NEET AIPMT Biology Chapter Wise Solutions – Plant Growth and Development
1. Auxin can be bioassayed by (AIPMT 2015)
(b) lettuce hypocotyl elongation
(c) Avena coleoptile curvature
2. What causes a green plant exposed to the light, on only one side, to bend towards the source of light as it grows? (AIPMT 2015, Cancelled)
(a) Light stimulates plant cells on the lighted side to grow faster.
(b) Auxin accumulates on the shaded side, stimulating greater cell elongation there.
(c) Green plants need light to perform photosynthesis.
(d) Green plants seek light because they are phototropic.
3. Typical growth curve in plants is (AIPMT 2015, Cancelled)
(a) stair-steps shaped
4. Dr. F. Went noted that if coleoptile tips were removed and placed on agar for one hour, the agar would produce a bending when placed on one side of freshly-cut coleoptile stumps. of what significance is this experiment? (AIPMT 2014)
(a) It made possible the isolation and exact identification of auxin.
(b) It is the basis for quantitative determination of small amounts of growth-promoting substances.
(c) It supports the hypothesis that IAA is auxin.
(d) It demonstrated polar movement of auxins.
5. A few normal seedlings of tomato were kept in a dark room. After a few days they were found to have become white-coloured like albinos.
Which of the following terms will you use to describe them? (AIPMT 2014)
6. Which one of the following growth regulators is known as ‘stress hormone’? (AIPMT 2014)
(a) Abscisic acid
(d) Indole acetic acid
7. During seed germination its stored food is mobilized by (NEET 2013)
8. The pineapple which under natural condition is difficult to blossom has been made to produce fruits throughout the year by application of (Karnataka NEET 2013)
(a) NAA, 2, 4-D
(b) Phenyl acetic acid
(d) IAA, IBA
9. Through their effects on plant growth regulators, what do the temperature and light control in the plants? (Mains 2012)
(a) Apical dominance
(c) Closure of stomata
(d) Fruit elongation.
10. Which one of the following gerterally acts as an antagonist to gibberellins? (Mains 2012)
11. Vernalization stimulates flowering in (Mains 2012)
12. Phototropic curvature is the result of uneven distribution of (Prelims 2010)
13. Photoperiodism was first characterised in (Prelims 2010)
14. Coiling of garden pea tendrils around any support is an example of (Prelims 2010)
15. One of the commonly used plant growth hormone in tea plantations is (Mains 2010)
(b) abscisic acid
(d) indole-3-acetic acid
16. Root development is promoted by (Mains 2010)
(a) abscisic acid
17. One of the synthetic auxin is (Prelims 2009)
18. Which one of the following acids is a derivative of carotenoids? (Prelims 2009)
(a) indole-3-acetic acid
(b) gibberellic acid
(c) abscisic acid
(d) indole butyric acid
19. Importance of day length in flowering of plants was first shown in (Prelims 2008)
20. Senescence as an active developmental cellular process in the growth and functioning of a flowering plant, is indicated in (Prelims 2008)
(a) annual plants
(b) floral parts
(c) vessels and tracheid differentiation
(d) leaf abscission.
21. The wavelength of light absorbed by Pr form of phytochrome is (2007)
(a) 680 nm
(b) 720 nm
(c) 620 nm
22. Which one of the following pairs, is not correctly matched? (2007)
(a) gibberellic acid – Leaf fall
(b) cytokinin – Cell division
(c) IAA – Cell wall elongation
(d) abscissic acid – Stomatal closure.
23. Opening of floral buds into flowers, is a type of (2007)
(a) autonomic movement of variation
(b) paratonic movement of growth
(c) autonomic movement of growth
(d) autonomic movement of locomotion.
24. An enzyme that can stimulate germination of barley seeds is (2006)
25. Farmers in a particular region were concerned that pre-mature yellowing of leaves of a pulse crop might cause decrease in the yield. Which treatment could be most beneficial to obtain maximum seed yield? (2006)
(a) application of iron and magnesium to promote synthesis of chlorophyll
(b) frequent irrigation of the crop
(c) treatment of the plants with cytokinins along with a small dose of nitrogenous fertilizer
(d) removal of all yellow leaves and spraying the remaining green leaves with 2, 4, 5-trichlorophenoxy acetic acid
26. How does pruning help in making the hedge dense? (2006)
(a) it releases wound hormones
(b) it induces the’ differentiation of n%w shoots from the rootstock
(c) it frees axillary buds from apical dominance
(d) the apical shoot grows faster after pruning
27. Treatment of seeds at low temperature under moist conditions to break its dormancy is called (2006)
28. The ability of the Venus Flytrap to capture insects is due to (2005)
(a) specialized “muscle-like” cells
(b) chemical stimulation by the prey
(c) a passive process requiring no special ability on the part of the plant
(d) rapid turgor pressure changes.
29. Cell elongation in internodal regions of the green plants takes place due to (2004)
(a) indole acetic acid
30. One set of the plant was grown at 12 hours day and 12 hours night period cycles and it flowered while in the other set night phase was interrupted by flash of light and it did not produce flower. Under which one of the following categories will you place this plant? (2004)
(a) long day
(b) darkness neutral
(c) day neutral
(d) short day.
31. Coconut milk factor is (2003)
(a) an auxin
(b) a gibberellin
(c) abscisic acid
32. Nicotiana sylvestris flowers only during long days and N.tabacum flowers only during short days. If raised in the laboratory under different photoperiods, they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offspring. What is the best reason for considering N.sylvestris and N.tabacum to be separate species ? (2003)
(a) they cannot interbreed in nature
(b) they are reproductively distinct
(c) they are physiologically distinct
(d) they are morphologically distinct
33. Plants deficient of element zinc, show its effect on the biosynthesis of plant growth hormone (2003)
(d) abscisic acid
34. Differentiation of shoot is controlled by (2003)
(a) high auxin cytokinin ratio
(b) high cytokinin auxin ratio
(c) high gibberellin auxin ration
(d) high gibberellin cytokinin ratio
35. Seed dormancy is due to the (2002)
(b) abscisic acid
36. Dwarfness can be controlled by treating the plant with (2002, 1992)
(b) gibberellic acid
37. Which plant is LDP? (2001)
(b) Glycine max
(c) Mirabilis jalapa
38. Which of the following prevents the fall of fruits? (2001)
39. Hormone responsible for senescence is (2001)
40. Which hormone breaks dormancy of potato tuber? (2001)
41. If the apical bud has been removed then we observe (2000)
(a) more lateral branches
(b) more axillary buds
(c) plant growth stops
(d) flowering stops.
42. By which action a seed coat becomes permeable to water (2000)
(d) all of the above.
43. Which hormone is responsible for fruit ripening? (2000)
(c) ethyl chloride
44. ABA is involved in (1999)
(a) shoot elongation
(b) increased cell division
(c) dormancy of seeds
(d) root elongation.
45. The closing and opening of the leaves of Mimosa pudica is due to (1999)
(a) seismonastic movement
(b) chemonastic movement
(c) thermonastic movement
(d) hydrotropic movement.
46. A plant hormone used for inducing morphogenesis in plant tissue culture is (1998)
(c) abscisic acid
47. The response of different organisms to the environmental rhythms of light and darkness is called (1998)
48. Which combination of gases is suitable for fruit ripening? (1998)
(a) 80 % CH4 and 20% CO2
(b) 80% CO2 and 20% O2
(c) 80% C2H4 and 20% CO2
(d) 80% CO2 and 20% CH2.
49. Phytochrome becomes active in (1998)
(a) red light
(b) green light
(c) blue light
(d) none of these.
50. Which one among the following chemicals is used for causing defoliation of forest trees? (1998)
(a) malic hydrazide
(d) phosphon D.
51. A pigment which absorbs red and far-red light is (1997)
52. What will be the effect on phytochrome in a plant subjected to continuous red light? (1997)
(a) phytochrome synthesis will increase
(b) level of phytochrome will decrease
(c) phytochrome will be destroyed
(d) first (b) then (a).
53. If a tree flowers thrice in a year (Oct., Jan. and July) in Northern India, it is said to be (1997)
(a) photo and thermo-insensitive
(b) photo and thermo-sensitive
(c) photosensitive but thermo-insensitive
(d) thermosensitive but photo-insensitive.
54. Gibberellic acid induces flowering (1997)
(a) in short day plants under long day conditions
(b) in day-neutral plants under dark conditions
(c) in some gymnospermic plants only
(d) in long day plants under short day conditions.
55. The twining of tendrils around a support is a good example of (1995)
(c) nastic movements
56. The closure of lid of pitcher in pitcher plant, is due to (1995)
(a) paratonic movement
(b) autonomous movement
(c) turgor movement
(d) tropic movement.
57. The movement of auxin is largely (1994)
(d) both (a) and (c).
58. If the growing plant is decapitated, then (1994)
(a) its growth stops
(b) leaves become yellow and fall down
(c) axillary buds are inactivated
(d) axillary buds are activated.
59. Removal of apical bud results in (1993)
(a) formation of new apical bud
(b) elongation of main stem
(c) death of plant
(d) formation of lateral branching.
60. The regulator which retards ageing/senescence of plant parts is (1993)
(d) abscisic acid.
61. The hormone produced during adverse environmental conditions is (1993)
(a) benzyl aminopurine
(b) bichlorophenoxy acetic acid
(d) abscisic acid.
62. Klinostat is employed in the study of (1993)
(b) growth movements
63. Which is produced during water stress that brings stomatal closure? (1993)
(b) abscisic acid
(c) ferulic acid
64. Flowering dependent on cold treatment is (1992)
65. Bananas can be prevented from over-ripening by (1992)
(a) maintaining them at room temperature
(c) dipping in ascorbic acid solution
(d) storing in a freezer.
66. Apical dominance is caused by (1992)
(a) abscisic acid in lateral bud
(b) cytokinin in leaf tip
(c) gibberellin in lateral buds
(d) auxin in shoot tip.
67. In short day plants, flowering is induced by (1992)
(a) photoperiod less than 12 hours
(b) photoperiod below a critical length and uninterrupted long night
(c) long night
(d) short photoperiod and interrupted long night.
68. Cytokinins (1992)
(a) promote abscission
(b) influence water movement
(c) help retain chlorophyll
(d) inhibit protoplasmic streaming.
69. Which is employed for artificial ripening of banana fruits? (1992)
70. Abscisic acid causes (1991)
(a) stomatal closure
(b) stem elongation
(c) leaf expansion
(d) root elongation.
71. The hormone responsible for apical dominance is (1991)
72. A chemical believed to be involved in flowering is (1991)
73. Twining of tendrils is due to (1991)
74. Hormone primarily connected with cell division is (1991, 1988)
(d) gibberellic acid.
75. Highest auxin concentration occurs (1990)
(a) in growing tips
(b) in leaves
(c) at base of plant organs
(d) in xylem and phloem.
76. Phytohormones are (1990)
(a) chemical regulation flowering
(b) chemical regulation secondary growth
(c) hormones regulating growth from seed to adulthood
(d) regulators synthesised by plants and influencing physiological processes.
77. Abscisic acid controls (1990)
(a) cell division
(b) leaf fall and dormancy
(c) shoot elongation
(d) cell elongation and wall formation.
78. Phototropic and geotropic movements are linked to (1990)
79. Which of the following movement is not related to auxin level? (1990)
(a) bending of shoot towards light
(b) movement of root towards soil
(c) nyctinastic leaf movements
(d) movement of sunflower head tracking the sun.
80. Which of the following hormones can replace vernalisation? (1989)
81. Leaf fall can be prevented with the help of (1989)
(a) abscisic acid
82. Mowing grass lawn facilities better maintenance because (1989)
(a) wounding stimulates regeneration
(b) removal of apical dominance and stimulation of intercalary meristem
(c) removal of apical dominance
(d) removal of apical dominance and promotion of lateral meristem.
83. Which one increases in the absence of light? (1989)
(a) uptake of minerals
(b) uptake of water
(c) elongation of intemodes
(d) ascent of sap.
84. Cut or excised leaves remain green for long if induced to root or dipped in (1988)
85. Gibberellins promote (1988)
(a) seed germination
(b) seed dormancy
(c) leaf fall
(d) root elongation.
86. Phytochrome is involved in (1988)
87. Movement of leaves of sensitive plant, Mimosa pudica are due to (1988)
Auxin bioassay is a quantitative test as it measures concentration of auxin to produce the effect and the amount of the effect. Avena Curvature is based upon experiments of Went (1928). 10° curvature is produced by auxin concentration of 150 mg/litre at 25°C and 90% relative humidity. The test can measure auxin upto 300 mg/litre. Auxin from a shoot tip or any other plant organ is allowed to diffuse in a standard size agar block (generally 2 x 2 x 1 mm). Auxin can also be dissolved directly in agar. 15- 30 mm long oat coleoptile grown in dark is held vertically over water. 1 mm tip of coleoptile is removed without injuring the primary leaf. After 3 hours a second decapitation is carried out for a distance of 4 mm. Primary leaf is now pulled loose and agar block supported against it at the tip of decapitated coleoptile. After 90-110 minutes, the coleoptile is found to have bent. The curvature is measured, ft can also be photographed and the curvature known from shadow graph.
Auxins induce cell elongation. In a differentially illuminated plant, they accumulate in the shaded part, causing elongation of the cells in the shaded part. This unequal elongation on two sides causes the plant to curve or bend towards the light source i.e., phototropic curvature.
Geometric growth cannot be sustained for long in natural condition. Limited nutrient availability slows down the growth. It leads to a stationary phase or even a decline. Plotting the growth against time, gives a typical sigmoid or S-curve. Sigmoid curve of growth is typical of most organisms in their natural environment including plants. The exponential growth can be expressed as:
W1 = W0ert ;
W2 = final size (weight, height, number etc.)
W0 = initial size at the beginning of the period
r = growth rate t = time of growth
e = base of natural logarithms An idealised sigmoid growth curve is drawn below:
Etiolation is the abnormal form of growth observed when plants grow in darkness or severely reduced light. Such plant characteristically have branched leaves and shoots, excessively long shoots and reduced leaves and root systems.
Abscisic acid prepares plants to cope with stress conditions like drought etc. by inducing stomatal closure and other reactions. Hence it is named stress hormone.
Gibberellins are plant growth substances chemically related to terpenes and occurring naturally in plants and fungi. They promote elongation of stems, e.g., bolting in cabbage plants, and the mobilization of food reserves in germinating seeds and are influential in inducing flowering and fruit development.
NAA (a-Naphthalene acetic acid) and 2,4 D (2, 4 dichlorophenoxy acetic acid) are synthetic auxins. Normally, auxins inhibit flowering. In lettuce, it delays flowering and the plants can be kept in their vegetative phase for longer periods of time. In litchi and pineapple, however, application of auxin promotes flowering.
Light and temperature may affect flowering in plants in various ways. The effect of photoperiods or daily duration of light hours-(and dark periods) on flowering is called photoperiodism. For example, in short day plants flowering occurs when day length is below critical period, e.g., dahlia, rice etc. In long day plants, flowering occurs when day length is above critical period, e.g., spinach, lettuce etc. In short-long day plants, short photoperiod is required for floral initiation and long photoperiod is required for blossoming and vice-versa for long-short day plants. Photoperiodic response is mediated by phytochrome, a pigment which perceives the photoperiod stimulus in leaves. It shows reversible change in red and far-red wavelength. Besides correct photoperiod, some plants require low temperature for flowering. These plants remain vegetative during the warm season flower in winters on receiving low temperature. This was found by Lysenko, that the cold requiring annual and biennial plants can be made to flower in one growing season by providing low temperature treatment (vernalization). Stimulus of vernalization is perceived by meristematic cells, e.g., shoot | tips, root apex, etc., and is named as vemalin.
Abscisic acid or ABA is an antagonist to gibberellins. This is discussed in the table given below:
|Abscisic acid||Gibberellic acid|
|It inhibits growth.||It promotes growth.|
|It promotes the dormancy of seeds, buds and tubers.||It overcomes the natural dormancy of seeds, tubers etc.|
|It inhibits the synthesis of RNA and proteins.||It promotes the synthesis of RNA and proteins.|
|Causes abscission of flowers and fruits.||Promotes development of fruits.|
|Promotes leaf senescence.||Prevents leaf senescence.|
|Promotes stomatal closure.||Promotes stomatal opening.|
|Prevents amylase activity.||Promotes amylase activity during seed germination.|
Carrot is a biennial plant that requires stimulus of low temperature for flowering. It remains vegetative during the warm season and bears flowers and fruits only during winter.
It can be made to flower in one growing season by providing low temperature treatment to young plants or seedlings which is referred to as vernalization. Hence, vernalization stimulates flowering in carrot.
Phototropic curvature is the result of uneven distribution of auxin. Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass responded to unilateral illmination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile contain auxin that caused the bending of the entire coleoptile in relation to the direction of light.
Photoperiodism is the response to duration and timings of light and dark period. It was first studied by W.W. Gamer and H.A Allard (1920) in tobacco. They observed that Maryland Mammoth variety of tobacco could be made to flower in Summer by reducing the light hours with artificial darkening.
The growth movement in response to touch, or contact of a foreign body, in plants is called thigmotropism or thigmotropic movement. The stems and tendrils of the climbers are positively thigmotropic in their response. The coiling of garden pea tendrils around any support is an example of thigmotropism. Thigmonastic (haptonastic) movements are induced by some external stimuli. For example tentacles of Drosera leaf curve and the lamina lobes of Dionaea fold on coming in contact with an insect.
Indole-3-acetic acid (also called auxin) is a phytohormone which is generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. It is observed that the growing apical bud inhibits the growth of the lateral (axillary) bud. (apical dominance). Since apical meristem is the site of auxin synthesis, it is the physiological effect of the auxin which results in the phenomenon of apical dominance. When shoot tips is removed it usually results in the growth of lateral buds.
This phenomenon is widely applied in tea plantations and hedge-making because as in tea plantation and industries, the apical bud is plucked for tea processing which results in more lateral buds thus enhancing plantation and further industrial purposes.
Ethylene promotes root growth and root hair formation. In low concentration ethylene is used for initiation of roots and also of lateral roots.
17. (c & d)
Many auxins have been synthesized which have similar properties with natural auxin and some examples of synthetic auxins are indole-3-butyric acid (IBA), naphthalene acetic acid (NAA), 2,4-dichloro- phenoxy acetic acid(2,4-D), 2,4,5-trichloro-phenoxyacetic acid (2,4,5 -T).
Biosynthesis of abscisic acid (ABA) in most plants occur indirectly by degradation of certain carotenoids present in chloroplasts or other plastids. The biosynthetic pathway follow mevalonic acid pathway for their synthesis. The sites of synthesis are fruits, tissues, leaves, roots and seeds.
The effect of photoperiods or daily duration of light hours (and dark periods) on the growth and development of plants, especially flowering, is called photoperiodism. Photoperiodism was first studied by Gamer and Allard (1920). They observed that ‘Maryland Mammoth’ variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkening. It could be made to remain vegetative in winter by providing extra light.
Senescence is the process of ageing which is caused by cellular breakdown, increased metabolic failure, increased entropy etc. It occurs in the period between reproductive maturity and death. Cell division followed by cell enlargement and differentiation, precede the actual separation. Senescence of cell in distal region lead to lignification of cell wall. Tylose formation in tracheary element and callose deposition in sieve elements which occur in advance of abscission (i.e. senescence) finally, lead to actual separation. Thus, vessels and tracheid (trachery elements) differentiation indicates senescence.
Phytochrome is the photoreceptor pigment that controls flowering. It has two forms as Pr and Pfr . Pr is bluish phytochrome and it absorbs light at 660 to 680 nm of wavelength. Pfr is (far red) yellowish green and absorbs light at 730 nm of wavelength.
Gibberellic acid is a simple weakly acidic plant growth hormone which promotes cell elongation of both leaves and stems in general and intemodal length of genetically dwarf plants in particular. It is in general a growth promoting hormone and does not inhibit growth. So leaf abscission is not associated with gibberellic acid but with abscisic acid.
Movement may be of two types, movement of locomotion and movement of curvature. Movement of curvature is movement of individual parts in relation to other parts. Curvature movement may be growth movements and turgor movements. Growth movements are caused by differential growth in different part of an organ. Opening of floral buds into flowers is such a type of growth movement. Mostly floral bud opening shows thermonastic movement i.e. opening and closing are controlled by temperature. Sometimes movements are controlled by presence or absence of light, eg. Oxalis.
The process by which the dormant embryo of seed resumes active growth and forms a seedling is known as germination.
The initial step in germination process is the uptake of water and rehydration of the seed tissues by the process of imbibition. The first visible sign of germination is the emergence of the radicle from the seed. But this event is preceded by a series of biochemical reactions.
Imbibition of water causes the embryo within seed to produce α and β-amylases. These enzymes hydrolyze the starch stored in endosperm into glucose which is necessary for use both as a respiratory substrate and as a source of carbon skeletons of the molecules needed for growth.
Nitrogen is the fourth most abundant element. Chief source of Nitrogen for plants is nitrates of Ca and K. It is important for plants as it is a component of nucleic acids, proteins chlorophyll and cytochromes. Deficiency of nitrogen causes poor root development, lower respiration rate, chlorosis of older leaves etc. Cytokinins are also very important for plant development. They are associated with the control of apical dominance, fruit development, root growth, cambial activity. So a nitrogenous fertilizer like NPK and Cytokinins are most beneficial to the plant.
Prunning is the process of cutting shoot tips to promote lateral growth of branches. Removal of shoot tips involves removal of apical buds. In the shoot tips auxins are produced. Auxins are growth promoting phytohormones. They cause apical dominance by promoting the growth of apical buds and suppressing the growth of axillary buds. So when the auxins produced in the shoot tips are removed by decapitation it results in lateral growth and plants thus show bushy appearance. This is because of a relatively high concentration of auxin in the apical bud than in the lateral buds.
Vernalization is the method of promoting flowering by exposing young plants to cold treatment e.g., winter verieties of wheat, barley, oats and rye are given artificial cold treatment and planted in spring in areas of very harsh winters such as Soviet Union to promote flowering in them.
In most cereals optimum temperature for vernalization is 4°C. Receptive organ to chilling is the apical meristem. Chelation is the process by which certain micronutrients are treated to keep them readily available to a plant once they are introduced into the soil.
Stratification is a process by which seeds are pretreated to simulated winter conditions so that germination may occur. The degradation of the seed coat is called scarification. This process permits water to pass through the sead coat so that embryo can begin metabolism.
Thigmonastic movements are exhibited by some insectivorous plants such as Dionaea, venus fly trap, Drosera etc. These plants have tentacles, which are sensitive to the stimulus of touch. In the case of the venus flytrap turgor pressure changes occur in which hydrogen ions are rapidly pumped into the walls of cells on the outside of each leaf in response to the action potentials from the trigger hairs. The protons apparently loosen the
cell walls so rapidly that the tissue actually becomes flaccid so that cells quickly absorb water, causing the j outside of each leaf to expand and the trap to snap shut.
Gibberellins play a role in the elongation of intemodes in ‘rosette’ plants. Before reproductive stage there is too much elongation of intemodes but there is less leaf formation. An elongated intemode without leaves in called a “bolt” like structure and the process is called “bolting” flowering takes place after bolting.
Gibberellins induce cell division and cell elongation, when bolting takes place.
Plants require a day length or light period for flowering, this light period is called as photoperiod. It was ; first studied by Gamer and Allard (1920).
Short day plants (SDP’s) flower in photoperiods less than critical day length, e.g., Nicotiana tabacum, Glycine max (Soybean), Xanthium strumarium. Further these plants require long uninterrupted dark period and hence are called long night plants.
Long day plants (LDP’s) flower in photoperiod more than critical day length, e.g., Hyocyamus niger (Henbane), radish, Beta, spinach, Plantago, etc.
Day neutral plants flower in any photoperiod, e.g., tomato, maize, cucumber, etc.
Many experiments were done to sustain the proliferation of normal stem tissues in culture. The growth of culture was most dramatic when the liquid endosperm of coconut, also known as coconut milk, was added to the culture medium. This finding indicated that coconut milk contains a substance or substances that stimulate mature cells to enter and remain in the cell division cycle. Eventually coconut milk was shown to contain the cytokinin zeatin, but this finding was not obtained until several years after the discovery of the cytokinins. The first cytokinin to be discovered was the synthetic analog kinetin.
According to biological species concept a species is a natural inbreeding or panmictic species or group of natural populations which have essentially similar morphological traits, they are genetically distinct and reproductively isolated from others. Since under laboratory conditions N. tabacum and N. sylvestris can produce self- fertile offspring so they are not reproductively isolated. They are considered as separate species because since their flowering periods are different so cross pollination is not possible between them naturally. This makes them different species.
Zinc is available to the plants for absorption in the divalent form. It occurs in the form of minerals as hornblende, magnetite, biotite etc., from where it is released by weathering. It is involved in the synthesis of Indole-acetic acid in plants. It is an activator in the enzyme tryptophan synthetase. Tryptophan is the precursor of Indole-acetic acid.
The major physiological function of cytokinin is to enhance cell division. In tissue culture the undifferentiated mass of cells formed in the culture tubes is called callus.
The callus may remain in the undifferentiated condition or differentiation may take place in this. If it is differentiated, then root and shoot may be formed. Skoog and miller had reported that cytokinins induce shoot formation and auxins induce root formation (figure).
Later however it was proved experimentally that –
(a) High cytokinins/auxin ratio – Differentiation of shoot.
(b) Low cytokinins/auxin ratio-Differentiation of root.
(c) Intermediate cytokinins/auxin ratio – Differentiation of both root and shoot.
(d) Intermediate cytokinins/low auxin – Callus formation.
Viable seeds of some plants are unable to germinate even after getting all the” necessary conditions. This inability of viable seeds to germinate even under favorable conditions, is called dormancy of seeds. This is considered to be due to some barriers or blocks inside the seeds.
Some common causes of seed dormancy are
- Mechanically hard seed coat, which does not allow proper growth of embryo inside it, e.g., Brassica campestris.
- Presence of impermeable (impervious) seed coat to H20, e.g. many seeds of legumes.
- Presence of impermeable seed coat to oxygen, e.g. Xanthium (cocklebur).
- Presence of germination inhibitors like ABA (abscisic acid) and phenolics, etc., in seed coat or fruit pulp, e.g., in tomoto, inhibitor is present in fruit pulp. Seed coats may contain relatively high concentrations of growth inhibitors that can suppress germination of the embryo. Abscisic acid (ABA) is a common germination inhibitor present in the seed coats. Repeated washing and heavy rainfall removes such substances from the seed coat.
Giberellins helps in the reversal of dwarfism in many genetically dwarf plants. External supply of Gibberellic acid causes rapid elongation of growth, e.g. Rosette plant of sugarbeet when treated with GA3 undergoes marked longitudinal growth of axis.
Refer answer 30.
a-Naphthalene acetic acid (NAA) is a synthetic or exogenous auxin. It prevent the formation of abscission layer, which is a layer of dead cells in the petiole and pedicel that causes fall of leaf or fruit. NAA prevents formation of this layer and so it prevents fall of leaf or fruit.
Abscisic acid is a growth inhibiting phytohormone. It induces senescence in leaves by promoting the degradation of chlorophyll and proteins. Auxin is a growth promoting phytohomone that results in cell division, cell enlargement and apical dominance. Cytokinin is a growth promoting phytohormone that causes cell division, morphogenesis and seedling growth. Gibberellins are growth promoting phytohormone that results in overcoming seed dormancy and bolting.
Gibberellin is the hormone that breaks seed/ bud dormancy. The tubers of potato reproduce vegetatively to give rise to new plants. So the dormancy of these tubers can be overcome by applying gibberellins.
Apical dominance is the phenomenon by which presence of apical bud does not allow the nearby lateral buds to grow. When apical bud is removed, the lateral buds sprout.
Scarification means the application of those methods by which the hard seed coat is ruptured or softened so that it becomes permeable to water, gases and the embryo can expand. There are two methods of scarification as mechanical scarification and chemical scarification. This helps in overcoming seed dormancy.
Ethylene is largely a growth inhibiting phytohomone but is also involved in some growth promotion activities.
It has been established that ethylene is a fruit ripening hormone. Ethylene stimulates all the biochemical changes which take place at the time of fruit ripening.
Abscisic acid (ABA) is a growth inhibiting phytohormone. Abscisic acid is found in vascular plants, some mosses, some fungi and some green algae. If ABA is applied exogenously, seed germination is inhibited. It has been suggested that ABA inhibits the synthesis of some enzymes, for germination. These enzymes are synthesised under the direction of nucleic acids. A view has been expressed that the translation of a particular messenger RNA is inhibited by ABA and the result is that protein synthesis is blocked.
Nastic movements occur in response to a stimulus. It is independent of its direction. They are shown by bifacial organs (leaves, sepals, petals) and are in response to diffused external stimulus.
Seismonastic movements are due to shock or touch stimulus. Such movements are shown by Mimosa pudica. The leaf of Mimosa is compound having four pinna and each pinna is having many pinnules. If a terminal pinnule is touched, the whole leaf droops down. This is due to fact that at the bases “of pinnules, pinna and whole leaf, swollen structure called pulvinus is present and drooping is due to loss of turgidity of lower portion of pulvinus.
Cytokinins are growth promoting phyto- homones. Cytokinin plays an important part in organ formation (morphogenesis) with auxin. Different auxin/ cytokinin ratio decides the development of root shoot ratio. The major physiological function of cytokinins is to enhance cell division. If cytokinin to auxin ratio is low, then root formation takes place but if the ratio of cytokinin to auxin is high, then, there is formation of meristematic cells in the callus.
Photoperiodism is the response of plants to relative length of light and darkness. Phototaxis is plant movement where the stimulus is light. Phototropism is tropism in which stimulus is light. Vernalization is application of cold treatment to plants to effect flowering. It is a process in which a plant responds to the relative durative duration of daily light and dark periods. Low temperature, 0°C to 10°C or 18°C – 22°C (according to species) treatment of certain species for a specified period for the induction of ability to promote flowering is called vernalisation. It is a tropic movement, which is induced by light, when supplied unidirectionally. The plants, bending towards the source of light are called positively phototropic. Stems in general, are positively phototropic. Bending away from the source of light is called negative phototropism. Some roots show this response as in sunflower.
In most of the plants, there is a sharp rise in respiration rate near the end of the development of fruit, which sets in progress those changes, which are involved in ripening of fruit. The ripening on demand can be induced in these fruits by exposing them to normal air containing about 1 ppm of ethylene. Suitable combination of gases in atmosphere for fruit ripening is 80% ethylene (C2H4) and 20% CO2.
Phytochrome is a bright blue or bluish green pigment which was first of all isolated from plasma membrane of alga Mougeotias. It is a photoreceptive pigment. Phytochrome has a light absorbing or light detecting portion (the chromophore) attached to small protein of about 1,24,000 daltons. Phytochrome occurs in 2 forms, i.e.. P and P (i.e., red light and far red light absorbing forms) and these 2 forms are interconvertible.
Red light ^ p R ‘ Far red light FR
It is involved in the perception of photoperiodic stimuli controlling flowering and other morphogenetic phenomenon in plants.
2,4-D is a famous herbicide or weedicide which especially kills broad leaved weeds. It kills weeds perhaps by over stimulated root growth. Other auxins like 2, 4, 5 -T have also been used as defoliants during early sixties.
Refer answer 49.
Phytochrome is the photoreceptive pigment. It has a light absorbing or light detecting portion (the chromophore) attached to small protein of about 1,24,000 daltons. Phytochrome occurs in 2 forms, i.e., PR and PFR (i.e., red light and far red light absorbing forms) and these 2 forms are interconvertible.
When continous red light is given the level of PR decreases as most of it is converted to PPFR form. When the concentration of PR reaches below a critical value, it starts synthesis of more phytochromes in the PR form so that there is an equilibrium betweem synthesis and destruction of PR form.
Flower formation is a transitional phase in the life eycle of plant. During flowering, vegetative shoot apex is converted into reproductive shoot apex.
The physiological mechanism for flowering is controlled by 2 factors photoperiod or light period, i.e., photoperiodism and low temperature, i.e., vernalization.
Gibberellins are growth promoting phytohormones. Some of plants species flower only if the light period exceeds a critical length, and others flower only if this period is shorter than some critical length. Gibberellins can substitute for the long-day requirement in some species, showing an interaction with light.
Thigmotropism involves nastic movements resulting from touch. It occurs in tendrils which coil around support and help the plant in climbing. Phototropism is a paratonic movement in response to unidirectional light stimulus.
Chemotropism is a directional paratonic movement that occurs in response to a chemical stimulus.
Plants have the capacity of changing their position, in response to external or internal stimuli which are known as plant movements. The movements which occur due to internal stimuli are called autonomic movements and those that occur due to external stimuli are called paratonic movements.
Nepenthes (pitcher plant) is an insectivorous plant. In this the leaf lamina is modified to form a pitcher and leaf apex forms a coloured lid. When the insect enter the pitcher it is an external stimuli, so the closure of the lid is paratonic movement.
Auxin is a growth promoting phytohormone. It moves mainly from the apical to the basal end (basipetally). This type of unidirectional transport is termed polar transport. Auxin is the only plant growth hormone known to be transported polarly. Recently it has been recognized that a significant amount of auxin transport also occur acropetally (from basal end to the apical end) in the roots.
Decapitating a growing plant means removing shoot apex of the plant. Auxin, a growth promoting phytohormone present in apex inhibits the growth of axillary buds so that only the apex continues to grow. When the apex containing auxin is removed or decapitation is done, then axillary buds show their growth, this is because the apical dominance is removed. This practice of removal of apical dominance is applied in tea gardens, hedges, rose gardens etc.
Refer answer 58.
Richmond and Lang, 1967 observed that degradation of protein and chlorophyll was delayed in the detached leaves, if there was cytokinin in the medium. The senescence in the detached leaves was controlled by cytokinin first by keeping the stomata open thus allowing more CO, to enter. This suppresses the action of ethylene which promotes senescence.
Abscisic acid is a hormone which produced during adverse environmental condition. It also causes the closure of stomata under conditions of water stress as also under high concentration of CO, in the guard cells. ABA plays an important role in plants during water stress and drought conditions. The concentration of ABA increases in the leaves of plants facing such stresses, hence it is called as a stress hormone.
Klinostat is used to study growth movements. Klinostat comprises a disc with pot which are rotated by an axial rod with the help of a motor. Auxin get uniformly distributed on all sides and, hence the stem grows horizontally forward.
Abscisic acid is produced during water stress that brings stomatal closure. Abscisic acid is a stress hormone and it is produced during water scarcity, when the transpiration rate exceeds absorption, the plant faces water stress condition. As a result, incipient wilting occurs. Under water stress condition ABA increases which induces stomatal closure.
In several plants, particularly bienniales and perennials, light does not seem to be the only factor controlling the process of flowering. Temperature, particularly the low temperature treatment induce flowering. Vernalization means ability of low temperature to convert winter cereal into spring cereal as a result of satisfaction of their low temperature requirement.
In artificial ripening of banana, uncontrolled application of ethylene gas may cause over ripening of banana. It can be prevented from over-ripening by dipping in ascorbic acid solution.
According to Thimann and co-workers, auxin is responsible for the dominance of apical bud. The apical dominance is due to interaction between auxin and cytokinin. If the auxin concentration is higher than cytokinin, the apical bud will dominate the growth.
No flowering takes place if the dark period is less than the critical day length. The flowering is inhibited if weak intensity of light is given during the dark period. If the dark period is interrupted mid way by even a single flash of light, no flowering takes place. If this flash is given in the beginning or near the end of the dark phase, they produce flowers.
