Question
The site of perception of light in plants during photoperiodism is [NEET 2021]
(a) shoot apex
(b)stem
(c)axillary bud
(d) leaf
Answer/Explanation
Ans. (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.
Question
What is the site of perception of photoperiod necessary for induction of flowering in plants? [NEET (National) 2019]
(a) Pulvinus
(b) Shoot apex
(c) Leaves
(d) Lateral buds
Answer/Explanation
Ans. (c)
For the induction of flowering in plants, photoperiod stimulus is percieved 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.
Question
Phytochrome is a [NEET 2016, Phase II]
(a) flavoprotein
(b) glycoprotein
(c) lipoprotein
(d) chromoprotein
Answer/Explanation
Ans. (d)
Phytochrome is a chromoprotein which exist in two forms, $P_r$ and $P_{f r^{-}}$. These are inter-convertible. When plants get red right this protein gets converted into $P_{\text {tr }}$ form $P_{\text {t }}$ and vice versa. It controls the photoperiodism in the plants.
Question
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? [CBSE AIPMT 2014]
(a) Mutated
(b) Embolised
(c) Etiolated
(d) Defoliated
Answer/Explanation
Ans. (c)
Etiolation is a process in which flowering plants are grown in partial or complete absence of light. Etiolation is mainly characterised by long and weak stem and smaller, sparse pale yellow. calour of leaves due to the longer internodes. Thus due to this tomato seeding became white coloured.
Question
Importance of day length in flowering of plants was first shown in [CBSE AIPMT 2008]
(a) Lemna
(b) tobacco
(c) cotton
(d) Petunia
Answer/Explanation
Ans. (b)
Photoperiodism was first discovered by Garner and Allard (1920, 1922). They observed that maryland mammoth variety of tobacco could be made to flower only by reducing the light hours with artificial darkning. On the basis of photoperiodic response to flowering plants have been divided into short day plants (tobacco), long day plant (e.g. wheat, hanbane), short long day plants. (e.g. Campanula), long short day plants (e.g. Bryophyilum) intermediate plants(e.g. wild kidney bean)and day neutral plants (e.g. cotton).
Question
The wavelength of light absorbed by $P_r$ form of phytochrome is [CBSE AIPMT 2007]
(a) $640 \mathrm{~nm}$
(b) $680 \mathrm{~nm}$
(c) $720 \mathrm{~nm}$
(d) $620 \mathrm{~nm}$
Answer/Explanation
Ans. (b)
When $P$, absorbs red light $(650-670 \mathrm{~nm})$ it is converted into $P_t$ form and when $P_{\mathrm{ft}}$ absorbs far red light $(730-735 \mathrm{~nm})$ it is converted back into $P$, form.
Question
One set of a plant was grown at 12 $\mathrm{hr}$ day and $12 \mathrm{hr}$ 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? [CBSE AIPMT 2004]
(a) Long-day
(b) Darkness neutral
(c) Day neutral
(d) Short-day
Answer/Explanation
Ans. (d)
The condition shows that the plant require photo-period shorter than the critical day length. This plant needs uninterrupted dark period for flowering. Therefore, it is a short-day plant and these do not flower if the dark period is interrupted with flashes of light.
Question
Which one is a long-day plant? [CBSE AIPMT 2001]
(a) Tobacco
(b) Glycine max
(c) Mirabilis jalapa
(d) Spinach
Answer/Explanation
Ans. (d)
Plants which require long-day photoperiod for flowering and a small dark period for vegetation are known as long-day plants, e.g. spinach.
Question
Proteinaceous pigment which control activities concerned with light [CBSE AIPMT 2001]
(a) phytochrome
(b) chloraphyll
(c)anthocyanin
(d) carotenoids
Answer/Explanation
Ans. (a)
Phytochromes are the plant chromoproteins, containing protein pigment existing in two inter-convertible forms-P (absorbs red light- $660 \mathrm{~nm}$ ) and $P_4$ (absorbs far red light-730 nm). It controls flowering, seed dormancy, etc.
