5.1.5a plant responses (Finished?)

Question 1

Why do plants have a responses to the environment?

Answer 1

To limit abiotic stress, e.g. the consequences of water shortage

To deter herbivores which eat the leaves of the plant;

To increase rate of photosynthesis, e.g. by more effective competition for light

To decrease the spread of pathogens and limit the damage they cause

Question 2

when does abiotic stress occur?

Answer 2

Abiotic stress occurs when there are negative consequences to the level of an abiotic (nonliving, environmental) factor being too high or too low.

Question 3

give some examples of abiotic stress

Answer 3

temp, light itensity and water level

Question 4

why do platns respond to abiotic stress?

Answer 4

Plants carry out responses to abiotic stress, such that damage is prevent or limited, in such a way that the chances of survival of the plant are likely to be increased

Question 5

what does herbivory cause?

Answer 5

Herbivory causes loss of leaf surface area, and hence decrease in photosynthesis rate. Moreover, the herbivores may be the vectors of pathogens and the damaged tissues provide an opportunity for pathogens to easily enter the plant (without the obstruction of the waxy cuticle, which is usually an effective barrier).

Question 6

what are chemical defences towards herbivory? TAPV

Answer 6

Tannins: these have a bitter taste which deters larger herbivores; tannins are toxic to insects;

Alkaloids, including caffeine, nicotine, cocaine, morphine: these also have a bitter taste which deters larger herbivores; furthermore they disrupt the metabolism (e.g. nervous system function) of the herbivore, and in sufficient dose are toxic;

Pheromones: these are chemicals released into the atmosphere by individual, causing changes to the behaviour of other members of the same species; for example, the leaves of maple trees under insect attack release pheromones which cause other leaves (on the same or other trees) to put preemptive defences in place;

Volatile organic compounds (VOCs): these are chemicals released into the atmosphere by an individual, which attract or repel members of the other species.

Question 7

what are physical defences towards herbivory?

Answer 7

Permanent structures which deter herbivores, e.g. thorns, spines, hairy leaves, stings

The folding, curling or collapse of leaves in response to touch, e.g. as seen in the so‐called ‘sensitive plant,’ Mimosa pudica: the sudden movement of the leaves will dislodge insects that have settled on the leave and may frighten off larger herbivores.

Question 8

what is tropism?

Answer 8

The directional growth response of the shoots or roots of a plant, towards or away from an environmental stimulus

Question 9

what is positive tropsism?

Answer 9

growth of shoots or roots towards the stimulus

Question 10

what is negative tropsism?

Answer 10

growth of shoots or roots away from the stimulus.

Question 11

how did different tropisms come about, and what does it help the plant with?

Answer 11

natural selection, and it increases the plant chance of survival and reproduction

Question 12

What is an adaptive value?

Answer 12

something which increases the chances of survival of an organism

Question 13

what is Phototropism, stimulus and adaptive value

Answer 13

stimulus:
Directional light

Adaptive value:
Positive phototropism of the shoots of a plant gives more success when there is competition for light. This in turn allows a higher rate of photosynthesis and so increased growth.
Negative phototropism of roots causes them to grow into the soil (region of lowest light intensity), anchoring the plant in position and increasing opportunities for water and mineral ion absorption.

Question 14

what is Geotropism, stimulus and adaptive value

Answer 14

stimulus:
gravity

Adaptive value:
Positive geotropism of roots causes them to grow down into the soil, anchoring the plant in position and increasing opportunities for water and mineral ion absorption.
Negative geotropism of shoots causes them to grow upwards, in a direction which increases the likelihood of greater light availability for the leaves.

Question 15

what is Thigmotropism, stimulus and adaptive value

Answer 15

stimulus:
touch or contact

Adaptive value:
Positive thigmotropism of the stems/tendrils of soft‐stemmed climbing plants allows them to coil around solid structures, supporting the plant as it grows taller and thus allowing its leaves more access to light.

