Plants need to carry out gas exchange because they use aerobic cellular respiration (like animals). As a result, they need to obtain molecular oxygen and release carbon dioxide. In addition to aerobic cellular respiration, plants also need to obtain carbon dioxide to carry out photosynthesis and to release the molecular oxygen that is the product of this reaction.
In the covering of the leaves and of the primary structure of the stem, gas exchange is carried out through the cuticle and pores of the epidermis. In the covering of the secondary structure of the stem of woody plants, gas exchange is carried out through the lenticels of the periderm (small breaches in cork). Gas exchange in plants is carried out via simple diffusion.
Transpiration is the loss of water from the plant to the atmosphere into the form of vapor.
Transpiration occurs through the cuticle of the epidermis (cuticular transpiration) or through the ostioles of the stomata (stomatal transpiration). The most important of the two is stomatal transpiration, since it is more intense and is physiologically regulated.
Stomata (singular, stoma) are small specialized passageways for water and gases present in the epidermis of plants. As the plant needs to lose more or less water and heat, the stomata respectively close or open, preventing or allowing the movement of gases via diffusion.
A stoma is made of a central opening, called the ostiole, or slit, surrounded by two guard cells responsible for closing and opening. A substomatal chamber is located under the ostiole.
The opening and the closing of stomata depend upon the plant's need to lose water and heat through transpiration (the exit of water vapor means the elimination of heat). When the plant has excessive, water the guard cells become turgid and the ostiole opens. When little water is available, the guard cells become flaccid and the ostiole closes.
Water enters and exits stomata via osmosis.
Other factors such as light intensity and carbon dioxide concentration in the leaves influence the opening and the closing of stomata. When luminosity is high the photosynthesis rate increases and the stomata open to absorb more carbon dioxide from the environment and release heat; when luminosity is low, stomata tend to close. When the carbon dioxide concentration in the photosynthetic parenchyma is low, stomata open to absorb more of the gas to make photosynthesis possible; when its concentration is high, stomata tend to close.
If plants from a moister region are transferred to a drier region, it is likely that their stomata will remain closed for a longer time, because the time during which stomata are open will be reduced to lower the loss of water via transpiration.
During the day in dry habitats, guard cells become flaccid and stomata close; as a result, carbon dioxide is unable to move along to participate in diurnal photosynthesis. Some plants from dry regions solve this problem through the method of nocturnal carbon dioxide fixation. At night, when water loss by transpiration is lower, the stomata open, carbon dioxide enters and it is stored within parenchymal tissues. During the day the stored gas is mobilized to be used in photosynthesis.
In some plants whose leaves receive too much sunlight, stomata concentrate in the inferior epidermis. As a result, they contain less heat, and less water is lost via stomatal transpiration. In other plants adapted to dry environments, the stomata group in certain regions of the leaf, as over the surface of these areas, the water concentration of the air is higher compared to in the environment and the loss of water via transpiration is thus reduced. Some plants from dry climates also have stomata within cavities.
Plants do not only lose water in the form of vapor, as is the case in transpiration. Leaves also lose liquid water through a phenomenon known as guttation. Guttation takes place through structures called hydathodes, which are similar to stomata. Guttation mainly occurs when transpiration is difficult due to high air humidity or when the plant is placed in watery soil.
When air humidity is high, transpiration decreases. Since transpiration is a simple diffusion process, it depends on the concentration gradient of water between the plant and the environment. If the atmosphere has too much water vapor, the gradient becomes low or even reversed.
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During the day, the volume of water transpired is higher than the volume absorbed by the roots. At night, the situation reverses and the roots absorb more water than the volume of water transpired.
It can be observed that the volume of water transpired and the volume of water absorbed practically equal over the course of a day.
In bryophytes, substance transport is carried out by diffusion. Tracheophytes (pteridophytes, gymnosperms and angiosperms) contain specialized conducting vessels: xylem, which carries water and mineral salts, and phloem, which transports organic materials (sugar).
Carbon dioxide and oxygen are not transported through xylem or phloem. These gases reach the cells and exit the plant via diffusion through intercellular spaces or between neighboring cells.
The cells that constitute xylem ducts are dead cells killed by lignin deposition. Phloem cells are living cells.
Lignin is important because it is deposited on the cell wall of xylem cells, providing impermeability and rigidity to xylem vessels.
