Past Entrance Papers

Hint

(c) \(2,4 \mathrm{D}\) hormone is used as a herbicide to destroy weeds.
Auxins like IAA and IBA are used to induce parthenocarpy. IAA also stimulates nodule formation. Auxin like NAA is used to increase dwarf shoots.

Hint

(c) Plant plasticity refers to the ability to modify itself by forming a different kind of structures to adapt and cope with changes in its environment. It can be intrinsic plasticity or extrinsic plasticity. In both the cases plants shows heterophylly along with other morphological features, e.g. in the leaves Larkspur and buttercup.

Hint

(d) The response of plants to periods of day/night is termed as photoperiodism. The site of perception of photoperiod is leaf. The hormone florigen is responsible for inducing flowering as it migrates from leaves to shoot apices on induction of required photoperiods.

Hint

(c) Presence of inhibitory substances in dormant seeds can be removed by subjecting seeds to
(i) growth hormones like gibberellic acid, cytokinins,
(ii) by stratification in which seeds requiring low temperature are first allowed to imbibe water and then exposed to low temperature.
(iii) low concentration application of nitrates because it promotes seed germination.
Ascorbic acid is known to cause seed dormancy as it is a potent chemical inhibitor.

Hint

(c)  The function of different phytohormones include ethephon is a commercial derivative of ethylene and is used to ripen fruits. \(2,4-D\) is a synthetic auxin and it is used as herbicide against dicotyledonous weeds. Gibberellic acid induces bolting
in plants, i.e. the promotion of internodal elongation just prior to their reproductive phase.

Abscisic acid is a stress hormone as it the plants to overcome unfavourable conditions by inhibiting growth.

Hint

(b) Eichhornia crassipes, (water hyacinth) was introduced in several tropical
countries including India for ornamentation but later this exotic species became a trouble-some aquatic weed.

Hint

(d) Gibberellic acid breaks seed dormancy. It activates the synthesis of alpha-amylase which breakdown starch into simple sugar.

The Phytohormones gibberellic acid (GA) has essential signaling functions in multiple processes during plant development. In rice, GA signaling plays an important role in spikelet fertility.

Hint

(a) The correct option is (a) because spraying on sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.

Hint

(c) Removal of shoot tips is a very useful technique to boost the production of tea leaves. This is because effect of auxin is removed and the growth of lateral bud is enhanced. This phenomenon in most higher plants in which growing apical buds (shoot tips) inhibit the growth of lateral buds due to the effects of auxin is called apical dominance.

Hint

(c) In order to increase the yield of the sugarcane crop, gibberellins should be sprayed.Sugarcane stores carbohydrates as sugar in its stems. Spraying sugarcane crop with gibberellins increase the length of the stem, thus increasing the yield by as
much as 20 tonnes per acre.

Hint

(d) Auxin and ethylene can be applied to artificially induce flowering in pineapple plants throughout the year to increase yield.

Hint

(c) For the induction of flowering in plants, photoperiod stimulus is perceived by the leaves of plants. As a result, floral hormones are produced in the leaves which are then translocated to the apical part and subsequently cause the initiation of floral primordial growth.

Hint

(b) In low concentrations, auxins such as 2,4 \(\mathrm{D}(2,4\) – Dichlorophenoxy acetic acid) is useful in preventing, pre-harvest fruit drop and leaf drop.

Hint

(b) Cytokinin and auxin are two plant hormones that are supplied to the tissue culture medium in definite proportions. They bring about cell division and differentiation of callus. A low auxin to cytokinin ratio promotes shoot formation whereas a high auxin to cytokinin ratio promotes rooting of callus.

Hint

(d) Phytochrome is a chromoprotein, plant pigment that can detect the presence or absence of light and is involved in regulating many processes that are linked to day length (photoperiod), such as seed germination and initiation of flowering. It consists of a light detecting portion, called a chromophore, linked to a small protein and exists in two inter-convertible forms with different physical properties.

Hint

(a) Avena curvature test is a test based on the experiments of Went (1928) which can measure auxin up to \(300 \mathrm{mg}\) /litre. Indole 3-Acetic Acid (IAA) is a universal natural auxin and the Avena curvature test may serve as an accurate bioassay for this plant hormone.

