Senescence and Abscission
Short Questions and Answers
One mark questions with answers
1. In temperate regions trees change colour of leaves during autumn. It is due to
(a) synthesis of carotenoids
(b) synthesis of xanthophylls
(c) degradation of chlorophyll pigments
(d) All the above
Answer: (c) degradation of chlorophyll pigments
2. During abscission of leaves the separation layer is formed on the
(a) proximal side
(b) distal side
(c) at the leaf apex
(d) none of the above
Answer: (b) distal side
3. During abscission and senescence nutrients
(a) move into mature leaves
(b) move out of mature leaves
(c) relocated in the young leaves
(d) relocated in the woody part
Answer: (d) relocated in the woody part
4. During abscission
(a) auxin concentration is more in the distal part of the leaf
(b) auxin concentration is less in the distal part of the leaf
(c) there is no auxin gradient
(d) auxin has no role in abscission
Answer: (b) auxin concentration is less in the distal part of the leaf
5. Abscission layer is formed when the concentration of
(a) auxin increases
(b) auxin decreases
(c) gibberellins decreases
(d) gibberellins increases
Answer: (b) auxin decreases
6. Leaf fall occurs when content of
(a) abscisic acid decreases
(b) auxin decreases
(c) auxin increases
(d) gibberellins increases
Answer: (b) auxin decreases
7. Which one demonstrates process associated with abscission of a leaf?
(a) in the leaf concentration of both auxin and ABA decreases
(b) in the leaf concentration of both auxin and ABA increases
(c) reduction in ABA concentration and increase of auxin concentration in the leaf
(d) reduction in concentration of auxin and increase of concentration of ABA in the leaf
Answer: (d) reduction in concentration of auxin and increase of concentration of ABA in the leaf
8. The senescence inducing genes are called
(a) senescence associated genes
(b) senescence inducing genes
(d). Both a and c
Answer: (d). Both a and c
Very short answer questions
1. Name the two layers of the abscission zone
Answer: protective layer and separation layer
2. Which genes are associated with senescence?
Answer: Senescence associated genes
3. Name one compound which is used to keep the leaves green and fresh for longer duration
4. Name one compound which stimulates senescence
Answer: Abscissic acid
5. Type of senescence where entire plant dies after a single reproductive cycle is known as
Answer: Monocarpic senescence
Two marks questions with answers
1. What is senescence?
Answer: Senescence can be defined as the collective, progressive and degenerative processes that ultimately lead to termination of functional activities of an organ or organism. It may also be defined as a period between reproductive maturity and death of plants or plant parts.
2. What are senescence-associated genes?
Answer: Senescence shows the presence of several hydrolytic enzymes such as proteases, nucleases, lipases, and chlorophyll degrading enzymes. The synthesis of these enzymes is controlled by certain specific genes. Such senescence inducing genes are called senescence-associated genes and these genes code for specific hydrolytic enzymes.
3. Which area is called as the abscission zone?
Answer: The area where the leaf petiole detaches from the stem is structurally different from surrounding tissues. This area is called as the abscission zone. It can be distinguished externally by its pale colour and slightly constricted walls. It is composed of thin walled parenchyma cells and is anatomically weak. Abscission zone can be differentiated into two regions; the separation layer and the protective layer.
4. Mention two possible causes of senescence.
Answer: (1) Competition for nutrients: H. Molisch (1928) suggested that the senescence is due to nutrient depleting action of sexual reproduction.
(2). Production of inhibitors: Carl Leopold (1959) suggested that reproductive organs possibly produce substances that are transported to vegetative tissue where they cause senescence. Flowering in wheat, for example, causes sudden decrease in photosynthetic capacity, not only of old or mature leaves but even the young ones. This is due to the production of inhibitors by the flowers.
5. Define abscission. What is leaf abscission?
Answer: The process that scissors away (i.e., cuts or removes) is called abscission. Leaves of woody dicotyledons and gymnosperms and less commonly those of herbaceous dicotyledons separate from the stem without causing any injury to the stem by a special process called leaf abscission.
Three marks questions with answers
1. What is whole plant senescence?
Answer: Whole plant senescence is the characteristic of monocarpic plant species which flower and fruit only once in their life cycle and as such, senescence of whole plant occurs in them. The process of senescence begins with the reproductive maturity and the whole plant dies after seed production. A signal is sent out from the developing seed, progressively causing senescence of leaves, stems and the whole plant ultimately forcing the plant to die. Examples of this type include annual and biennial and some monocarpic perennials such as Bambusa, Agave etc.
Shoot senescence: In this type of senescence, the shoots die after flowering while the underground stems and roots survive and produce new shoots. Shoot senescence is noticed in many perennials such as Gladiolus, Zingiber, Curcuma, Musa, Chrysanthemum, Dahlia etc.
2. What is organ senescence?
Answer: In this type of senescence, the lateral organs such as leaves and the fruits die much prior to the death of the whole plant. Organ senescence may be of the following types.
(1). Simultaneous or synchronous: This type of senescence is noticed in temperate deciduous trees like maple, shed all their leaves in autumn. This is also called seasonal senescence.
(2). Sequential senescence: It is a progressive senescence of lower older leaves while the new ones are added to the growing shoots in a sequence. Because the leaves have limited life span, here the senescence takes place in a sequential manner depending on the age of the leaf.
3. Briefly describe the chemical changes accompanying senescence.
Answer: The young leaves have higher rates of respiration, more synthesis of RNA and protein, and relatively high content of growth hormones. The young leaves transport solutes to other organs as compared to the full grown leaves. The photosynthetic capacity of the leaves is maximum at about the time the leaf is completely expanded. With the approach of autumn and the onset of senescence, starch, protein, chlorophyll and RNA contents in the leaves decline. It is possibly due to synthesis of hydrolytic enzymes in the aging leaves which breakdown these products. The breakdown products are transported to young growing parts where they are used in synthetic processes. The onset of these breakdown process is stimulated by elevated temperatures, short days, nutrient deficiency and drought periods.
The senescing leaves turn red, golden, orange, purple or yellow partly due to unmasking of the yellow and orange carotenoid pigments as chlorophyll is lost. The bright red colours are due to an accumulation of anthocyanin pigments.
4. What anatomical changes lead to abscission ?
Answer: The primary anatomical cause of leaf abscission is the separation layer which differentiates days or even weeks before the leaf fall. It is composed of smaller cells with starch grains, the cells are also impregnated with substances like suberin and lignin. The vessels are short. Gradually, the vessel elements get blocked by tylosis. Just before leaf fall, enzymes dissolve middle lamella. Consequently, there is nothing to hold the cells together and a sudden breeze is enough to break the leaf from the stem.
5. How can you delay the process of senescence?
Answer: Cytokinins very effectively delay senescence in the leaves of various plants. Available evidences indicate that decrease in the cytokinin transport of the roots to shoots leads to the senescence of the shoots. It is evident by the fact that in rapidly growing sunflower plants, the cytokinin content of root exudates in high, but as the plants reach maturity and bear flowers and fruits there is many fold decrease in the hormone content of the leaves. Like cytokinins, certain auxins and gibberellins also retard leaf senescence.