Test: Plant Growth and Development - Assertion and Reasoning - NEET MCQ

The shoot tip tissues in plants are meristematic.

Nutrients are raw materials required for synthesis of protoplasm as well as source of energy. It should be rich in nitrogenous components to increase the synthesis of protoplasm and carbohydrates for energy and cell wall synthesis. All types of micronutrients and macronutrients should be available for proper growth. Water is required for cell elongation, maintenance of turgidity of growing cells and for providing medium for enzyme action. Even slight deficiency of water reduces growth. It may, however, promote differentiation. Water stress completely stops growth.

The root apex, shoot apex and many other regions consists meristematic tissue. By mitotic divisions of the meristematic cells new cells are produced. These cells have cellulosic cells walls, contains large nucleus and rich in protoplasm. It is called the phase of cell formation or cell division.

Growth is unique in plants because plants retain the capacity for unlimited growth throughout their life. These ability of the plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have the ability to divide and self-perpetuate. The root apical meristem and shoot apical meristem are indeterminate meristems, because these meristems continue thier activity throughout life of the plant. Root apical meristem and shoot apical meristem cause primary growth of plants and also causes the elongation of the plants along their axis.

An S-shaped or sigmoid curve is obtained, if total growth is plotted against time. It consists of four parts-lag phase, log phase, phase of diminishing growth and stationary phase. Growth is slow in the lag phase, rapid during log or exponential phase, slow again during the phase of diminishing growth. During the stationary phase growth stops completely. Log phase is also called as grand phase of growth due to fast growth in this phase.

During different phases of growth as well as in response to environment different structures are developed. To cope up with the changes in environment. The plant organs also change their structures, which is called plasticity. Heterophylly is the occurrence of different types of leaves on the same plant in different growth phases or under different environmental conditions. In case of environmental plasticity shown by aquatic butter cup Ranunculus flabellaris, the submerged leaves are highly dissected while the emerged leaves are broad and lobed.

In the life of a plant, growth, differentiation and development are closely related events. Development is sum of growth and differentiation. Development in plants is under control of both intrinsic and extrinsic factors. Intrinsic factors include intracellular genetic factors and intercellular factors growth regulators. Extrinsic factors are light, temperature, water, oxygen and nutrition, etc.

Secondary roots and shoots are plagiogeotropic i.e., they grow to a position at an oblique angle (45°) to the gravitational force. Root and stem branches lie at an angle other than 90° to the direction of gravity.

Removal of shoot tip increases the lateral dominance not apical dominance. Accumulation of auxin in the lateral part elongates the cells and increases growth.

F.W. Went isolated auxin from tips of coleoptiles of oat seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers.

The first selective herbicides to be discovered and used widely were 2,4-D and its derivatives and are very potent auxins. 2,4-D and 2,4,5-T destroy dicot weeds. They block their sieve elements and disturb mitosis. The plant is ultimately destroyed. Agent orange, which was used in the war in Vietnam as a defoliant is an effective mixture of free 2,4-D and the N-butyl ester of 2,4,5-T.

Auxin is produced by shoot tip that acts as a growth promoter for apical bud and inhibit the growth of lateral buds. When a terminal bud is removed, the nearest axillary buds begin to grow and the plant branches rapidly. In case of mulberry plants, pruning is done to remove apical dominance and causes more branching of the main body of the plant.

Auxin delays abscission of young leaves and fruits. Its effect is through nonformation of abscission zone below a leaf or fruit. Abscission zone cuts off nutrients and water supply. However, auxin promotes the abscission of mature or older leaves and fruits.

The apical bud is not the only source of auxins. Young developing leaves also produce auxins and it has been shown that auxins from this source may inhibit lateral bud growth. When the apical bud is removed, the lateral buds sprout. However, if a paste of auxin is painted on the cut end of the decapitated shoot, the lateral buds remain inhibited, as if the apical bud is present.

Application of GA can induce flowering in long day plants. It has no favourbale effect on size of flower and fruit in certain plants. The genetically dwarf plants can be made tall by application of GA.

Stratification of seeds may affect the disappearance of inhibitors and the buildup of germination promoters such as the gibberellins and cytokinins. Natural stratification occurs when seeds shed in the fall are covered with cold soil, debris and snow. In artificial stratification, layers of seeds are alternated with layers of moistened Sphagnum sand or some other appropriate material and stored at low temperatures.

Gibberellins are applied to increase the number and size of several fruits e.g., grape, tomato. Size and shape of apple frutis is enhanced by application of GA₄ and GA₇ mixture. Gibberellins (e.g., GA₃) increase the yield of malt from barley grains i.e., malting.

The first cytokinin was discovered from degraded autoclaved Herring sperm DNA. It is called kinetin (6-furfuryl amino-purine).

Kinetin does not occur naturally and is a synthetic hormone. Cytokinins help to overcome the apical dominance. It is the ethylene that breaks seed and bud dormancy.

When cytokinins are added directly to the abscission layer, senescence of the zone is retarded. They delayed the degradation of protein and chlorophyll of the plant parts and hence delay senescence. As they act as anti senescent, they act as antagonistic to ethylene which accelerate senescence.

In most fruits, the rate of respiration will undergo a sharp rise and then fall near the end of ripening. Kidd and West termed this phenomenon "climacteric rise". The climacteric acts as a trigger that sets in progress those changes that rapidly transform the fruit from an unripe to a ripe condition. Finally, application of ethylene to unripe fruit will bring on a premature climacteric and accelerate ripening.