Plant Physiology – Q&A Review
1. Why do plants need to exchange gases with the environment?
Plants need to do gas exchange because they create aerobic cellular respiration (like animals) and they need to get molecular oxygen and to release carbon dioxide. Besides aerobic cellular respiration plants still need to get carbon dioxide to make photosynthesis and to release the molecular oxygen that is the product of this reaction.
2. What are the main gas exchange organs of the plants? How is the process accomplished?
In the covering of the leaves and of the primary structure of the stem gas exchange is made through the cuticle and pores of the epidermis. In the covering of the secondary structure of the stem of woody plants gas exchange is made through the lenticels of the periderm (small breaches of the cork). The gas exchange in plants is accomplished by simple diffusion.
3. What is plant transpiration? What are the two main types of plant transpiration process? Which of them is more significant in volume?
Transpiration is the loss of water from the plant to the atmosphere in 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 is stomatal transpiration since it is more intense and physiologically regulated.
4. What are stomata? How do these structures participate in the plant transpiration?
Stomata (singular, stoma) are small specialized passages for water and gases present in the epidermis of the plants. As the plant needs more or less to lose water and heat the stomata respectively close or open preventing or allowing the passage of gases by diffusion.
5. What are the elements that constitute the stomata?
The stoma is made of a central opening, the ostiole, or slit, delimited by two guard cells responsible for its closing or opening. A substomatal chamber is located under the ostiole.
6. How do plants control the opening and the closing of the stomata?
The opening and the closing of the stomata depend upon the necessity of the plant to lose water and heat through transpiration (exit of water vapor means 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 goes out the stomata by osmosis.
Other factors like light intensity and carbon dioxide concentration in the leaves influence the opening and the closing of the 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 the stomata tend to close. When the carbon dioxide concentration in the photosynthetic parenchyma is low the stomata open to absorb more of the gas and make photosynthesis possible; when such concentration is high the stomata tend to close.
7. Do plants placed under an environment drier than the habitat where they are used to living have an increase or a reduction in the time during which their stomata remain open?
If plants from a moister region were transferred to a drier region probably their stomata would remain closed for a longer time, i.e., the time during which stomata are open will be reduced to lower the loss of water by transpiration.
8. Why do some plants adapted to a dry environment open their stomata only at night?
During the day in dry habitats the guard cells become flaccid and the stomata close; concurrently carbon dioxide is disallowed to pass 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 the parenchymal tissues. During the day the stored gas is mobilized to be used in photosynthesis.
9. How has the position of the stomata changed in some plants to prevent excessive water loss by transpiration?
In some plants that have leaves that receive too much sunlight the stomata concentrate in the inferior epidermis, so their heating is lower and less water is lost by stomatal transpiration. In other plants of dry environments the stomata group in some regions of the leaf; over the surface of these areas the water concentration of the air is higher comparing to the environment and the loss of water by transpiration is thus reduced. Some plants of dry climates also have stomata within cavities.
10. Is transpiration the only way through which leaves lose water?
Plants do not only lose water as vapor, as by transpiration. The leaves also lose liquid water by a phenomenon known as gutation. Gutation takes place through structures called hydathodes similar to stomata. Gutation mainly occurs when transpiration is difficult due to high air humidity or when the plant is placed in a watery soil.
11. When air humidity is high does the plant transpiration increase or lower?
When air humidity is high transpiration diminishes. Since transpiration is a simple diffusion process it depends upon the concentration gradient of water between the plant and the environment. If the atmosphere has too much water vapor the gradient becomes lower or even reversed.
12. How do the water absorption volume and the water transpiration volume comparatively vary in plants in a day? What is the final comparative balance of these processes?
During the day the transpired volume of water is higher than the volume absorbed by the roots. At night the situation reverses and the roots absorb more water than the transpired volume.
It is observed that the transpired and the absorbed volumes practically equal in a day.
13. How do plants solve the problem of transporting substances throughout their tissues?
In bryophytes the substance transport is done by diffusion. In tracheophytes (pteridophytes, gymnosperms and angiosperms) there are specialized conductive vessels, the xylem that carries water and mineral salts and the phloem that conducts organic material (sugar).
14. Is transportation of gases in tracheophytes made through the vascular tissues?
Carbon dioxide and oxygen are not transported through the xylem or phloem. These gases reach the cells and exit the plant by diffusion through intercellular spaces or between neighboring cells.
