chapter 16 pt 2 Flashcards
Gibberellins
pt 1
gibberellins are involved in the germination of seeds.
They are also important in the elongation of plant stems during growth.
Gibberellins affect the length of the internodes - the regions between the leaves on a stem.
Gibberellins were discovered because they are produced by a fungus from the genus
Gibberella that affects rice.
The infected seedlings grew extremely tall and thin. Scientists investigated the rice and isolated chemicals - gibberellins - which produce the same spindly growth in the plants.
Gibberellins
pt 2
It was then discovered that plants themselves produce the same compounds.
Plants that have short stems produce few or no gibberellins.
There are well over a hundred different naturally produced gibberellins.
Scientists have bred many dwarf varieties of plants where the gibberellin synthesis pathway is interrupted.
Without gibberellins the plant stems are much shorter.
This reduces waste and also makes the plants less vulnerable to damage by weather and harvesting.
different concentrations affect different tissues diagram
Investigating the effect of hormones on plant growth of the shoots, the roots, and the germination of seeds.
- growing seedlings hydroponically (in nutrient solution rather than soil) in serial dilutions of different hormones, or applying different concentrations of hormones to the cut ends of stems or roots and observing the effects.
- In most experiments, it is important to make serial dilutions to observe the effects of different concentrations of the hormones, as they can have different effects on growth at different concentrations.
- Experiments investigating the effect of hormones on plant growth usually involve large numbers of plants.
- When you have completed your measurements, the spread of data from each experimental group should be measured using standard deviation.
synergism.
Most plant hormones do not work on their own but by interacting with other substances.
In doing so, very fine control over the responses of the plant can be achieved.
If different hormones work together, complementing each other and giving a greater response than they would on their own, the interaction is known as synergism.
antagonism.
If the substances have opposite effects, for example one promoting growth and one inhibiting it, the balance between them will determine the response of the plant.
This is known as antagonism.
Abiotic stresses include
changes in day length, cold and heat, lack of water, excess water, high winds, and changes in salinity.
Plants need to be able to cope with these changes.
lant responses involve both physical and physiological adaptations.They may have very thick cuticles, hairy leaves, sunken stomata or a wilting response in hot, dry or extremely windy conditions, or develop aerenchyma if they grow in an aquatic environment.
Leaf loss in deciduous plants as a response to abiotic stress:
pt 1
Plants that grow in temperate climates experience great environmental changes during the year.
For example, the range of daylight hours in parts of northern Scotland ranges from about 6.5 hours midwinter to just under 18.5 hours midsummer.
Temperatures vary as well - in England the mean temperature is 3-6°C in winter and 16-21°C in summer.
As light and temperature affect the rate of photosynthesis, seasonal changes have a big impact on the amount of photosynthesis possible.
Leaf loss in deciduous plants as a response to abiotic stress:
pt 2
The point comes when the amount of glucose required for respiration to maintain the leaves, and to produce chemicals from chlorophyll that might protect them against freezing is greater than the amount of glucose produced by photosynthesis.
In addition, a tree that is in leaf is more likely to be damaged or blown over by winter gales.
This means deciduous trees in temperate climates lose all of their leaves in winter and remain dormant until the days lengthen and temperatures rise again in spring.
Daylength sensitivity:
1
Scientists have discovered that plants are sensitive to a lack of light in their environment - This is known as photoperiodism.
For many years it was assumed that plants responded to the length of daylight, but more recent evidence suggests that it is lack of light that is the trigger for change.
Many different plant responses are affected by the photoperiod including the breaking of the dormancy of the leaf buds so they open up, the timing of flowering in a plant and when tubers are formed in preparation for overwintering.
Daylength sensitivity:
2
The sensitivity of plants to day length (or dark length) results from a light-sensitive pigment called phytochrome.
This exists in two forms - Pr and Pfr
Each absorbs a different type of light and the ratio of Pr to Pfr changes depending on the levels of light.
Abcission or leaf fall:
1
- period of dormancy during the winter months.
The falling light levels result in falling concentrations of auxin.
The leaves respond to the falling auxin concentrations by producing the gaseous plant hormone ethene.
At the base of the leaf stalk is a region called the abscission zone, made up of two layers of cells sensitive to ethene.
Ethene seems to initiate gene switching in these cells resulting in the production of new enzymes.
Abcission or leaf fall:
2
These digest and weaken the cell walls in the outer layer of the abscission zone, known as the separation layer.
The vascular bundles which carry materials into and out of the leaf are sealed off.
At the same time fatty material is deposited in the cells on the stem side of the separation layer.
This layer forms a protective scar when the leaf falls, preventing the entry of pathogens.
Cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer layer.
Then further abiotic factors such as low temperatures or strong autumn winds finish the process - the strain is too much and the leaf separates from the plant.
A neat, waterproof scar is left behind.
Abcission or leaf fall diagram
Preventing freezing:
1
Another major abiotic factor which affects plants is a decrease in temperature.
If cells freeze, their membranes are disrupted and they will die.
Many plants, however, have evolved mechanisms that protect their cells in freezing conditions.
The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point.
Some plants produce sugars, polysaccharides, amino acids, and even proteins which act as antifreeze to prevent the cytoplasm from freezing or protect the cells from damage even if they do freeze.
graph of Stomatal control:
