormones in plants?

Answer 2

• auxins: control cell elongation, prevent leaf fall (abscission), maintain apical dominance, involved in tropisms, stimulate the release of ethene and involved in fruit ripening
• gibberellins: cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
• ethene: causes fruit ripening, promotes abscission in deciduous trees
• ABA (abscisic acid): maintains seed and bud dormancy, stimulates cold protective responses e.g. antifreeze production, stimulates stomatal closing

Question 3

Why are some plant responses still not fully understood?

Answer 3

Plant hormones work at low concentrations so isolating them and measuring changes isn’t easy. There are also multiple interactions, making the isolation of the role of a single chemical in a specific response difficult.

Question 4

What are the stages of a seed germinating?

Answer 4

1. sees absorbs water, activates embryo and begins to produce gibberellins
2. stimulation of enzymes which break down food stores found in the seed *
3. The emrbyo plant uses these food stores to produce ATP for building materials to grow and break out through the seed coat**

*in cotyledons, food store is found in the cotyledons in the dicot seeds and in the endosperm in the monocot seeds

**- gibberellins switch on genes which code for amylase and protease - digestive enzymes required for germination
ABA acts as an antagonist to gibberellins (interferes with gibberellin action) and it is the relative levels of these hormones which decides when a seed with germinate

Question 5

What is experimental evidence for role of gibberellins?

Answer 5

• mutant varieties of seeds have been breed without the gene to code for gibberellins and these seeds don’t germinate, if gibberellins are applied to the seed externally, they begin to germinate
• if gibberellin biosynthesis inhibitors are applied to seeds, they don’t germinate since need gibberellins to break dormancy, when inhibition is removed/ gibberellins are applied, the seeds germinate

Question 6

What is the role of auxin?

Answer 6

• auxins such aas indoleacetic acid (IAA) are growth stimulants
• made in cells at the tip, roots and shoots and in the meristems
• can move down the stem and up the root and from cell to cell in the transport tissue
• effect depends on conc. and interactions with other hormones

Question 7

What is the role of auxins in apical dominance?

Answer 7

• stimulate growth of the main apical shoot
• presence of auxins increases plasticity of cell wall as they can stretch more easily
• auxin molecules bind to specific receptors sites on plasma membrane; pH falls to about 5 (optimum for enzymes to keep cell walls very flexible and plastic)
• as cells mature, auxin is destroyed
• as hormone levels fall, pH rises so the enzymes maintaining plasticity become inactive = wall becomes rigid and more fixed in shape + size so cells no longer expand

Question 8

What is synergy and antagonism?

Answer 8

• synergism is different hormones working together to complement each other and give a greater response than they would on their own
• if the substances have opposite effects e.g. one promotes growth and one inhibits it, balancing them will determine the response of the plant

Question 9

What are examples of abiotic stresses?

Answer 9

• changes in day length
• temperature
• water availability
• wind speed
• changes to salinity
  plants need to cope with these chances and such adaptations may include a thick waxy cuticle, hairy leaves, sunken stomata or a wilting response in a hot and dry/ extremely windy conditions/ develop aerenchyma if they grow in an aquatic environment

Question 10

Leaf loss in deciduous plants as a response to abiotic stress

Answer 10

• plants that grow in temperature climate undergo great changes e.g. range of daylight hours could have 12 hour difference depending on season
• since temp and light have impact on photosynthesis, seasonal changes can impact the amount of photosynthesis possible
  = can come to a point to where demand ( of amount of glucose required to maintain leaves, chemicals from chlorophyll that have antifreezing properties) is greater than rate of photosynthesis
• trees can lose all their leaves due to damage from wind gales so in temperate climates, deciduous trees lose all their leaves in winter and remain dormant until days lengthen and temp rises again in spring

Question 11

daylength sensitivity

Answer 11

• plants are sensitive to lack of light - photoperiodism
• affecting many plant responses e.g:
• breaking of dormancy of the leaf buds to allow them to open
• timing of flowering plants
• when tubers are formed in preparation for overwintering
• sensitivity to day length/ dark length comes from light sensitive pigment called phytochrome which exists in 2 forms - Pr and Pfr, each absorbs different type of light and ratio of Pr to Pfr changes depending on level of light