Hint

(c) Auxin is a phytohormone that is often bioassayed by Avena coleoptile curvature test. The angle of curvature of a decapitated oat coleoptile is measured after placing an agar block containing auxin on one side. The ability of auxin to stimulate shoot growth is then measured.

Hint

(b) 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.

Hint

(c) Geometric growth cannot be sustained for long in natural conditions. 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.

An idealised sigmoid growth curve is drawn below:

Hint

(b) It is the basis for the quantitative determination of small amounts of growth-promoting substances.
Dr F. Went isolated Auxin from Avena coleoptile tip. His experiment demonstrated the polar movement of auxins. Tryptophan is the precursor of Indole acetic acid (IAA, natural auxin). Important biassay of auxins are the Avena curvature test and cross root inhibition test.

Hint

(c) Etiolation is the abnormal form of growth observed when plants grow in darkness or severely reduced light. Such plants characteristically have branched leaves and shoots, excessively long shoots, and reduced leaves and root systems.

Hint

(a) 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.

Hint

(b) 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.

Hint

(a) NAA ( \(\alpha\)-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, the application of auxin promotes flowering.

Hint

(b) Light and temperature may affect flowering in plants in various ways. The effect of photoperiods or the daily duration of light hours (and dark periods) on flowering is called photoperiodism. For example, in short-day plants flowering occurs when the day length is below the critical period, e.g., dahlia, rice, etc. In long-day plants, flowering occurs when day length is above the 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.

Hint

(c) Abscisic acid or ABA is an antagonist to gibberellins. 

\(
\begin{array}{|l|l|}
\hline \text { Abscisic acid } & \text { Gibberellic acid } \\
\hline \text { It inhibits growth. } & \text { It promotes growth. } \\
\hline \begin{array}{l}
\text { It promotes the } \\
\text { dormancy of seeds, } \\
\text { of buds and tubers. }
\end{array} & \begin{array}{l}
\text { It overcomes the } \\
\text { natural dormancy } \\
\text { seeds, tubers etc. }
\end{array} \\
\hline \begin{array}{l}
\text { It inhibits the synthesis } \\
\text { of RNA and } \\
\text { proteins. }
\end{array} & \begin{array}{l}
\text { It promotes the } \\
\text { of RNA and } \\
\text { synthesis proteins. }
\end{array} \\
\hline \begin{array}{l}
\text { Causes abscission of } \\
\text { flowers and fruits. }
\end{array} & \begin{array}{l}
\text { Promotes } \\
\text { development of } \\
\text { fruits. }
\end{array} \\
\hline \begin{array}{l}
\text { Promotes leaf } \\
\text { senescence. }
\end{array} & \begin{array}{l}
\text { Prevents leaf } \\
\text { senescence. }
\end{array} \\
\hline \begin{array}{l}
\text { Promotes stomatal } \\
\text { closure. }
\end{array} & \begin{array}{l}
\text { Promotes stomatal } \\
\text { opening. }
\end{array} \\
\hline \begin{array}{l}
\text { Prevents amylase } \\
\text { activity }
\end{array} & \begin{array}{l}
\text { Promotes amylase } \\
\text { activity during seed } \\
\text { germination. }
\end{array} \\
\hline
\end{array}
\)

Hint

(c) 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 carrots.

Hint

(d) Phototropic curvature is the result of the uneven distribution of auxin. Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of the coleoptile contain auxin that caused the bending of the entire coleoptile in relation to the direction of light.

Hint

(a) Photoperiodism is the response to the duration and timings of the light and dark period. It was first studied by W.W. Garner 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. 

Hint

(c) 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.

Hint

(d) 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 plantations and industries, the apical bud is plucked for tea processing which results in more lateral buds thus enhancing plantation and further industrial purposes.

Hint

(d) Ethylene promotes root growth and root hair formation. In low concentration, ethylene is used for initiation of roots and also of lateral roots.

Hint

(d) NAA (Naphthalene Acetic Acid) and 2,4-D (2.4-dichlorophenoxy acetic acid) are synthetic auxins. The term auxin is applied to the indole3-acetic acid (IAA) and to other natural and synthetic compounds having certain growth-regulating properties. IAA and IBA (Indole Butyric Acid) have been isolated from plants. All these auxins have been used extensively in agricultural and horticultural practices.

Hint

(c) Biosynthesis of abscisic acid (ABA) in most plants occurs indirectly by degradation of certain carotenoids present in chloroplasts or other plastids. The biosynthetic pathway follows the mevalonic acid pathway for their synthesis. The sites of synthesis are fruits, tissues, leaves, roots, and seeds.

