Plant life cycle and its photoperiod requirements

The Plant life cycle begins with a seed. A seed is a very important plant structure because within it lies continuity. The seed grows into an immature plant called a seedling, which grows further into a mature plant that can produce seeds for the next generation.

The seeds are borne by many plants within the ovary found in the flower which further develops into a fruit or as in the case of non flowering plants, the seeds or spores lie exposed on leaf-like structures waiting to be dispersed. Seeds can be dispersed by wind, animals, water, or humans to places conducive for germination into a mature plant. As the plant matures from being a seedling to an adult, it requires different photoperiods for each stage of its life cycle.

Photoperiod refers to the period of time each day during which an organism is exposed to or receives light from the sun or an artificial source in relation to the period of darkness its exposed to. This unique relationship a plant has with light and darkness is quite complex and is not the same for every plant. Generally, in an enclosed environment, vegetative growth (leaves and roots) requires about 18 hrs of light and 6 hrs of darkness while flowering requires equal periods of light and darkness of 12 hrs each; with other factors such as soil quality,  moisture and temperature being constant.

However, plants can be categorised based on their response to varying degrees of exposure to light and darkness as follows:

• Short day plants (SDP)
• Long day plants (LDP) and
• Neutral day plants (NDP).

For Short day plants, more than 12 hrs of darkness (<12 hrs light) is required for them to flower, while for long day plants, less than 12 hrs of darkness (>12 hrs light) will produce a bloom. A neutral day plant does not flower based on photoperiod but on maturity or reaching a certain age. Below are examples of plants in each category:

• SDP: rice, strawberry, chrysanthemum, onion, soybean, viola, marigold, dahlia, cotton, tobacco, cosmos etc.
• LDP: cabbage, oat, spinach, lettuce, hibiscus, foxglove, radish, beet, wheat, barley, petunia etc
• NDP: tomatoes, cucumber,sunflower, maize, dandelion, begonia etc

Therefore, if a plant is raised for its exotic flowers, or extensive roots or beautiful foliage, knowledge of its photoperiod requirement will greatly assist in its propagation.