Question 16

what is Hydrotropism, stimulus and adaptive value

Answer 16

stimulus:
water

Adaptive value:
Positive hydrotropism of roots causes the roots to grow into areas of higher water potential, allowing increased uptake of water into the roots by osmosis.

Question 17

what are the six steps of positive photostropism of shoots?

Answer 17

1. Photoreceptors (called phototropins) in the shoot tip detect the direction of blue light.
2. Cells in the shoot tip produce the plant hormone, auxin.
3. The auxin is transported downwards from the shoot tip, but accumulates more on the shaded side of the shoot’s stem.
4. In the zone of cell elongation (the region below the shoot tip), the higher concentrations of auxin on the shaded side cause increased cell elongation:
   a. auxin binds to complementary receptors;
   b. this activates enzymes which cause breakage of cross‐links between cellulose microfibrils;
   c. the cell walls become more stretchy;
   d. cells with more stretchy cell walls elongate more when they take in water by
   osmosis.
5. The different extents of cell elongation on the lit and shaded sides of the stem cause the stem to bend as it grows, towards the light source.
6. The growth of the shoot towards the light makes it more competitive for light, giving a higher rate of photosynthesis and therefore glucose production and growth.

Question 18

what are plant hormones

Answer 18

Plant hormones are cell‐signalling molecules in plants that coordinate growth, reproduction
and responses to herbivores, pathogens and abiotic stress.

Question 19

why are plant hormones hard to research?

Answer 19

are effective in very low concentrations, hence it is difficult to measure in vivo

concentrations or to isolate the hormones to identify which are causing an effect;

may quite different (even opposite!) effects depending on their concentration, e.g. auxin stimulates root development at moderate concentration, but inhibits it at high concentration;

may have very different effects when present in combination with other plant
hormones:
if two different plant hormones have similar effects and together have a stronger effect, they are described as synergistic with one another;
if two different plant hormones have opposing effects they are described as
antagonistic to one another, and their relative concentrations will determine
the extent/direction of the response;

have different effects depending on the responding cell/tissue type and stage of development;

have different effects in different plant species.

Question 20

how do plant hormones move through the plant?

Answer 20

plant hormones often move directly from cell‐to‐cell by simple diffusion via plasmodesmata, or use carrier or channel proteins to cross plasma membranes by facilitated diffusion or active transport.

Question 21

what is stomatal closures

Answer 21

The closure of stomata is a response to water stress (abiotic stress).
This response can be highly effective since
a very high proportion of transpirational water loss occurs as diffusion of water vapour through open stomata.

Question 22

what happens if the stomata doesnt close?

Answer 22

An imbalance in water which leads to loss of cell turgor and hence the wilting of stems and leaves. Wilting causes decreased photosynthesis rate, as leaves are no longer held up to the light optimally.

Question 23

what is the mechanism for stomatal closure?

Answer 23

1. The hormone ABA is released by leaf cells that suffering a decrease in water potential and a loss of turgor.
2. ABA acts as a signal to guard cells, which have complementary receptors in their plasma membranes that ABA binds to.
3. Guard cells respond to ABA by losing their cell turgor via water loss by osmosis to surrounding cells, such that both guard cells in a pair collapse and seal the stoma between them.
4. Once stomata are closed, there is a decrease in loss of water vapour by transpiration, hence more water retained in the plant.

Question 24

what is apical dominance?

Answer 24

The shoot tip (apex) of a plant contains the apical meristem. This is responsible for the upward (vertical or primary) growth of the main stem, caused by repeated mitosis of cells in the apical meristem.

Apical dominance is a phenomenon seen in many plants whereby the presence of the apical meristem suppresses growth of side shoots (branches) from the lateral buds that occur at intervals down the stem.

If apical dominance is operating, all the plant’s resources are used to fuel upward growth at the highest possible rate, rather than for outward (lateral) growth.