Hint

(d) The effect of photoperiods or the 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 Garner and Allard (1920). They observed that the ‘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.

Hint

(c) Senescence is the process of aging 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 cells in the distal region leads to lignification of the 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 indicate senescence.

Hint

(a) Phytochrome is the photoreceptor pigment that controls flowering. It has two forms as \(P_{r}\) and \(P_{f r} . P_{r}\) is bluish phytochrome and it absorbs light at 660 to \(680 \mathrm{~nm}\) of wavelength. \(P_{f r}\) is (far red) yellowish green and absorbs light at \(730 \mathrm{~nm}\) of wavelength.

Hint

(a) Gibberellic acid is a simple weakly acidic plant growth hormone which promotes cell elongation of both leaves and stems in general and internodal 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.

Hint

(c) Movement may be of two types, movement of locomotion and movement of curvature. The movement of curvature is the movement of individual parts in relation to other parts. Curvature movements may be growth movements and turgor movements. Growth movements are caused by differential growth in different parts 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 the presence or absence of light, e.g., Oxalis.

Hint

(a) Gibberellins(GA) were first observed from the fungus Gibberella fujikuroi, the causal organism of foolish seedling disease of rice plants in Japan by Kurasawa in 1926.

Hint

(b) The process by which the dormant embryo of seed resumes active growth and forms a seedling is known as germination.

The initial step in the 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 \(\alpha\) – and \(\beta\)-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.

Hint

(c) Nitrogen is the fourth most abundant element. Chief source of nitrogen for plants is nitrates of \(\mathrm{Ca}\) and \(\mathrm{K}\). 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, and cambial activity. So a nitrogenous fertilizer like NPK and cytokinins are most beneficial to the plant.

Hint

(c) Prunning is the process of cutting shoot tips to promote lateral growth of branches. Removal of shoot tips involves the 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 a bushy appearance. This is because of a relatively high concentration of auxin in the apical bud than in the lateral buds.

Hint

(c) Vernalization is the method of promoting flowering by exposing young plants to cold treatment e.g., winter varieties of wheat, barley, oats, and rye are given artificial cold treatment and planted in spring in areas of very harsh winters such as the Soviet Union to promote flowering in them.

In most cereals optimum temperature for vernalization is \(4^{\circ} \mathrm{C}\). The 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 seed coat so that embryo can begin metabolism.

Hint

(d) 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 outside of each leaf to expand and the trap to snap shut.

Hint

(c) Gibberellins play a role in the elongation of internodes in ‘rosette’ plants. Before the reproductive stage, there is too much elongation of internodes but there is less leaf formation. An elongated internode 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.

Hint

(d) Plants require a day length or light period for flowering, this light period is called as photoperiod. It was first studied by Garner 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.

Hint

(d) 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.

Hint

(a) According to the 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.

Hint

(a) Zinc is available to the plants for absorption in the divalent form. It occurs in the form of minerals such 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.

Hint

(b) A high cytokinin to auxin ratio triggers shoot formation but inhibits root induction, whereas, a low ratio produces the opposite effect. Plant growth regulators (cytokinin, auxin) are widely used to control differentiation in cultured plant cells and tissues.

Hint

(b) Ethylene breaks the dormancy of different plants. Abscisic acid induces dormancy in seeds, buds, and underground storage organs.

Hint

(b) Gibberellins help in the reversal of dwarfism in many genetically dwarf plants. The external supply of Gibberellic acid causes rapid elongation of growth. E. \(g\)., rosette plant of sugarbeet when treated with \(\mathrm{GA}_{3}\) undergoes marked longitudinal growth of axis.

Hint

(c) Phytochrome has a light-absorbing or light detecting portion (the chromophore) attached to small protein of about 1,24000 daltons. Phytochrome occurs in 2 forms, i.e., \(P_{R}\) and \(P_{F R}\) (i.e., red light and far-red light-absorbing forms) and these 2 forms are interconvertible. \(j\) Cytochromes are electron transferring proteins. They contain iron porphyrin or copper porphyrin as prosthetic groups. Chlorophyll is the fundamental green pigment of photosynthesis. It is localized on the chloroplasts. Carotenoids are lipid compounds and they are yellow, orange, purple, etc. in colour. These are found in higher plants red algae, green algae, fungi, and photosynthetic bacteria.