Root pressure is the pressure that forces water from the soil to be absorbed by xylem in the root. It is caused by the osmotic gradient between the interior of the root and the soil.
Capillarity is the phenomenon through which water moves inside extremely thin tubes (capillaries) aided by the attraction force between water molecules and the capillary wall. The phenomenon of capillarity is possible because water is a polar molecule that forms intermolecular hydrogen bonds. Therefore, there is an electrical attraction (adhesion force) between the capillary wall and the water molecules, which then pull each other (cohesion force), since they are bound. Other liquids may also move inside capillaries via capillarity, and not just water.
Capillarity is not particularly relevant for the transport of water in plants. It only contributes to a few centimeters of ascent.
Water enters the roots due to the root pressure and a water column is maintained within xylem from the roots to the leaves. The most important factor that makes water go up is transpiration, mainly in the leaves. As the leaves lose water via transpiration, their cells tend to attract more water, creating suction inside xylem. The cohesion property of water that keeps its molecules bound (one pulls the other) by hydrogen bonds helps in the process.
Malpighi’s girdling, or tree girdling, is the removal of a complete external girdle containing the phloem (which is more external) from a stem, all the while preserving the xylem (which is more internal).
When a girdle is removed below the branches like that, the plant dies because organic food (sugar) is unable to move into the region below the girdle and, as a result, the roots die from the lack of nutrients. When the roots die, the plant does not obtain water or mineral salts and dies as a result.
Plant hormones, also called phytohormones, are substances that control embryonic development and growth in adult plants.
The main natural plant hormones and their respective effects are the following:
Auxins (the best known natural auxin is IAA, indoleacetic acid): their function is to promote plant growth, distension and cellular differentiation. Gibberellins: their effect is similar to that of auxins (growth and distension); they stimulate flowering and fruit formation and activate seed germination. Cytokinins: they increase the cellular division rate and, together with auxins, help growth and tissue differentiation and slow the plant aging process. Ethylene (ethene): this is a gas released by plants, which participates in the growth process and has a noteworthy role in ripening fruit and leaf abscission.
The coleoptile is the first (one or more) aerial structure of the sprouting plant that emerges from the seed. It encloses the young stem and the first leaves, protecting them.
The top of the coleoptile is generally the region where auxins are produced. If this region is removed, plant growth stops, since auxins are necessary to promote growth and tissue differentiation.
Indolacetic acid (indolyl-3-acetic acid), or IAA, is the main natural auxin produced by plants. It promotes plant growth and cellular differentiation.
Synthetic auxins, such as indolebutyric acid (IBA) and naphthalenic acid (NAA), are substances similar to IAA (a natural auxin) but which are artificially produced. Some are used to accelerate methods of asexual reproduction (such as grafting or budding) and others are even used as herbicides since they selectively kill some plants (mainly dicots).
Auxins are produced and found in large amounts in the apical buds of the stem and shoots as well as in young leaves.
Parthenocarpic fruits are those produced without fertilization. Some plants produce parthenocarpic fruits naturally, such as the banana tree, stimulated by their own hormones.
Angiosperms that do not naturally produce parthenocarpic fruits may do so if auxins are applied to flowers before fertilization. Therefore, even without fertilization, the ovaries grow and fruits are formed, although they are seedless.
In some parts of the plant (the stem, roots, lateral buds), there are auxin concentration ranges in which the hormonal action is positive (it stimulates growth). It has been observed that concentrations over the upper limit of those ranges have the opposite effect (the inhibition of growth).
Apical dominance is the phenomenon through which high (over the positive range limit) auxin concentrations due to auxins from the apical bud moving down the stem inhibit the growth of the lateral buds of the plant. At the beginning of stem development, apical dominance causes plant growth to be longitudinal (upwards), since the growth of lateral buds remains inhibited. As the lateral buds become more distant from the apex, the auxin concentration in these buds lowers and shoots grow more easily.
The growth of tree branches can be stimulated by preventing apical dominance through the removal of the apical bud.
Gibberellins are plant hormones that stimulate plant growth, flowering and fruit formation (also parthenocarpy) and the germination of seeds. There are more than 70 known types of gibberellins. Gibberellins are produced in the apical buds and young leaves.
Cytokinins are phytohormones active in the promotion of cellular division. They also slow down the aging of tissues and act together with auxins to stimulate plant growth. Cytokinins are produced by the root meristem and are distributed through the xylem.
