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Plant movements & plant hormones
Plant movement : The movements in plants are not as apparent as in case of animals. Plants generally show movements at a very slow rate. The higher plants are fixed to the substratum by means of roots. They cannot move from one place to another. Therefore, they show movement of their organs only.
Classification of plant movements :
Plant movements are broadly classified into two types :
1. Tropic movements :- Directional movements or orientations of specific part of a plant in response to external stimuli such as light, force of gravity, chemicals, water are called tropisms or tropic movements.
Tropic movements are very slow. The movement of the plant part can be either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).
(i) Phototropism :- Definite direction movement in relation to light.
(ii) Geotropism :- Definite direction movement in relation to gravity.
(iii) Chemotropism :- Definite direction movement in relation to chemicals.
e.g. - Movement of pollen tubes and fungal hyphae.
(iv) Hydrotropism :- Definite direction movement in relation to water.
e.g. - Roots of seedlings.
(v) Thigmotropism (Haptotropism) :- Definite direction movement in relation to contact or support. The pea plant develops tendrils which help it to climb up other plants or fences or some other support. These tendrils are sensitive to touch.
e.g. - Tendrils, haustoria of Cuscuta
(2) Nastic movements :- Induced by external stimuli such as light, temperature, touch. However, these are not directional movements. Here, the part of the plant does not respond towards or away from the stimulus. Nastic movements include :-
(i) Seismonasty :- The turgor changes occur in thin-walled cells of pulvinus, causing folding and drooping of the compound leaves. Such movements occur in response to touch (shock). These movements are very quick and are best seen in 'touch-me-not' plant (Mimosa pudica), also called 'Chhui-mui' or 'Lajwanti' or 'sensitive plant'. If we touch the leaves of the chhui-mui plant with our finger, we find that all its leaves immediately fold up and droop. After sometime, the leaves regain their original status. Here, no growth is involved. Instead, plant cell changes shape by changing the amount of water in them (turgor changes), resulting in folding up and drooping of leaves.
(ii) Nyctinasty :- Sleep movements - Due to day and night.
e.g. - Daily movement of flowers, leaves, stomata.
|Direction of movement is in the direction of the stimulus.||Direction of movement is not determined by the direction of stimulus.|
|Growth takes place.||Growth does not take place.|
|Movements are slow.||Movements are fast.|
|e.g.Growth movement of the shoot towards light||e.g. The folding up and drooping of leaves in the sensitive plants.|
Chemical Coordination in Plants
(iv) Abscisic acid
(i) Auxins :-
Functions of Auxins
(ii) Gibberellins (GA) :-
Functions of Gibberellins:
(iii) Cytokinins (CK) :-
Functions of Cytokinins:
Promote cell division and elongation
(iv) Abscisic Acid (ABA) :- It is also known as stress hormone or dormin.
Functions of Abscisic Acid (ABA)
(v) Ethylene (Gaseous hormone) :-
Functions of Ethylene (Gaseous hormone)
Do you know ?
Photoperiodism :- Effect or requirement of relative length of day (photoperiod) and night (dark phase) on flowering of plants is called as photoperiodism. The phenomenon was first discovered by Garner and Allard on Maryland mammoth (a mutant variety of tobacco) and they classified plants into following groups -
(i) SDP (Short Day Plants) :- They need short day length for flowering.
e.g. - Tobacco, Soybean. Strawberry, Dahlia, Sugarcane.
(ii) LDP (Long Day Plants) :- They need long day length for flowering.
e.g. - Potato, Radish, Carrot, Wheat, Spinach.
(iii) DNP (Day Neutral Plants) :- They do not need any specific light period for the flowering.
e.g. - Zea, Cotton, Tomato, Sunflowers, Cucumber.
Phytochrome :- It is light sensitive pigment responsible for flowering which was discovered by Borthwick and Hendricks.
Vernalization (Yarovization) :- "Acceleration of ability to produce flower by chilling treatment is called vernalisation".