Hint

(d) All plants need sunlight for their growth or photosynthesis or photoperiodism. Plants show specific biological responses and reactions when they are exposed to a specific period of sunlight and darkness. This phenomenon is called as photoperiodism. Depending on the amount of light or darkness required for survival and development, plants are divided into three main categories: Short-day plants, long-day plants, and neutral-day plants. Long-day plants flower and grow when the daylight they obtain is more than their critical (required) photoperiod, like in the summer season. They need more light than short-day plants and usually flower when the temperature becomes warmer. Examples of long-day plants are spinach, lettuce, radish, sugar beet, etc. Thus, the correct answer is ‘Spinacia oleracea (Spinach).’

Hint

(b) \(\alpha\)-Naphthalene acetic acid (NAA) is a synthetic or exogenous auxin. It prevents the formation of the abscission layer, which is a layer of dead cells in the petiole and pedicel that causes the fall of leaf or fruit. NAA prevents the formation of this layer and so it prevents the fall of leaf or fruit.

Hint

(a) 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.

Hint

(a) 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.

Hint

(a) Phytochromes are the plant chromoproteins, containing protein pigment existing in two inter-convertible forms- \(P_{r}\) (absorbs red
light- \(660 \mathrm{~nm}\) ) and \(\mathrm{f}_{\mathrm{tr}}\) (absorbs far red light- \(730 \mathrm{~nm}\) ). It controls flowering,
seed dormancy, etc.

Hint

(a) Apical dominance is the phenomenon by which the presence of an apical bud does not allow the nearby lateral buds to grow. When the apical bud is removed, the lateral buds sprout.

Hint

(a) 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.

Hint

(a) Ethylene is largely a growth-inhibiting phytohormone 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.

Hint

(d) Apical dominance is the phenomenon in which the presence of apical bud does not allow the nearby lateral buds to grow. When the apical bud is removed the lateral buds sprout.

Hint

(b) Skoog(1954-1956) observed that coconut milk contained a substance which stimulated cell division. The substance was later on called cytokinin. The most widely occurring cytokinin in plants is isopentanyladenine (IPA). IPA has been isolated from Pseudomonas tumefaciens.

Hint

(b) Abscisic Acid (ABA) is called stress hormone or dormin. It is a growth retarding hormone that induces dormancy and promotes aging and abscission of fruits, leaves, and flowers. It also causes closure of stomata and overcomes the conditions of stress.

Hint

(c) Ethylene is a plant hormone which has the most remarkable effect of stimulation of fruit ripening.

Hint

(c) Abscisic acid (ABA) is a growth-inhibiting phytohormone. Abscisic acid is found in vascular plants, some mosses, some fungi, and some green algae.

If \(\mathrm{ABA}\) is applied exogenously, seed germination is inhibited. It has been suggested that \(\mathrm{ABA}\) 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.

Hint

(a) 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. Fig.: Seismonastic movement 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 the 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 the lower portion of the pulvinus.

Hint

(a) Cytokinins are growth-promoting phytohomones. Cytokinin plays an important part in organ formation (morphogenesis) with auxin. Different auxin/cytokinin ratio decides the development of the root shoot ratio. The major physiological function of cytokinins is to enhance cell division. If the cytokinin to auxin ratio is low, then root formation takes place but if the ratio of cytokinin to auxin is high, then, there is the formation of meristematic cells in the callus.

Hint

(b) Photoperiodism is the response of plants to the relative length of light and darkness. Phototaxis is plant movement where the stimulus is light. Phototropism is tropism in which the stimulus is light. Vernalization is the application of cold treatment to plants to effect flowering.

Hint

(c) 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 the 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 \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)\) and \(20 \% \mathrm{CO}_{2}\).

Hint

(a) Important plant responses regulated by the phytochrome system include photoperiodic induction of flowering, chloroplast development, leaf senescence, and leaf abscission. Phytochrome occurs in two forms: Pr (red light) and Pfr (far-red light) and both are interconvertible. It is considered that during daytime Pfr form accumulates in plants which is inhibitory to flowering in SDP. During night Pr form accumulates in plants which is stimulatory to flowering.

