plant responses to abiotic stress Flashcards
Leaf loss in deciduous plants as a response to abiotic stress
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. 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
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.
The sensitivity of plants to day length (or dark length) results from a light-sensitive pigment called phytochrome. This exists in two forms - Pf and Pfr Each absorbs a different type of light and the ratio of Pf to Pfr changes depending on the levels of light.
abcission or leaf fall
After a summer of long days, short nights, and warm temperatures, the nights lengthen, days shorten, and temperatures fall as autumn develops. The lengthening of the dark period triggers a number of changes, including abscission or leaf fall and a 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. 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 leat separates from the plant. A neat, waterproof scar is left behind.
preventing freezing
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.
Most species only produce the chemicals which make them hardy and frost resistant during the winter months. It appears that different genes are suppressed and activated in response to a sustained fall in temperatures along with a reduction in day length, effectively preparing the plants to withstand frosty conditions. A sustained spell of warm weather along with extended day length reverses these changes in the spring.
stomatal control
As you have already learnt, heat and water availability are major abiotic stresses for plants. One of the major ways in which plants can respond to these stresses is to open the stomata to cool the plant as water evaporates from the cells in the leaves in transpiration, or to close the stomata to conserve water.
The opening and closing of the stomata in response to abiotic stresses is largely under the control of the hormone ABA. The leaf cells appear to release ABA under abiotic stress, causing stomatal closure.
However, scientists now think that the roots also provide an early warning of water stresses through ABA. So, for example, when the levels of soil water fall 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.
ABA activates changes in the ionic concentration of the guard cells, reducing the water potential and therefore turgor of the cells. As a result of reduced turgor, the guard cells close the stomata and water loss by transpiration is greatly reduced.