This is generally advantageous as it brings the leaves of the plant above those of other plants, giving a competitive advantage in light absorption. Furthermore, apical dominance prevents the plant becoming too ‘bushy,’ which could result in overcrowding of leaves such that they shade one another (limiting light absorption and therefore photosynthesis rate).

Question 25

what is the mechanism for apical dominance?

Answer 25

1. The apical meristem in the shoot tip produces very high auxin concentrations.
2. The auxin diffuses down the stem.
3. If the lateral buds experience high enough auxin concentrations, their growth is suppressed, i.e. they are kept in a dormant state, and no side shoots grow.
4. All the plant’s resources are used to fuel upward growth only.
5. Eventually, once the plant becomes very tall, the lowest lateral buds may no longer experience sufficiently high auxin levels to maintain their dormancy.
6. Hence the lowest lateral buds are the first to begin side shoot growth.

Question 26

experimental evidence for the role of auxins in the control of apical dominance

Answer 26

Removal of the shoot tip results in the growth of side branches from the lateral buds, due to loss of apical dominance (as the auxin levels experienced by lateral buds fall low enough for growth, once the site of auxin production is removed);

However, if a paste containing auxin is immediately applied in place of the removed shoot tip, side shoots do not grow as the apical dominance is maintained by the auxin diffusing down the stem in high enough quantities to suppress growth from lateral buds.

If a plant that shows apical dominance is turned and kept upside down, side branches start to grow, as apical dominance is lost (suggesting auxin is not transported towards the lateral buds in high enough quantities to suppress their growth, if it has to move against gravity).

Question 27

why does lead abscission occur in deciduous plants?

Answer 27

In the winter, the days are short, the light intensity is low (as the sun does not get as high in the sky) and the temperature is low;

In these conditions, the rate of photosynthesis would be extremely low;

If leaves were present they would not be giving the plant much benefit and yet they would still use up glucose in respiration and lose water vapour in transpiration, at a time when water absorption to replace the lost water vapour might be impossible (if
the soil has frozen);

Hence having leaves (with huge total surface area) becomes a disadvantage in the winter and it is adaptive to lose them;

Despite an absence of glucose production by photosynthesis, the remaining cells in the plant must still respire: they do so using sugars obtained via the hydrolysis of stored starch in the roots, translocated in the phloem to where sugars are needed

Question 28

what is the mechanism for lead abscission occur in deciduous plants?

Answer 28

1. Through the summer, high levels of auxin secretion by (young) leaves inhibit ethene production and therefore prevent abscission. Auxin is antagonistic to ethene.
2. In autumn, triggered by the increased length of the dark period (night), auxin levels fall and ethene production therefore increases.
3. As a consequence of higher ethene levels in the leaf, chlorophyll is broken down (resulting in the loss of green colour from the leaf; the red/orange/yellow colours result from the increase visibility of accessory pigments called carotenoids).
4. The magnesium ion (Mg2+) from the centre of each chlorophyll molecule is transported in the phloem to cells elsewhere in the plant for storage, so it can be reused in the spring for synthesis of new chlorophyll. These ions are too valuable to be lost in the leaves that are about to fall!
5. Once withdrawal of valuable ions from the leaf is complete, a protective layer of waterproof, impermeable suberin (a waxy material) is produced in abscission zone, in the part of stem adjoining the base of the petiole.
6. This protective layer will seal the wound left when the petiole breaks away, and in particular blocks the vascular tissue (xylem and phloem). This removes the opportunity for pathogens to enter the wound, and prevents leakage of sugars and evaporation of water from exposed vascular tissue.
7. Ethene also stimulates synthesis of cellulase and pectinase enzymes, resulting in digestion of the cellulose and pectin in cell walls and middle lamellae (sticky material holding cells together) specifically in the abscission layer (also called the separation layer: the band of cells at the base of the petiole).
8. This cellulose and pectin digestion creates a line of weakness at the petiole base.
9. The final ‘push’ for the leaf to fall will usually be a gust of wind.

Question 29

what is stem elongation?

Answer 29

Gibberellins stimulate the elongation of plant stems. More specifically, they cause an increase in the length of internodes, which are the regions of stem between nodes (which are the location of meristem tissue and hence lateral buds and/or leaves).

Question 30

how do gibberellins work?

Answer 30

they stimulate cell division by mitosis and also stimulate cell elongation. Therefore, in plants with higher levels of gibberellins, in which the internodes appear longer, each internode contains a greater number of cells and each individual cell is more elongated.

Question 31

Experimental evidence for the role of gibberellin in the control of stem elongation

Answer 31

In dwarf (genetically short) plants, lower gibberellin levels are detected, fewer cells are present between nodes and these cells are less elongated;

Supplying gibberellin to these dwarf plants (e.g. in the water given to the plant, or via a stem graft with a stem from a tall plant) causes increased stem elongation;

Genetic analysis of the dwarf plant usually reveals a mutation in a gene for a gibberellin‐synthesis enzyme (or a mutation in a gene for a gibberellin receptor).

Question 32

seed germination mechanism

Answer 32

1. Water is absorbed through the testa (seed coat) into the seed (in a process called imbibition).
2. This triggers increased synthesis of the plant hormone, gibberellin, by the embryo within the seed.
3. ABA concentration in the seed must be low enough to allow gibberellin to stimulate the events that follow, since ABA is antagonistic to gibberellin.
4. Gibberellin activates transcription of the genes for amylase and protease.
5. Amylase and protease enzymes are synthesised.
6. Amylase hydrolyses stored starch into maltose, for use in respiration.
7. Protease hydrolyses stored proteins into amino acids, for use in translation.
8. Metabolic processes including respiration increase in rate, allowing increased cell division by mitosis, followed by cell differentiation.
9. A first shoot (radicle) and first root (plumule) emerge from the seed, breaking through the testa. This is possible due to the high hydrostatic pressure of elongating cells.

Question 33

Experimental evidence for the role of gibberellin in the control of seed germination

Answer 33

Seeds from plants with a mutation in the gene coding for a gibberellin synthesis enzyme cannot make gibberellin upon imbibition of water;

such seeds will fail to germinate, suggestion gibberellin synthesis is essential for germination.

If gibberellin‐synthesis inhibitors are applied to (normal) seeds, they fail to germinate. If gibberellin is applied externally to the mutant or inhibited seeds, they will germinate normally.

Question 34

what are the commerical uses of plant hormones?

Answer 34

Ethene to promote fruit ripening:
o Fruits such as bananas are picked when unripe, transported in this unripe state
(as hard fruit are less easily bruised and damaged) and then exposed to ethene once near/at point of sale.
o The ethene causes the fruit to rapidly ripen.
o Ensuring this happens only once the fruit is about to be sold reduces food waste
due to damage in transit or the oversupply of ripe fruits in shops.

Auxin in rooting powders:
o To stimulate root growth in a stem cutting, the base of the cutting is dipped in
a ‘hormone rooting powder,’ containing auxin.
o This increases the chance of successful vegetative propagation as the auxin stimulates growth of roots at the base of the cutting.
o The cutting will now grow into a complete plant, a genetically identical clone of
the plant the cutting was taken from.

Synthetic auxins as hormonal weedkillers:
o Synthetic auxins act as selective weedkillers that kill broad‐leaved weeds but not narrow‐leaved cereal crops (or grasses).
o These chemicals are relatively cheap to obtain and are generally not toxic to humans and other animals;
o This means that farmers can spray their crops (and gardeners their lawns) to selectively kill the weeds that compete with the crop (or grass) for light, space, water and mineral ions, increasing yield.
o The effect of the weedkiller is to cause rapid, unsustainable growth of the stems of the weeds, which become too weak to support themselves, collapse and die.