15. Are the xylem and the phloem made of living cells?
The cells that constitute the xylem ducts are dead cells killed by lignin deposition. The cells of the phloem are living cells.
16. What is the importance of lignin for the xylem formation?
Lignin is important because it is deposited on the cell wall of the xylem cells providing impermeability and rigidity to the xylem vessels.
17. What is root pressure?
Root pressure is the pressure that forces water from the soil to be absorbed by the xylem of the root. It is due to 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 between water molecules and the capillary wall. The capillarity phenomenon is possible because water is a polar molecule and forms intermolecular hydrogen bonds. Therefore there is electrical attraction (adhesion force) between the capillary wall and the water molecules that then pull each other (cohesion force) since they are bound. Not just water but other liquids may move inside capillaries by capillarity.
Capillarity is not too relevant for the transport of water in plants. It contributes only to a few centimeters of ascension.
19. What are the forces that make water to flow within the xylem from the roots to the leaves?
Water enters the roots due to the root pressure and a water column is maintained within the xylem from the roots to the leaves. The most important factor that makes water ascend is transpiration, mainly in the leaves. As leaves lose water by transpiration their cells tend to attract more water creating suction inside the xylem. The cohesion property of water that keeps its molecules bound (one pulls the other) by hydrogen bonds helps the process.
20. What is tree girdling? What happens to a plant when that girdle is removed from the stem (below the branches)?
Malpighi’s girdling, or tree girdling, is the removal from a stem of a complete external girdle containing the phloem (that is more external) but preserving the xylem (that is more internal).
When a girdle like that is removed below the branches the plant dies because organic food (sugar) is disallowed to pass to the region below the girdle and thus roots die from the lack of nutrients. Since roots die the plant does not get water and mineral salts and dies too.
21. What are plant hormones?
Plant hormones, also called phytohormones, are substances that control the embryonic development and the growth of the adult plant.
22. What are the main natural plant hormones and what are their respective effects?
The main natural plant hormones and their respective functions 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: have action similar to auxins (growth and distension), stimulate flowering and fruit formation and activate seed germination. Cytokinins: increase cellular division rate and together with auxins help growth and tissue differentiation, slow the plant aging. Ethylene (ethene): a gas released by plants that participates in the growth process and has noteworthy role in fruit ripening and in leaf abscission.
23. What is the plant coleoptile? Why does the removal of the coleoptile extremity disallow plant growth?
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 generally is the region where auxins are made. If this region is removed, plant growth stops since auxins are necessary to promote growth and tissue differentiation.
24. What is indolacetic acid (IAA)?
Indolacetic acid (indolyl-3-acetic acid), or IAA, is the main natural auxin made by plants. It promotes plant growth and cellular differentiation.
25. What are synthetic auxins and what are their uses?
Synthetic auxins, like indolebutyric acid (IBA) and naphthalenic acid (NAA) are substances similar to IAA (a natural auxin) but artificially made. Some are used to accelerate methods of asexual reproduction (like grafting or budding) and others are even used as herbicides since they selectively kill some plants (mainly dicots).
26. Where in plants is a large amount of IAA found?
Auxins are produced and found in large amount in the apical buds of the stem and shoots and in the young leaves.
27. How do phytohormones help the development of parthenocarpic fruits?
Parthenocarpic fruits are those produced without fecundation. Some plants naturally make parthenocarpic fruits, like the banana tree, stimulated by their own hormones.
Angiosperms that do not naturally produce parthenocarpic fruits may do it if auxins are applied to flowers before fecundation. Therefore even without fecundation the ovaries grow and fruits are formed although seedless.
28. How do auxins act helping the lateral (secondary) growth of the stem?
Auxins stimulate the formation of conductive vessels, xylem and phloem, promoting the thickening of the stem.
29. What happens when the auxin concentration in some structures of the plant is over the action range of the hormone?
In some parts of the plant (stem, roots, lateral buds) there are auxin concentration ranges in which the hormonal action is positive (stimulate growth). It is observed that concentrations over the superior limit of those ranges have the opposite effect (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 downward the stem inhibit the growth of the lateral buds of the plant. At the beginning of the stem development the apical dominance causes the plant growth to be longitudinal (upwards) since the growth of the 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 preventing the 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 made in the apical buds and in young leaves.