Hint

(b) 2,4 -D is a famous herbicide or weedicide which especially kills broad-leaved weeds. It kills weeds perhaps by overstimulated root growth. Other auxins like \(2,4,5\)-T have also been used as defoliants during the early sixties.

Hint

(c) Phytochrome is a light-sensitive pigment in plants and some bacteria and fungi. It exists in two interconvertible forms- Pr and Pfr. Pfr absorbs far-red light of \(730 \mathrm{~nm}\) wavelength. When it absorbs far-red light, it converts to its original form, Pr. Thus Pfr is the active form of phytochrome which inhibits flowering.

Hint

(d) When continuous red light is given the level of \(P_{R}\) decreases as most of it is converted to \(P_{F R}\) form. When the concentration of \(P_{R}\) reaches below a critical value, it starts the synthesis of more phytochromes in the \(P_{R}\) form so that there is an equilibrium between synthesis and destruction of \(P_{R}\) form.

Hint

(a) Flower formation is a transitional phase in the life cycle of plant. During flowering, the vegetative shoot apex is converted into the 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.

Hint

(d)Gibberellins are growth-promoting phytohormones. Some 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 interaction with light.

Hint

(a) Some light-sensitive seeds like Lectuca sativa (Lettuce) germinate in complete darkness with the help of Gibberellins which otherwise require specific light conditions.

Hint

(d) 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 a unidirectional light stimulus.

Chemotropism is a directional paratonic movement that occurs in response to a chemical stimulus.

Hint

(a) 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 the leaf apex forms a coloured lid. When the insect enters the pitcher it is an external stimuli, so the closure of the lid is paratonic movement.

Hint

(c) Thigmotropism movements are due to the contact with a foreign body. In twiners and lianas, there is less growth on the idea of contact and more growth on the side of branch away from the contact. Coiling of garden pea tendrils around any support is an example of thigmotropism.

Hint

(c) Climacteric fruits are fleshy fruits which show a sudden sharp rise of respiration rate at the time of ripening. Ethylene is used to induce artificial ripening of these fruits, e.g. apple, mango, and banana.

Hint

(a) The phenomenon of thigmotropism is a response to contact with a solid object. It is exhibited by tendrils of climbing plants that grow around a pole or the stem of another plant.

Hint

(b) 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 occurs acropetally (from the basal end to the apical end) in the root.

Hint

(d) Decapitating a growing plant means removing the shoot apex of the plant. Auxin, a growth-promoting phytohormone present in the 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.

Hint

(b) Abscisic acid promotes reversal of \(\mathrm{H}^{+} \rightleftharpoons \mathrm{K}^{+}\)pump and increasing availability of \(\mathrm{H}^{+}\)inside the guard cell cytoplasm. Loss of \(\mathrm{K}^{+}\)decreases
the osmotic concentration of guard cells as compared to adjacent epidermal cells. Due to the exosmosis, the turgidity of guard cells decreases and it closes the pore of the stoma.

Hint

(a) Three types of growth are present in multicellular animals :
(i) Auxetic growth: Increase in cell volume only, not in number.
(ii) Multiplication growth: Increase in number, not in volume.
(iii) Accretionary growth: Growth is due to mitotic division of some special type of cells occurring in specific locations of the body.

Hint

(d) 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 a unidirectional light stimulus. Chemotropism is a directional paratonic movement that occurs in response to a chemical stimulus.

Hint

(a) 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 \(\mathrm{CO}_{2}\) to enter. This suppresses the action of ethylene which promotes senescence.

Hint

(d) Abscisic acid is a hormone which is produced during the adverse environmental conditions. It also causes the closure of stomata under conditions of water stress as also under high concentration of \(\mathrm{CO}_{2}\) 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 a stress hormone.

Hint

(b) 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 gets uniformly distributed on all sides and, hence the stem grows horizontally forward.

Hint

(b) 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 conditions. As a result, incipient wilting occurs. Under water stress conditions ABA increases which induces stomatal closure.

Hint

(d) 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 induces flowering. Vernalization means the ability of low temperature to convert winter cereal into spring cereal as a result of satisfaction of their low-temperature requirement.

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(c) 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.

Hint

(d) According to Thimann and co-workers, auxin is responsible for the dominance of apical bud. The apical dominance is due to the interaction between auxin and cytokinin. If the auxin concentration is higher than cytokinin, the apical bud will dominate the growth.

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(b) No flowering takes place if the dark period is less than the critical day length. The flowering is inhibited if the 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.

