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Light is another climatic factor which is of vital importance to plant life as it is the source of energy for them. Effect of light on plant life depends upon its quality or intensity, the quality or kind of light, and the number of day length hours or photoperiod.
- Plants vary in their requirement for light.
- Plants like Rose and Carnation require light of high intensity (500 – 1000 ft candles)
- Some plants like Ficus, Ferns, Anthurium and African violets need light of low intensity, and partial shade.
- The intensity of light also affects the rate of photosynthesis, respiration, and transpiration.
- The newly planted young growing plants are thus shaded to check transpiration.
- In some plants, the flower colour may fade due to intense light.
- A sun-loving plant growing in shade or partial shade will have lanky and sickly growth.
Most of the important floricultural crops need the morning sun for the production of satisfactory blooms. Thus if a flower has to be grown successfully, it must receive right amount of light.
Quality of light:- The composition of light is made up of the visible spectrum containing violet, indigo, blue, green, yellow, orange and red which may be further divided into a red and invisible spectrum e.g., ultraviolet and infrared.
- In certain plants white light (the entire visible spectrum) is much better for growth than if certain portions are eliminated.
- Certain plants when grown in red or blue lights produce greater dry weight compared to plants grown in white light.
- The green light inhibits plant growth.
- Red light promotes seed germination in certain plants and is suitable for optimum growth of seedlings in many plants.
- The invisible spectrum (Ultraviolet and infrared) of light has no useful role in plant growth and development.
- The glass of the greenhouse absorbs most of the ultraviolet and infrared light but even then the crops grown inside are of excellent quality.
Photoperiod and its effects: - The duration of light during the course of day is termed as photoperiod. The photoperiod has a profound effect on the vegetative growth, flowering, and the blub formation in a large number of plants. Plants can be sub-divided into three groups according to photoperiodic requirements as
- Long-day plants require long light periods combined with short dark periods to form flower buds, Examples are Delphinium, Hibiscus, China aster etc.
- Short-day plants need short light periods combined with long dark periods. Examples are Cosmos, Chrysanthemum, Poinsettia and Gardenia.
- Day-neutral plants flower any time irrespective of the number of dark or light periods. Examples are African violet, Tuberose, Carnation, Zinnia, etc.
- If short-day plants such as Chrysanthemum are allowed to grow in long-day they will continue to make vegetative growth and fail to form flower buds.
- Similarly, long-day-plants will not flower unless they get the minimum number of hours of daily light.
- Long-day plants would need 8-10 hours of continuous dark period
- Most of the short-day plants will need 10-14 hours of continuous dark period to produce flower buds.
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