Richmond and Land, 1967 observed that degradation of protein and chlorophyll was delayed in the detached leaves, if there was cytokinin in the medium. The senescence in the detached leaves was controlled by cytokinin first by keeping the stomata open thus allowing more CO, to enter. This suppresses the action of ethylene which promotes senescence.
Ethylene is a hormone which is used for ripening of fruit. In case of unripe banana, it can be made to ripe before proper time if they are kept in ethylene atmosphere. Uncontrolled application of this gas many spoil the fruits. Ethylene is produced in mature but unripe fruits and then it initiates a chain of reactions that finally lead to ripening.
Absisic acid is a hormone which produced during adverse environmental condition. It also causes the closure of stomata under conditions of water stress as also under high concentration of CO, in the guard cells. Abscisic acid inhibits the K~ uptake by guard cells and promotes the leakage of malic acid. It results in reduction of osmotically active solutes so that the guard cells become flaccid and stomata get closed.
In vascular plants especially the taller ones, if the apical bud is intact and growing of the lateral bud remains suppressed, removal of apical bud causes fast growth of lateral buds. This influence of apical bud in suppressing the growth of lateral buds is termed as apical dominance.
Indote-3-acetic acid (IAA) is a natural auxin which is responsible for apical dominance.
GA is Giberrellic acid causes rapid elongation growth. ABA (abscisic acid) is a powerful growth inhibitor. Florigen is a chemical involved in flowering.
Chailakhyan in 1937 gave the view that flower hormone namely florigen is synthesized in the leaves under favourable photoperiodic conditions. This hormone is transmitted to the growing point where the flowering occurs.
Refer answer 55.
Cytokinin has a very specific effect on cell division (cytokinesis), hence the name cytokinin. They contain kinetin and related compound generally called as kinins chemically, cytokinins are degradation product of adenine, ATP, NAD and NADP. Cytokinins are essential for cytokinesis though chromosome doubling can occur in their absence. Cytokinins bring about division even in permanent cells.
Auxins are well known to promote elongation of stem and coleoptile. However when exogenous auxin is given to intact plants this is not observed because the required amount of auxin is already present in plants. When the apex of shoot is removed, then the exogenous application of auxin promotes growth, this clearly indicates that growing apex, having meristematic cells, is the site where endogenous auxins are present in sufficient amount, once the apex is removed the source of auxin is also removed.
Growth hormones or phytohormones are defined as organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes. Phytohormones are chemical substances which are synthesized by plants and are naturally occurring.
Before a leaf fall, a special zone of cells is formed at the base of the pedicel or petiole. This zone is called as abscission zone. It is delimited by a protection layer on the stem side and a separation layer on the organ side. The leaf is ultimately separated and phenomenon is called abscission. ABA is also a growth inhibitor. It regulates the dormancy of seeds and buds perhaps by inhibiting the growth process. The ABA level decreases in the whole seed as their dormancy is broken.
Auxin regulates some of the important plant growth movements like photoropism and geotropism. Phototropism means growth of plants in response to light and geotropism means growth of plants towards gravity.
Nyctinastic leaf movement is not related to auxin level. It is a movement of leaves of many species from nearly horizontal leaves during the day and nearly vertical at night. This movement are controlled by biological clock, while the other conditions bending of shoot towards light, movement of root towards soil and movement of sunflower head tracking the sun are the conditions related to auxins.
Gibberellin is a hormone that replaces vernalisation. Vernalization is a period of cold treatment for plants, usually perennials or trees. Some plants won’t bloom without it. In vernalization plants are exposed to low temperature in order to stimulate flowering or to enhance seed production. The biennials form their vegetative body in the first year. Then they pass through a winter season and then produce flowers and fruits in the second year. By exogenous application of gibberellins many biennials can be induced to behave as annuals and they no more require the natural chilling treatment for their flowering.
Cytokinins delay senescence of leaves and other organs by mobilisation of nutrients. Abscissic acid promotes abscission of flowers and fruits. Its excessive presence stops protein and RNA synthesis in the leaves and hence stimulates their senescence.
Mowing grass lawn facilities better maintenance because of removal of apical dominance and stimulation of intercalary meristem.
Stem elongation takes place in the absence of light due to etiolation. But uptake of minerals, uptake of water and ascent of sap all this process are related to photosynthesis which takes place only in the presence of light.
Cytokinin are plant growth hormones which are basic in nature cytokinins induce formation of new leaves chloroplasts in leaves, which intum keeps the leaves green for a longer duration of time. Cytokinins applied to marketed vegetables can keep them fresh for several days. Shelf life of cut shoots and flowers is prolonged by employing the hormones.
Gibberellins promote seed germination. Giberellins are weakly acidic growth hormones having ring structure and which cause cell elongation of intact plants in general and increased intemodal length of genetically dwarfed plants. Giberellins are synthesized in the apical shoot buds, root tips and developing seeds.
During seed germination, especially of cereals giberrellin stimulates the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases, proteases. The enzymes solubilize the reserve food of the seed.
Phytochrome is a photoreceptor molecule which mediates several developmental and morphogenetic responses of plants to light. This is called photoperiodism. Borthwick, Hendicks and Parker in 1952, discovered phytochrome, which is a pigment received light existed in two inter convertible forms active form and inactive form.
Refer answer 45.