Hint

(c) 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 \(\mathrm{CO}_{2}\), to enter. This suppresses the action of ethylene which promotes senescence.

Hint

(c) Ethylene is a hormone that is used for the ripening of fruit. In the case of unripe bananas, it can be made to ripe before the proper time if they are kept in an ethylene atmosphere. Uncontrolled application of this gas may spoil the fruits. Ethylene is produced in mature but unripe fruits and then it initiates a chain of reactions that finally lead to ripening.

Hint

(a) Abscisic acid is a hormone produced during adverse environmental conditions. It also causes the closure of stomata under conditions of water stress as also under high concentration of \(\mathrm{CO}_{2}\) in the guard cells. Abscisic acid inhibits the \(\mathrm{K}^{+}\)uptake by guard cells and promotes the leakage of malic acid. It results in the reduction of osmotically active solutes so that the guard cells become flaccid and stomata get closed.

Hint

(a) In vascular plants especially the taller ones, if the apical bud is intact and growing of the lateral bud remains suppressed, removal of the 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.

Indole-3-acetic acid (IAA) is a natural auxin which is responsible for apical dominance.

GA is Giberrellic acid that causes rapid elongation growth. ABA (abscisic acid) is a powerful growth inhibitor. Florigen is a chemical involved in flowering.

Hint

(c) 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.

Hint

(a) 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 a unidirectional light stimulus. Chemotropism is a directional paratonic movement that occurs in response to a chemical stimulus.

Hint

(c) 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 products 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.

Hint

(c) In the absence of light stem becomes excessive long due to ‘etiolation’. This causes cells of internodes to get enlarged.

Hint

(a) 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 the shoot is removed, then the exogenous application of auxin promotes growth, this clearly indicates that the growing apex, having meristematic cells, is the site where endogenous auxins are present in sufficient amounts, once the apex is removed the source of auxin is also removed.

Hint

(d) 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 that are synthesized by plants and are naturally occurring.

Hint

(b) 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 the 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 its dormancy is broken.

Hint

(c) Auxin regulates some of the important plant growth movements like phototropism and geotropism. Phototropism means the growth of plants in response to light and geotropism means the growth of plants towards gravity.

Hint

(c) 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 is controlled by a biological clock, while the bending of shoot towards the light, movement of root towards the soil, and movement of sunflower head tracking the sun are the conditions related to auxins.

Hint

(c) 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.

Hint

(d) Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients. Abscisic acid promotes the abscission of flowers and fruits. Its excessive presence stops protein and RNA synthesis in the leaves and hence stimulates their senescence.

Hint

(b) Mowing grass lawn facilitates better maintenance because of removal of apical dominance and stimulation of intercalary meristem.

Hint

(c) Stem elongation takes place in the absence of light due to etiolation. But uptake of minerals, uptake of water, and the ascent of sap all this process are related to photosynthesis which takes place only in the presence of light.

Hint

(c) Leaves of monocots are characterised as isobilateral (equally green on both the surfaces), amphistomatic (stomata on both surface), dumbbell-shaped guard cells.
The upper epidermis possesses groups of larger-sized thin-walled vacuolate cells called bulliform or motor cells. Bulliform cells help in the rolling of leaves during water stress or drought.

Hint

(b) Cytokinin are plant growth hormones which are basic in nature. Cytokinins induce the formation of new leaves chloroplasts in leaves, which in turn keeps the leaves green for a longer duration of time. Cytokinins applied to marketed vegetables can keep them fresh for several days. The shelf life of cut shoots and flowers is prolonged by employing the hormones.

Hint

(a) Gibberellins promote seed germination. Gibberellins are weakly acidic growth hormones having ring structure and which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants. Gibberellins are synthesized in the apical shoot buds, root tips, and developing seeds.

During seed germination, especially of cereals, gibberellin stimulates the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases, and proteases. The enzymes solubilize the reserve food of the seed.

Hint

(c) Phytochrome is a photoreceptor molecule that 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 that received light and existed in two interconvertible forms: active form and inactive form.

Hint

(b) In seismonastic movement, the response is made to mechanical shocks such as blows, shaking, or pressure. In Mimosa, turgor changes occur in thin-walled cells of pulvinus(lower side) and pulvinnules (upper side), causing the folding of pinnules, and drooping of compound leaves.