- Written by Nadia O’Carroll
A major key in regulating plant activity is the change in length of day (ratio of light to darkness in a 24-hour period) throughout the year.
The seasons and length of day are linked because they are a result of the change in the orientation of the Earth relative to the Sun. The Earth’s axis is tilted at an angle of 23.5 degrees. When the top half of the Earth tilts towards the sun it is Northern Hemisphere summer/Southern Hemisphere winter, when the lower half of the Earth tilts towards the sun it is Northern Hemisphere winter/Southern Hemisphere summer. Days are longer in summer and shorter in winter. On the equator there is very little change in position so length of day is an almost constant 12 hours. At the poles there is only one sunrise and sunset per year. These occur about the time of solstice, 21 June and 21 December, and mark the time of maximum tilt of the equator and consequently they are the dates of the longest and shortest days.
Photoperiodism is the term used to describe an organism’s ability to detect changes in the length of day. In plants it is a complex process and the mechanics are not totally understood. The plant has to perceive the ratio of darkness to light in a 24-hour period and then determine the marginal increase of light or darkness. Plants accomplish this through photoreceptors found in their leaves, these light sensitive protein pigments are called phytochromes and cryptochromes. Two forms of phytochrome react differently to the length of darkness and the change in ratio of these forms determines whether light or darkness has increased. The plant then has to communicate this information to another part of the plant to produce the response required. A message protein is produced in the leaf, it then moves through a nerve-like network of chloroplasts and specialised cells to other parts of the plant, such as the shoot tips and triggers the complicated gene activity which produces floral buds.
Plants that flower in spring have many active pollinators such as insects, birds and bats, and also have an opportunity to set fruit and seed, which mature before the following winter. However there are many plants that flower in other seasons, many temperate species even require a cold spell, called vernalisation, before they will flower, no matter how favourable other conditions are.