Question
The method that renders the seed coat permeable to water so that embryo expansion is not physically retarded, is [CBSE AIPMT 2000]
(a) vernalisation
(b) stratification
(c)denudation
(d) scarification
Answer/Explanation
Ans. (d)
In many plants, the seed coats are quite tough and provide mechanical resistance to the growth of the embryos. Scarification done by abrasion through machine, threshing, filing, etc this process is done to rupture or weaken the seed coat and promote germination.
Question
The response of different organisms to environmental rhythms of light and darkness is called [CBSE AIPMT 1998]
(a) phototaxis
(b) photoperiodism
(c) phototropism
(d) vernalisation
Answer/Explanation
Ans. (b)
Photoperiodism is the term to denote a biological response to changes in the ratio of light and darkness in a 24 hrs cycle.
Question
The pigment, that absorbs red and far-red light in plants, is [CBSE AIPMT 1995, 2002]
(a) xanthophyll
(b) cytochrome
(c) phytochrome
(d) carotene
Answer/Explanation
Ans. (c)
Phytochrome is a type of pigment which absorbs red or far-red light and its absorbing region is closely associated with protein. The phytochrome pigment is found to be present in two photoreversible forms $P_r\left(P_{600}\right)$ and $P_{14}\left(P_{730}\right)$.
Question
What will be the effect on phytochrome in a plant subjected to continuous red light? [CBSE AIPMT 1997]
(a) Level of phytochrome decreases
(b) Phytochrome is destroyed
(c) Phytochrome synthesis increases
(d) Destruction and synthesis of phytochrome remain in equilibrium
Answer/Explanation
Ans. (b)
Continuous exposure to red light causes (a) conversion of $P_t P_t$ which is rapidly destroyed, (b) inhibition of synthesis of $P_r$. Thus, total amount of phytochrome is decreased.
Question
If a tree, flowers thrice in a year (Oct., Jan. and July) in Northern India, it is said to be [CBSE AIPMT 1997]
(a) photosensitive but thermoinsensitive
(b) thermosensitive but photoinsensitive
(c) hoto and thermosensitive
(d) photo and thermoinsensitive
Answer/Explanation
Ans. (d)
Since, flowering can take place during any part of the year, therefore, the plant is not sensitive to photoperiod and temperature.
Question
In short-day plants, flowering is induced by [CBSE AIPMT 1992]
(a) photoperiod less than $12 \mathrm{hrs}$
(b) photoperiod below a critical length and uninterrupted long night
(c) long night
(d) short photoperiod and interrupted long night
Answer/Explanation
Ans. (b)
In short-day plants, flowering is induced when the day length do not exceed a certain critical value, the day length required is less than a certain critical length. Short-day plants may be more correctly called long night plants as a certain minimum of uninterrupted dark period in $24 \mathrm{hrs}$ is necessary for their flowering. Short-day plants will not flower if the dark period is less than a critical length.
Question
Flowering dependent on cold treatment is [CBSE AIPMT 1992] (
a) cryotherapy
(b) cryogenics
(c) cryoscopy
(d) vernalisation
Answer/Explanation
Ans. (d)
Vernalisation is a process of shortening of juvenile or vegetative phase and hastening flowering by a previous cold treatment.
Question
Which one increases in the absence of light? [CBSE AIPMT 1989]
(a) Uptake of minerals
(b) Uptake of water
(c) Elongation of internodes
(d) Ascent of sap
Answer/Explanation
Ans. (c)
In general intense light retards growth in plants. High light intensities induce dwarfening of the plant. Absence of light reduces the overall growth, photosynthesis, uptake of minerals and ascent of sap. However, the elongation of internodes is seen to occur in the absence of light.
Question
Phytochrome is involved in [CBSE AIPMT 1988]
(a) phototropism
(b) photorespiration
(c) photoperiodism
(d) geotropism
Answer/Explanation
Ans. (c)
Phytochrome is a chromoprotein (photosensitive pigment) that exists in two states, $P_r$ (red) or $P_{800}$ and $P_{\text {fr }}$ (far red) or $\mathrm{P}_{730}$. Phytochrome is involved in photomorphogenetic responses, seed germination, bud dormancy, synthesis of gibberellin and ethylene and photoperiodism.