Question 12

Abcission or leaf fall

Answer 12

• the lenghtening of the dark period triggers a number of changes including abcission/ leaf fall and a period of formancy during the winter months
• the falling light levels lead to a fall in auxin conc. = leaves then produce ethene, at the base of the lead stalk is the abscission zone (made up of two layers of cells sensitive to ethene)
• ethene initiates gene switching in cells leading to production of new enzymes which digest and weaken the cell wall of outer abscission zone (separation layer)
• vascular bundles carrying materials in and out of the cell are sealed off and fatty materials are deposited in the cells on the stem side of the separation layer (which forms the protective scar when the leaf falls, preventing entry of pathogens
• cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the weakened outer layer, further abiotic stresses such as low temp and strong winds lead to too much strain and the leaf separates from the plant, leaving behind a scar

Question 13

How do plants prevent freezing?

Answer 13

• if cells freeze, the plasma membrane is disrupted and they will die
• cytoplasm of the plant cell and cell sap in vacuoles contains solutes which lower the freezing point
• some plants produce sugars, polysaccharides, amino acids and proteins which act as antifreeze to prevent cytoplasm freezing
• species only produce chemicals to become hardy and frost resistant as a response to a fall in temp and in other seasons, the genes are suppressed

Question 14

How do plants have stomatal control?

Answer 14

• when too hot, stomata open to cool the plant and water evaporates in transpiration
• opening and closing of stomata is controlled by ABA which is produced by leaf cells
• roots can also help as when level of soil water falls and transpiration is under threat, plant roots produce ABA which is transported to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells and reduce the water potential = reducing turgor which in turn causes the guard cells to close the stomata and water loss by transpiration is reduced

Question 15

What are chemical defenses against herbivores?

Answer 15

• tannins - bitter taste which outs animals off eating the leaves and toxic to insects as bind to digestive enzyme in saliva and inactivate them
  -alkaloids - affect metabolism of animals and poison them. Alkaloids include caffeine (toxic to fungi and insects and in coffee bush seedlings, spreads through soil and prevents germination of the seeds so it reduces plant competition too) nicotine ( toxin made in the roots of tobacco plants and released in leaves when eaten) morphine and cocaine
• terpenoids - act as toxins to fungi and insects that might attack the plant e.g. chrysanthemums produce pyrethin which acts as an insect neurotoxin (interferes with nervous system), some act as insect repellent

Question 16

What are pheromones?

Answer 16

a chemical made by an organism which affects the social behaviour of other members of the same species. In plants
- when a tree is attacked, it releases pheromones which are absorbed by leaves on other branches, which make callose to help protect against attack
- plants are thought to communicate via root systems and alert each other of water stress
- plants produce volatile organic compounds (VOG), which act like pheromones and diffuse through the air in and around the plant

Question 17

What are examples of gene switching due to VOG?

Answer 17

-when cabbages are attacked by caterpillers, the release a chem signal which attracts a parasitic wasp which lays eggs on the caterpillers and these are then eaten alive, protecting the plant
- when spider mites attack apple trees, they produce VOG which attracts predatory mites to destroy the pest
- some types of wheat seedlng produce VOG when they are attacked by aphids and these repel other aphids

Question 18

Folded in response to touch

Answer 18

• some plants eaten by herbivores contain a toxic alkaloid and the stem has sharp prickles, if the leaves are touched, they fold down and collapse, frightening off larger herbivores and dislodging smaller insects
• the leaf falls in a few seconds and recovers over 10-12 minutes as a result of potassium ion movement into specific cells followed by osmotic water movement

Question 19

what types of tropism are there?

Answer 19

• geotropism: response to gravity
• phototropism: response to light coming from one direction
• hydrotropism: response to water, roots are positively hydrotropic
• chemotropism: response to chemicals
  other:
• thigmotropism - response to contact with an object
• thermotropism - response to temperature

Question 20

tropism as a response to environmental cues

Answer 20

• once a seed germinates, the shoots must grow up towards the light source for photosynthesis, roots must grow downwards into the soil for support and minerals and water for the plant

Question 21

What is phototropism?

