Understanding Circadian Rhythms in Plants

The California poppies in my garden open wide when the sun is high and close down near dusk. They might be a perfect example of circadian rhythms.

The term sounds like a dance band, but it refers to cycles of activities in living things that occur with timing close to (Latin: circa) the 24-hour day (dies).

Evolution Through Gravity

Evolution has occurred on a rotating world—sunrise, sunset—with drastic but predictable changes from day to dark.

Our 24-hour cycle is tied to the Earth’s spin and a whole solar system that conserves the angular momentum in the cooling primordial gas cloud out of which the sun, planets, and thousands of asteroids and smaller objects were pulled together by gravity.

Day and night cycles are far older than life. From archaic bacteria to our favorite plants to a new baby, every cell carries billions of years of biochemical adaptation.

Maybe you thought 12/12 or 6/18 lighting requirements could tell the hourly story of your plants, but there’s way more to it.

Though it was the late 19^th century before circadian rhythms in animals were studied, observations in plants are far older. During the marches of Alexander the Great in the fourth century BCE, Androsthenes noted cyclic and predictable leaf movements of the tamarind tree on the island of Tylos (now Bahrain).

French scientist Jean-Jaques de Mairan is credited with the earliest scientific observation of the phenomenon. In 1729, he took notes on daily cycles of leaf movements in Mimosa, the “sensitive plant,” and found that they persisted even when plants were kept indoors in constant dark.

It’s All About Available Energy

Your personal circadian clock lives inside your brain. More specifically, in the supra-chiasmatic nucleus,a bilateral nerve cluster of about 20,000 neurons in your hypothalamus.

For humans, it seems, those who went to bed instead of stumbling in the dark survived longer. For plants, it’s all about utilizing available energy in warm bright light while waiting out the chilly dark.

Plants are richly rhythmic. Leaf movement is only one of many daily rhythms that include flower opening, resistance to infection, fragrance emission, germination, photosynthetic activity, growth, enzyme activity, gas exchange, and stomatal movement.

All life attunes to the planet’s daily cycles. Widespread circadian regulation of gene expression underlies many of these rhythms. Once genes are activated, transcription produces the proteins that regulate day-night activities via a genetic feedback loop. Botanists know time of day influences outcomes of experiments.

In addition to response to light’s energy, the circadian clock regulates response to cold, drought, and osmotic stress, and provides a timing mechanism that lets plants recognize changing seasons and prepare for coming conditions. They can thus regulate growth, reproduction, and activities such as dormancy and leaf abscission.

“Processes with 24-hour oscillations are more generally called diurnal rhythms,” says world expert on the topic, C. Robertson McClung, now at Dartmouth College. “Strictly speaking, they should not be called circadian rhythms unless their endogenous nature is confirmed.” So, to be called circadian, patterns must be built-in and self-sustaining without any external stimuli, as first appreciated in the leaf movement rhythms of plants.

Attributes of Circadian Rhythm

The four defining attributes of circadian rhythms—persistence in constant conditions without external stimuli, endogenous origin, entrainment, and temperature compensation—affect other rhythms proven to continue in unchanging light, including patterns in the rate of cell division, photosynthesis, and hormone production. Endogenous origin of the patterns is proven when behavior breeds true, showing up dependably in offspring.

Entrainment refers to how the patterns can be entrained or pulled closer to another pattern by external clues, called zeitgebers (time givers) in German. The zeitgebers can include light, temperature, and redox cycles that are part of normal metabolism. The fourth requirement, temperature compensation, refers to the pattern being unaffected by heat and cold. That’s tricky to pull off, since the kinetics of enzymatic reactions are affected by temperature.

Modern experimental approaches using systems biology have identified many unique components in biological clocks. Cell autonomous oscillators exist in almost every cell in a plant. Scientists are working on how these oscillators are coordinated.

They know that redox oscillations as seen in peroxiredoxin rhythms could be the grandfather clock, with genetic feedback circuits the major output mechanisms to control behavior via cell and tissue physiology.

The current biochemical model of the clock sees it as a result of an interaction between both genetic transcription circuits and non-transcriptional elements such as redox oscillations and protein phosphorylation cycles.

How Circadian Rhythm Affects Your Crop

For most growers, the major concern is how all this affects their crop. Factors closely related to circadian rhythms can have an effect as important as the timing of light and dark. Some plants, for example, track the sun and anticipate arrival of dusk and dawn. Some varieties are best adapted to a gradual increase and decrease of light, as occurs at both ends of the natural day.

Plants also respond to the angle of light and changes in wavelength, or light color. Closer to the poles, and in early spring and fall, nature delivers long wavelengths of lower-energy reddish light. At all seasons, as the sun rises and sets, its light passes at a low angle through many miles of atmosphere, scattering much of the blue.

Closer to the equator, nearer the summer solstice and around midday, sunlight includes more direct, high-energy blue light. For some plants, bluer light leads to growth while redder light, as in late summer or fall, leads to flowering.

Plant breeders select for desirable traits, but most of them ignore genetic circadian rhythms, so figuring out what your plants need in that area can be hard. But matching indoor conditions to nature is a good bet for better health and yield.

Lighting systems that can provide for all variables are rare. Some advanced LED lighting systems contain programmable elements to simulate sunrise and sunset, as well as offering the option of fine-tuning lighting cycles based on intensity and color spectrum variation at specific latitudes and times of year.

Understanding the circadian rhythms of the plants you have chosen to grow will provide better insight on how they function, what they need to thrive, and when to provide it.