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Unto light

© Narayan Dattatray Wadadekar

Plants are affected by light. Light is a complex signal. It comes from the external environment and it varies in many of its aspects. The presence or absence of light, as well as its intensity i.e. strength, duration i.e the period for which the light is available and also the direction of light - all these factors keep on changing during a day and in different seasons.

Plants have specialized structures to sense light. These are the photoreceptors. They can be sensor pigments like – chlorophylls, carotenes, flavonoids, phytochromes etc .in all higher plants and in addition to these chemicals cell organelles like eye spots, in some algae etc.

A very wide range of responses are given by the plants as a result of receiving of light such as – (1) developing stouter stems, (2) development of colored pigments like chlorophyll and anthocyanins (3) germination of seeds, (4) opening of stomata, (5) bending of stem tips towards light, (6) flowering and (7) movement of chloroplasts or (8) even entire plant organism etc.

For more details on phototaxis, photoperiodism, flower induction please go to - http://www.biologie.uni-hamburg.de/b-onl...

At least sixteen families of flowering plants (all known so far are dicots with the sole exception of the family of monocots - the Marantaceae) show heliotropic movement i.e. show solar tracking of leaves, leaflets, or finer subdivisions of the leaves.

In Phaseolus i.e. bean plants and in many other plants leaflets close at sunset and open to expose their photosynthetic machinery to sunlight at day break. There is a small knob like part called pulvinus at the base of many such leaves and the stiffness of this structure raises or allows leaves to droop. The stiffness of pulvinus depends on the entry or exit of water into it. And the signal of light especially blue light drives this function.

Shoot tips, especially the green ones, perceive difference in the light available from two sides and grow towards the light. A plant hormone called, auxin gathers more in the shaded part than in the lit part of stem tip. Auxin makes the tissues grow and therefore the part less illuminated grows longer resulting into bending towards light source. Auxin is also known to cause cell wall loosening. This appears to help in bending of a plant organ like the shoot tip.

Euglena gracilis (gracile = slender and graceful) a unicellular green alga, moves towards a source of light if the light is dim and away if the light is strong.

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