Tropisms in plants

Tropisms -

As various organs are formed, their movements are of special interest. Why is it that root always go into the soil, or the stem always moves towards light? The movements of plant roots, stems, leaves and other parts in response to certain directional fluxes or gradients in environmental factors are known as paratonic movements of growth or tropisms.

They are of following types –

Phototropism (Heliotropism) –

The curvature induced in plant organs in response to the unidirectional light is called phototropism. The unidirectional light causes the stem apex to move towards the source of light and in root away from it. Thus the stem shows positive phototropism and the root negative phototropism.

           

The positive and negative curvatures are the result of the unequal growth of the illuminated and shaded sides of the apex. In case of stem, the growth is more on the shaded side, while in the root it is more on the illuminated side. Greater growth of the shaded side of the stem is due to more accumulation of auxin on the shaded side. This accumulation may be due to -

1) Translocations of auxins from illuminated side to shaded side.

 2) Inhibition of auxin synthesis in the illuminated side. 

 

Geotropism –

The growth and orientation of stem and roots in response to the force of gravity is called geotropism. Different parts respond differently to the stimulus of gravity. The stem shows negative geotropism i.e. it grows away from the force of gravity and root shows positive geotropism. Thus if a plant is kept horizontally, its stem will bend upward and roots in downward direction.

           

Geotropism can also be explained on the basis of auxin concentration. In a horizontally placed plant the auxin accumulates on the lower side of the stem and root apices due to gravity. In the stems, higher conc. of auxin on the lower side of the apex stimulates its growth on that side. As a result, the stem grows in upward direction showing negative geotropism. In case of root, the higher conc. of auxin on the lower side inhibits the growth of that side and the root apex grows downwards. i.e. show positive geotropism.

           

Different plant parts respond to gravity in three ways -

1) Orthogeotropic - Primary roots grow towards the force of gravity and are called positively orthogeotropic. The main stem which grows away from the gravity is said to be negatively orthogeotropic.

2) Plagiotropic - Secondary roots and branches arising from the main stem are at the angle of 45-1800 from the   main axis. Thus, these organs are placed obliquely to the force of gravity. Such organs are called plagiotropic. The secondary roots are positively plagiogeotropic and the branches are negatively plagiogeotropic.

3) Diageotropic - Tertiary roots, horizontally placed branches and leaves arise at an angle of 900 from the main axis. They are placed horizontally to the force of gravity and are said to be diageotropic.

Hydrotropism –

Growth movements in plant organs in response to variations in the amount of moisture are known as hydrotropism. The curvature of organ concerned is due to unequal growth on its two sides. Roots are positively hydrotropic.

Chemotropism –

The movement of plant organs due to the unilateral stimulus of chemicals is called chemotropism. The growth of pollen tube from stigmatic surface into the style and ovary is due to chemical stimulus. Similarly, hyphae of many fungi show positive chemotropism, they grow towards sugar and other nutrient substances.

 

Thigmotropism (Haptotropism) –  

The growth movement of plant organs in response to unilateral stimulus of touch is known as thigmotropism. Tendrils of many cucurbitaceous plants nutate in the air and when come in contact of any support, they coil around it. The coiling occurs because the growth of the surface which comes in contact of the support is retarded while it remains normal (or is accelerated) on the opposite side.