31 Years NEET Previous Year Questions: Plant Growth And Development - 3

 Mimosa pudica (touch me not) leaves show seismonastic movement in response to physical contact. The movement is due to turgor changes due to ejection of water by cells of pulvinus (swollen leaf base) on touching it. It causes folding and drooping of leaves.

Borthwick & Hendricks (1952) gave the concept of phytochrome. Phytochrome is a coloured protein pigment located in plasma membrane to perceive  the photoperiodic stimulus in leaves. It may exist in two interconvertable forms Pr and Pfr (red and far red light absorbing forms)

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

 Because cytokinins help cells in retaining chlorophyll for longer time. Cytokinin delay the senescence (ageing) of leaves and other organs.

The most important function of cytokinins (zeatin is one of the cytokinins present in Zea mays - the corn) is the promotion of cell division. These are now established as true cell division factors.

Monocot leaves have some specialized cells in the epidermal cells of their leaves called bulliform cells (motor cells) which lose or gain turgidity due to which their leaves show coiling and uncoiling.

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.

Apical dominance is a common observation in vascular plants. If apical meristem is intact & growing, the lateral tips growth is suppressed. According to Thimann & Skoog the Auxin, synthesized in apical meristem, in translocated downwards & inhibits the growth of lateral tips. That is why mowing grass lawns facilitates better maintenance.

It is due to Richmond-Lang effect. Cytokinins prevent degeneration of chlorophyll, proteins and nucleic acids. They increase of inflow of auxin and amino acids. It reactivates leaf metabolism.

Vernalization is the low temperature treatment for flowering in plants. Plants are treated (seeds and seedlings) with very low temperature to promote germination. A Lang & co-workers demonstrated that the application of gibberellins can replace the cold treatment or vernalization in many plants.

All except nyctinastic movement are related to auxin. Nyctinastic (sleeping) movement of leaves are due to diurnal changes. (changes in day and night).

Phototropic movements are the plant responses towards light. Generally plant shoot and flower show positive phototropism while geotropism is the plant response to the gravity of earth. Generally roots are positively geotropic. All these movements are linked to Auxin.

Abscisic acid as the name suggests cause abscission due to which leaves fall. It also acts as a growth inhibitor & induce bud dormancy in various plants.

 Phytohormones are plant hormones which are synthesized by the plants and act as regulators and influence physiological process like growth, movements etc. auxins, gibberellins etc. are the phytohormones.

Highest concentration of auxins occurs at the growing tips was proved by Went in 1928 by his experiment on Avena coleoptile. It was demonstrated that auxins are synthesized at the tips and then translocated downwards where they cause growth.

Twining of tendrils is due to thigmotropism i.e. movement due to the contact with a foreign body (touch).

Chailakhyan (1936) a Russian  investigator who researched on photoperiodism proposed the term ‘florigen’. According to him ‘Florigen Complex’, the true flowering hormone includes two group of substances formed in leaves (i) Gibberellins which are necessary for the formation and growth of stem and (ii) Anthesins which are necessary for flower formation. Acting together they form the substance named ‘florigen’

Apical dominance is caused by the action of auxins & IAA (Indole-3-acetic acid) is a naturally occurring auxin.

Abscisic acid is a stress hormone. During desication abscisic acid is synthesized which reduces transpiration by closure of stomata. It conserves water.

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

Cytokinins cause delay in senescence (Richmond-Lang effect). Senescence is the phenomenon in which the mature leaves lose chlorophyll, turn yellow and fall off. 

A short day plant is one that flowers on photoperiods shorter than the critical day length. It generally flowers when treated by a short day and uninterrupted long night (more than 12 hrs.)

Apical dominance is the phenomenon by which presence of apical bud does not allow lateral buds to grow. Apical dominance is due to auxin. When shoot tip is cut; then lateral buds start growing.

Ascorbic acid is an organic acid with antioxidant properties. Its appearance is white to light yellow crystals or powder. It is water soluble. The L-enantiomer of ascorbic acid is commonly known as Vitamin C. Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions and as a preservative.

Vernalization is the method of inducing early flowering in plants by pretreatment of their seeds at low temperature.