(This is the online version of the paper booklet available at ticket offices in the Garden.)
The earth rotates on its axis every 24 hours, resulting in day and night cycles. It also orbits around the sun, causing annual seasonal shifts in day length, light levels, temperature, rainfall and other climatic conditions.
Plants develop rhythms in response to these cyclical changes. These rhythms can be as a result of direct responses to environmental cues, or the presence of an inbuilt clock. The latter gives them the innate ability to measure time and anticipate rhythmic changes in the environment, such as day and night. ‘Circadian rhythms’ are biological cycles completed in 24 hours, allowing plants to tell the time of day.
This trail takes a closer look at rhythms in plants, including cyclic changes in growth rates, flowering times, opening and closing of stomata, production of scent and leaf loss.
Download the Rhythms in Plants Trail brochure or map.
Rhythms are everywhere
The natural world is full of rhythms. The day/night cycle results in changes in light levels and temperature, while the yearly cycle gives alternation of wet and dry seasons at the equator, and progressions of temperature and day length with the seasons at latitudes further towards the poles. Responding to these rhythms is crucial to the survival of animals and plants. Animals can move, allowing them to shelter in the shade of a tree during the hot noon day sun, or migrate to a warmer climate for winter, while plants remain literally rooted to the spot, and must physiologically adapt to be able to cope.
These adaptations fall into two categories: direct responses to an environmental cue, or anticipatory responses controlled by an internal 24-hour circadian clock. An example of a direct response is when sensors in the leaf detect the light of dawn and cause leaf pores called stomata to open, allowing the intake of air for use in photosynthesis. Another is when cooling temperatures in autumn trigger leaf fall in deciduous trees. Relying solely on direct responses is inefficient and potentially dangerous: opening pores is slow and, if only triggered by an increase in light, part of the day where photosynthesis could occur would be wasted. Likewise, an unseasonably cool summer could cause trees to lose their leaves too early, resulting in months of lost photosynthesis and a smaller reserve of sugars for the tree to use as food the following spring. For these reasons, many responses are regulated both directly and by an internal clock to provide anticipation.
Plants supplement environmental cues with their own internal circadian clock, which anticipates the 24-hour cycle of light and dark and associated temperature changes. The clock takes the form of rising and falling levels of certain proteins within the plants’ cells. Production of one set of proteins causes the levels of a second set to fall, which turns off the production of the first set; levels of the second set then rise, allowing the first set to be produced again. This sets up an oscillating loop of protein levels: the basis of the circadian clock in plants. (For a fuller explanation of the mechanism, see the panels in the Circadian Beds at point 13 on the map.)
Using these cycles, the plant can keep track of when dawn is approaching, opening its leaf pores in anticipation of light levels rising. The appearance of light at dawn is also how the plant keeps its clock ‘in sync’ with the 24-hour days: light resets the clock at the start of every day.
The clock can also inform the plant about seasons, because it can be used to measure the length of the day. By combining information from light sensors in the leaves with protein levels which build up during the day, the plant can tell if it has been sunny for more than a certain number of hours: if so, then it must be spring and time to start growing. Likewise, shortening days indicate that it is approaching autumn and therefore the plant should start shedding leaves.
Research into plant rhythms
Circadian rhythms in plants are studied by Prof. Alex Webb and his research group in the Department of Plant Sciences. By growing plants in day/night cycles which are longer or shorter than the normal 24 hours, and by using plants defective in particular elements of the circadian clock, the scientists are looking at what keeps the clock ticking with its daily regularity. Changing the clock’s timing can mean plants grow slower or faster, so this research will help increase crop yields for food and bioenergy production.
