2. What are the main gas exchange organs in plants? How does this process take place?
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.
Plant Transpiration and Stomata
3. What is plant transpiration? What are the two main types of plant transpiration processes? Which of them has a greater volume?
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.
4. What are stomata? How do these structures participate in plant transpiration?
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.
- Plant Physiology Review - Image Diversity: stomata
5. What elements compose stomata?
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.
6. How do plants control the opening and the closing of stomata?
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.
7. Do the stomata of plants placed in a dryer than usual environment remain open for more or less time?
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.
8. Why do some plants adapted to a dry environment open their stomata only at night?
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.
9. How has the position of stomata changed in some plants to prevent excessive water loss via transpiration?
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.
10. Is transpiration the only way through which leaves lose water?
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.
- Plant Physiology Review - Image Diversity: guttation
11. When air humidity is high, does the transpiration of a plant increase or decrease?
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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12. How do the volume of water absorption and the volume of water transpiration vary in plants over the course of a day? Overall, how can these quantities be compared?
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.
13. How do plants solve the problem of transporting substances throughout their tissues?
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).
- Plant Physiology Review - Image Diversity: xylem
14. Is the transportation of gases in tracheophytes carried out through vascular tissues?
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.
15. Are xylem and phloem made of living cells?
The cells that constitute xylem ducts are dead cells killed by lignin deposition. Phloem cells are living cells.
16. What is the importance of lignin in xylem formation?
Lignin is important because it is deposited on the cell wall of xylem cells, providing impermeability and rigidity to xylem vessels.
17. What is root pressure?
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.
18. What is capillarity? How is this phenomenon chemically explained? What is the relevance of capillarity for water transport in plants?
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.
19. What forces cause water to flow from the roots to the leaves within xylem?
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.
20. What is tree girdling? What happens to a plant when the girdle is removed from the stem (below the branches)?
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.
21. What are plant hormones?
Plant hormones, also called phytohormones, are substances that control embryonic development and growth in adult plants.
22. What are the main natural plant hormones and what are their respective effects?
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.
23. What is the coleoptile? Why does the removal of the extremity of the coleoptile prohibit plant growth?
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.
- Plant Physiology Review - Image Diversity: coleoptile
24. What is indolacetic acid (IAA)?
Indolacetic acid (indolyl-3-acetic acid), or IAA, is the main natural auxin produced by plants. It promotes plant growth and cellular differentiation.
25. What are synthetic auxins and what are their uses?
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).
26. Where is a large amount of IAA found in plants?
Auxins are produced and found in large amounts in the apical buds of the stem and shoots as well as in young leaves.
27. How do phytohormones help the development of parthenocarpic fruits?
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.
29. What happens when the auxin concentration in certain structures of the plant is over that of the action range of the hormone?
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).
30. What is the phenomenon of apical dominance in plants? How can it be artificially eliminated?
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.
31. What are gibberellins? Where are they produced?
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.
32. What are cytokinins? Where are they produced?
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.
33. What plant hormone is remarkable its ability to stimulate flowering and fruit ripening? What are the uses and practical setbacks of this hormone?
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.
34. Are the development and growth of plants only influenced by plant hormones?
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.
35. What are plant tropisms?
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).
36. In which direction does the growth of one side of a stem, branch or root cause the overall structure to curve?
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.)
37. What is phototropism?
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.
38. What are the types of plant geotropisms? Why do the stem and the roots present opposite geotropisms?
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.
- Plant Physiology Review - Image Diversity: geotropism
39. What is thigmotropism?
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.
40. What is a photoperiod?
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.
41. What is photoperiodism?
Photoperiodism is the biological response of certain living organisms to their daily amount of light exposure (photoperiod).
42. What plant organs are responsible for the perception of variations in light? What pigment is responsible for this perception?
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.
43. How does photoperiodism affect the flowering of some plants?
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.
44. What is the critical photoperiod? How can the critical photoperiod of flowering be experimentally determined?
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.
45. How can plants be classified according to their photoperiodism-based flowering?
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.
46. Why do most plants present opposite phyllotaxis?
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.