The effect of light on growth can be studied under 3 headings light intensity, light quality and duration of light. Growth is generally favoured by darkness, but light is indispensable because of its role in the manufacture of food. Young plants growing in the absence of light develop elongated thin stems with narrow leaves and poorly developed shoot system. Such plants are known as etiolated.
In weak intensity of light the internodes are short and the leaves are expanded. In strong intensity of light, the plant assumes a normal height. Very low light intensity reduces the rate of overall growth of the plant, by lowering the rate of photosynthesis.
Growth in full spectrum of visible light is found to be better than the growth in any one of the different colours of light. Red colour seems to be the most favourable for growth.
The duration of light has a pronounced effect on the growth of vegetative as well as reproductive structures. The influence of duration of light is most marked in inducing or suppressing flowering. This phenomenon is termed as photo periodism.