The plant hormone notable for its ability to stimulate and accelerate fruit ripening is the gas ethylene (ethene). Because it is a gas, ethylene acts not only in the plant that produces it but also in neighboring ones.
Some fruit processing industries use ethylene to accelerate the ripening of fruit. On the other hand, if the intensification or acceleration of fruit ripening is not desirable, care must be taken to prevent mixing of ripe fruits that release ethylene with others.
Physical and chemical environmental factors, such as intensity and position of light in relation to the plant, gravitational force, temperature, mechanical pressures and the chemical composition of the soil and of the atmosphere, can also influence the growth and development of plants.
Tropisms are movements caused by external stimuli. In botany, the plant tropisms studied are: phototropism (tropism in response to light), geotropism (tropism in response to the gravity of earth) and thigmotropism (tropism in response to mechanical stimuli).
Whenever one side of a stem, branch or root grows more than the other side the structure curves towards the side that grows less. (This is an important concept for plant tropism problems.)
Phototropism is the movement of plant structures in response to light. Phototropism may be positive or negative. Positive phototropism is when the plant movement (or growth) is towards the light source and negative phototropism is when the movement (or growth) is opposite, moving away from the light source.
Phototropism is related to auxins since the exposure of one side of the plant to light makes these hormones concentrate in the darker side. This causes the effect of auxins on the stem to be positive, meaning that the growth of the darker side is more intense and the plant arcs towards the lighter side. In roots, (when subject to light, in general and experimentally) the effect of auxins is negative (over the positive range), the growth of the darker side is inhibited, and the root curves towards that side.
The types of geotropisms are positive geotropism, in which the plant grows in favor of gravitational force, such as in roots, and negative geotropism, which is against gravitational force, such as in the stem.
Root geotropism and stem geotropism are opposite due to the different sensitivities to auxin concentrations in these structures. The following experiment can demonstrate the phenomenon: Stems and roots are placed in a horizontal position (parallel to the ground) and auxins naturally concentrate along their bottom part. Under this condition, we can observe that the stem grows upwards and the root grows downwards. This happens because, in the stem, the high auxin concentration in the bottom makes that side grow (longitudinally) more and the structure arcs upwards. In the root, the high auxin concentration in the bottom inhibits the growth of that side and the upper side grows more, making the root curve downwards.
Thigmotropism is the movement or growth of a plant in response to mechanical stimuli (touch or physical contact), such as when a plant grows around a supporting rod. This occurs in grape and passionfruit vines, for example.
A photoperiod is the daily time period of light exposure of a living organism. The photoperiod may vary according to the time of the year.
Photoperiodism is the biological response of certain living organisms to their daily amount of light exposure (photoperiod).
Leaves are mainly responsible for the perception of light intensity in plants. The pigment that is able to perceive light variations, and which controls photoperiodism, is called phytochrome.
Flowering is a typical and easy to observe example of photoperiodism. Most flowering plants flower only during specific periods of the year or when placed under certain conditions of daily illumination. This occurs because their blossoming depends on the duration of the photoperiod, which in turn varies with the season of the year. Flowering is also affected by exposure to certain temperatures.
The critical photoperiod is the limit of the duration of the photoperiod after which some biological response occurs. This limit can be a maximum or a minimum, depending on the characteristics of the biological response and to the studied plant.
To determine the critical photoperiod of flowering, 24 groups of plants of the same species can be used and the following experiment can be carried out: Each group is subject to a different photoperiod: the first group receives 1 hour of daily exposure to light; the second 2 hours; the third 3 hours; and so on, until the last group is exposed to 24 hours. We can observe that beyond a specific duration of light exposure, plants present or do not present flowering, and the remainder submitted to a shorter photoperiod present the opposite behavior. The duration of the light exposure that separates these two groups is the critical photoperiod.
According to their photoperiodism-based flowering, plants can be classified as: long-day plants, which depend on longer photoperiods than the critical photoperiod to flower; as short-day plants, which depend on shorter photoperiods than the critical photoperiod to flower; and as indifferent plants, whose flowering does not depend on the photoperiod.
Phyllotaxis is the way leaves are arranged along shoots. Most plants have opposite phyllotaxis (alternating in sequence, one on one side of the shoot, the following on the opposite side) as a solution to prevent leaves from blocking the sun received by other leaves, thus improving the efficiency of photosynthesis.
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