Answer 21

• result of movement of auxin across the shoot or root if it is exposed to light that is stronger on one side than the other
• whether plants are grown in bright light or low light as long as it is even all over, they grow straight upwards
• if shoots (positively phototropic) are grown in unilateral light that only shines on one side, they will grow towards the light, exposed roots will grow away
• shoots do this as a survival response to receive as much light all around as possible and maximise photosynthesis

Question 22

Ways to test for phototropism

Answer 22

1. unilateral light (from one side) - shoot bends towards the light - shoot is positively phototropic and bending occurs behind the tip
2. light when shoot tip is removed - no response - the tip either detects the stimulus or produces the messenger so its removal prevents a response
3. lightproof cover is placed over intact tip of shoot - no response - the light stimulus is not detected by tip
4. thin, impermeable barrier of mica on side with light source - movement of chemical down shaded side + bends towards light - mica on illuminated side of shoot allows hormone to pass only down shaded side where it increases growth and causes bending
5. mica inserted on shade side - movement of chemical down the shaded side is prevented by mica so no response
6. tip removed, gelatin block inserted and tip replaced - movement of chemical down shaded side so it bends towards the light - as gelatin allows chemical to pass through, but not electrical messages, the bending occurs due to the chemical passing through the tip

Question 23

The effect of unilateral light

Answer 23

• when one side of a plant is exposed to light, it contains less auxin compared to the side which is shaded, the light causes the auxin to move laterally so there is higher conc. on shaded side = stimulates cell elongation and growth on the dark side
• once the shoot is growing towards the light, the unilateral stimulus is inhibited and and the transport of auxin stops so the shoot grows straight upwards again

Question 24

What are the effects on plants that are grown in the dark?

Answer 24

• gibberellins are responsible for the extreme elongation in the dark since the seedlings that break through the soil don’t have to compete for light and the plant needs to grow upwards rapidly to be able to photosynthesise
• farmers use this to grow some crops in the dark which causes rapid growth and is known as etiolation, these plants are thin and pale due to deprivation of light so little chlorophyll develops in the leaves

Question 25

What is geotropism?

Answer 25

-plants are sensitive to gravity, in normal conditions, plants always a unilateral gravitational stimulus since gravity always acts downwards
- shoots are negatively geotropic and roots are positively geotropic

Question 26

Plants can be used in control of ripening

Answer 26

• ethene is involved in ripening of climacteric fruits (ripen after being harvested) e.g. bananas, tomatoes, mangoes, ethene works by triggering a series of reactions that greatly increase respiration rate
• effect of ethene on climacteric fruit can be easily seen when part of a bunch of green bananas are placed next to a single ripe banana where the ripe banana will ripen faster than the rest of the bunch even in the same temperature due to the ethene being a stimulant
• climacteric fruits are used commercially and harvested when fully formed but not ripe, cooled, stored and transported, this allows each of the batch to ripen at the same rate as unripe fruit is harder and will not damage as easily and prevents wastage of fruits

Question 27

Hormone rooting powders and micropropagation

Answer 27

• a cutting is a small piece of the stem usually with some leaves on it, when placed in compost/ water, roots may eventually appear and new plant forms, dipping the cut stem into hormone rooting powder increases the chances of roots forming and a successful propagation, making it easier for horticulture and agriculture

Question 28

hormonal weedkillers

Answer 28

• if the balance of hormones is disrupted, it can impact the metabolism of the plant and may lead to death
• weeds interfere with commercial food crops by competing for light, space, water and minerals
• much of food crops are narrow leaved monocot leaves but weeds are wide leaved dicots, synthetic dicot auxins they are absorbed only by the broad leaved dicots and affects their metabolism where the growth rate rapidly increases and the plant dies.

Question 29

other uses of plant hormones

Answer 29

• auxins can be used in production of seedless fruit
• ethene is used to promote fruit dropping in plants such as cotton
• cytokinins are used to prevent ageing of ripened fruit and products such as lettuces and in micropropagation to control tissue development
• gibberellins can be used to delay ripening and ageing in fruit, to improve the size and shape of fruits and in beer brewing to speed up the malting process