32. What are cytokinins? Where are they made?
Cytokinins are phytohormones active in the promotion of cellular division, they slow down the aging of tissues and act together with auxins stimulating plant growth. Cytokinins are produced by the root meristem and distributed through the xylem.
33. What is the plant hormone remarkable for stimulating flowering and fruit ripening? What are the uses and practical inconveniences of that hormone?
The plant hormone notable for stimulating and accelerating fruit ripening is the gas ethylene (ethene). By being 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 fruit ripening. On the other hand, if the intensification or acceleration of fruit ripening is not desirable care must be taken to prevent the mixture of ripe fruits that release ethylene with the others.
34. Are the development and growth of plants only influenced by plant hormones?
Physical and chemical environmental factors, like intensity and position of light in relation to the plant, gravitational force, temperature, mechanical pressures and 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 stimulus. In Botany the studied plant tropisms are: phototropism (tropism in response to light), geotropism (tropism in response to the earth gravity) and thigmotropism (tropism in response to mechanical stimulus).
36. To which direction does the growth of one side of a stem, branch or root induce the 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 that in which the plant movement (or growth) is towards the light source and negative phototropism is that in which the movement (or growth) is inverse, away from the light source.
Phototropism relates to auxins since the exposition of one side of the plant to light makes these hormones concentrate in the darker side. This fact makes the auxin action upon the stem to be positive, i.e., the growth of the darker side is more intense and the plant arcs towards the lighter side. In the root (when submitted to light, in general experimentally) the auxin action is negative (over the positive range), the growth of the darker side is inhibited and the root curves towards this side.
38. What are the types of plant geotropisms? Why do the stem and the root present opposite geotropisms?
The types of geotropisms are the positive geotropism, that in which the plant grows in favor of the gravitational force, as for example in roots, and the negative geotropism, that against the gravitational force, for example, in the stem.
Root geotropism and stem geotropism are opposed due to different sensitivities to auxin concentration in these structures. The following experiment can demonstrate the phenomenon: Stem and root are placed in a horizontal position (parallel to the ground) and naturally auxins concentrate along their bottom part. Under this condition it is observed that the stem grows upwards and the root grows downwards. This happens because in the stem the high auxin concentration in the bottom makes this side grow (longitudinally) more and the structures arcs upwards. In the root the high auxin concentration in the bottom inhibits the growth of this side and the upper side grows more making the root to curve downwards.
39. What is thigmotropism?
Thigmotropism is the movement or growth of the plant in response to mechanical stimuli (touch or physical contact), as when a plant grows around a supporting rod. It occurs for example in grape and passionfruit vines, etc.
40. What is photoperiod?
Photoperiod is the daily time period of light exposure of a living being. The photoperiod may vary according to the period of the year.
41. What is photoperiodism?
Photoperiodism is the biological response presented by some living beings to their daily time of light exposure (photoperiod).
42. What are the plant organs responsible for the perception of light variation? What is the pigment responsible for this perception?
Leaves are mainly responsible for perception of light intensity in plants. The pigment that performs this perception and commands photoperiodism is called phytochrome.
43. How does the 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 some conditions of daily illumination. This occurs because their blossoming depends on the duration of the photoperiod that in its turn varies with the season of the year. Flowering is also affected by exposition to certain temperatures.
44. What is the critical photoperiod? How can the critical photoperiod relate to flowering be experimentally determined?
The critical photoperiod is the limit of the photoperiod duration for the occurrence of some biological response. This limit can be a maximum or a minimum, according to the characteristics of the biological response and to the studied plant.
To determine the critical photoperiod relating to flowering, 24 groups of plants of the same species can be taken and the following experiment can be done: Each group is submitted to a different photoperiod, the first group to 1 hour of daily exposure to light, the second to 2 hours, the third to 3 hours, and so on, until the last group is exposed to 24 hours. It is observed later that beyond a specific duration of light exposure plants present or do not present flowering and the remaining submitted to a shorter photoperiod present opposite behavior. The duration of the light exposure that separates these two groups is the critical photoperiod.
45. How do plants classify according to their photoperiodism-based flowering?
According to their photoperiodism-based flowering plants classify as long-day plants, those that depend on longer photoperiods than the critical photoperiod to flower, as short-day plants, those that 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 on shoots. Most plants have opposite phyllotaxis (alternating in sequence, one in one side of the shoot, the following in the opposite side) as a solution to prevent self shading of the leaves thus improving the efficiency of photosynthesis.