Transpiration (növényi párologtatás)
The xylem carries water and minerals up the stems of plants from the root to the leaves, because water is needed for photosynthesis to occur in the leaves. Evaporation of water through the stomata in the leaves is called transpiration.
The loss of water in the leaves causes a suction force in the xylem (called the transpiration stream), which helps to "pull" the water up the xylem. This force is effective up to 10m.
The rate of transpiration varies, depending on:
-number of stomata
- temperature
- humidity
- air pressure
- air movement
and other factors. Nonetheless, a "typical" oak tree might transpire about 68L of water in a "typical" day.
Root pressure (gyökérnyomás)
This is a way to move water up the xylem from the root, but it requires energy to occur. It is very important to start water flow in the spring before the leaves appear on the plants.
Capillarity (hajszálcsövesség)
Capillarity is a combination of the forces of adhesion (water molecules sticking to the xylem wall) and cohesion (water molecules sticking to other water molecules). In the very narrow tubes of the xylem, these two molecular forces work together causing the water to "crawl" up the xylem.
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Translocation
In the phloem sugars, and other products made by plant cells are carried either up or down to growing regions or to the roots for storage (sometimes we call the areas of plants that are for storage "sinks"). This is an active process, meaning that it requires energy to happen, but we do not know the exact mechanisms by which it happens (and what we do know, you don't need to learn for this class!!!)
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Nutrition
Plants are autotrophs (or self-feeders), which means they carry out photosynthesis to meet their energy needs.
For photosynthesis to occur, plants need light energy, which they gain from sunlight. They also require CO2 and H2O, which they use to form carbohydrates (in other words, sugar). Carbon dioxide they take up from the air via their stomata, water is absorbed by their roots from the soil.
Photosynthesis
6CO2 + 6H2O react in the presence of sunlight and enzymes to form C6H12O6 + 6O2
The rate of the reaction depends on light intensity, temperature and the concentration of CO2.
Glucose that is formed in this reaction is used by the plant to form:
-sucrose (found in fruit)
-starch (form in which plants store extra)
-fructose
-cellulose (forms cell walls)
-oil (found in seeds, cell membranes)
-amino acids (form proteins, but to be formed nitrogen and sulphur are required)
Plants also need inorganic minerals to form proteins and other compounds. Below are some of the minerals that are required in large amounts and what symptoms the plants exhibit when they are lacking the minerals.
| Mineral | Use | Lack |
| Nitrogen, as NO3-, NH4+ | In proteins, chlorophyll | Small, yellow |
| Phosphorus, as PO4- | In membranes, ATP | Dark green, except leaf edges are yellow, flower and fruit are delayed |
| Potassium (kálium) | Starch, cell walls | Brown dots |
| Calcium | Root growth | Root desintegration |
Other important minerals include magnesium, sulphur, iron, manganese, zinc, copper and chlorine.
Respiration
Respiration is the biological process, where living organisms breakdown glucose to produce energy, which can then be used to power other chemical reactions, such as the production of proteins, or processes, such as root pressure or translocation. This reaction occurs in the mitochondria of the the cells and is summarized as:
C6H12O6 + 6O2 react in the presence of enzymes to produce 6CO2 + 6H2O + 2830kJ of energy (which is held in molecules called ATP - more on that in gr. 11!)
Using information about the amount of CO2 produced and the amount of O2 consumed, it is possible to calculate a plant' s respiratory quotient (RQ), which lets us know how much photosynthesis or respiration is occurring.
RQ = CO2 produced/ O2 used
In the dark, when photosynthesis doesn't occur, but the plant is respiring, the uptake of oxygen is high as is the production of carbon dioxide. In dim light, when photosynthesis is occurring at a rate to match respiration, then there no measurable uptake or production of oxygen or carbon dioxide. In bright light, where the rate of photosynthesis is higher than the rate of respiration, then carbon dioxide is being taken up, while oxygen is produced.
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| This graph shows the relationship between respiration and photosynthesis. While respiration has a fairly constant level in the plant (blue line), the rate of photosynthesis depends on light intensity. |
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| This graph shows something similar to the above graph, but corresponds the rate of photosynthesis with the time of day (or, essentially, the intensity of sunlight) |
Plant Development
Plants can exhibit different types of development:
- annuals - live only one growing season, in that season, they grow, flower and produce seeds
- biennials - live for 2 growing seasons, in the first seasons they exhibit only vegetative growth and they flower and produce seeds in the second season
- perennials - live for many growing seasons, they may flower once or many times in their lifetime.
Stages of development (already been mentioned, but a quick review!)
- seed: product of double fertilization, it contains the endosperm and the zygote, which are surrounded by a seed coat
- germination: beginning of sprout growth in seed. It is induced by external factors, such as moisture, oxygen levels, temperature (some plant seeds need exposure to freezing temperatures before they will germinate) and sometimes light. It is also induced by internal factors, particularly hormones.
- vegetative stage: growth of the plant depends on environmental factors
- reproductive stage: this is the production of flowers and fruit. It is induced by temperature, moisture and day length (also called photoperiodism). Some plants are short-day plants, this means they will flower when there are 8-12 hours of sunlight. Others are long-day plants and will flower when there are 12-16 hours of sunlight/day. Yet others are day neutral and will flower irrelevant of day length.
Hormones (phytohormones or plant hormones)
1. Auxins
- stimulate cell elongation, promote fruit development and inhibit fruit dropping
- their highest concentrations are found at tips of shoots and roots, where it speeds up growth in the shoot and slows growth in the root, so they affect upward growth of the shoot and downward growth of the root.
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| In the presence of light from one side (like in the case of a plant in a window), it has been shown that more auxin in produced on the shaded side of the tip, resulting in increased elongation of the cells on the shaded side, which causes the plant to "bend" toward the light (Páal Árpad) |
2. Gibberelins
- make plants taller, increase the rate of germination and bud development
3. Cytokinins
- stimulate dividing tissue and help in wound healing
4. Absissic acid
- inhibits growth, promotes ageing, maintains dormancy
5. Ethylene
- speeds up fruit ripening, causes withering and dropping of leaves.
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