Light, Plants and Rhythms

by Bruce Scofield

(from Llewellyn’s 2008 Moon Sign Book)

Plants are more sensitive to light than most other kinds of life on our planet. Light is what they seek, use, and live by. Plants know from what direction light is coming from and they will move themselves out of shade. At the tip of a plant shoot are chemicals called auxins that influence cell division – with more light comes more cell division and consequently more growth and extension. In this way plants will always bend towards the light, something that is obvious to any indoor gardener. This response to light is called phototropism. While plant shoots are positively phototropic, the roots are negatively phototropic and grow away from light.

Light is essential to plants, and further, plants are essential to us. We are what are called heterotrophs, organisms that must get their energy from somewhere else. So are all other animals and all fungi. But plants are autotrophs, or self-feeders, that make their own energy from light and water. The secret of plants is in photosynthesis, the process that occurs in the chloroplasts of the plant cell. In these tiny organelles photons of light are used to split water and make carbohydrates that the plant uses to live. And of course we owe our lives to plants, directly as in a salad, or indirectly as in a cow that ate grass. Interestingly, the tiny chloroplasts found in all green plants are the remnants of formerly free-living bacteria, cyanobacteria to be exact. Cyanobacteria (sometimes incorrectly called blue-green algae for marketing purposes) have been on the Earth for at least 3 billion years and they are what initially made our oxygen-rich environment. These bacteria invented photosynthesis and plants merely carry on their tradition. The bottom line here is that we are living off cyanobacteria in one form or another – and ultimately, the Sun!

All plants have a daily cycle of photosynthesis that changes as the Sun moves through our sky and then beneath it. Plants know exactly when to turn this cycle on and off each day, and this is in response to the alternation of day and night that is caused by the rotation of the Earth. This cycle is an example of what are called circadian rhythms, a type of biological clock. There are many kinds of circadian rhythms in the cells of plants, and some plants have daily leaf motion rhythms as well that serve to limit water loss by evaporation. Circadian rhythms allow plants to know when to close their flowers at night to keep the pollen dry, and they use them to know when to give off special odors to attract insect pollinators.

The study of biological rhythms in modern times actually began with plants. The French astronomer Jean Jacques d’Ortous de Mairan observed in 1729 that the sleep movements of a species of heliotrope varied in a consistent way throughout the day. He thought that the plant was following the alternation of night and day, but to test this he placed the plant in continual darkness. Amazingly, its leaf movements persisted suggesting there was an internal clock in the plant itself. A century later, Charles Darwin wrote a book on this topic called The Power of Movement in Plants, and throughout the 20th century many researchers continued to probe the basis of these rhythms.

Another important adaptive rhythm found in plants is related to the cycle of the year. Many plants live at latitudes that have a yearly cycle of day-length change. In the northern hemisphere days are longer in summer and shorter in winter, and this becomes more extreme as one moves north. The reverse is true in the southern hemisphere. Tracking this seasonal rhythm is central to the reproductive cycle of the plant and in order to do so, plants must measure changing differences in day and night length. The first report of an organism measuring the length of day was published in 1920. Researchers writing for the U.S. Department of Agriculture, concerned with efficient tobacco propagation, discovered that the flowering of the tobacco plant at specific times in the year was influenced by the day/night ratio within the cycle of the year. They named this property photoperiodism.

During the course of the year, the days vary in length. In spring (in the northern hemisphere) the days lengthen until they reach a maximum at midsummer’s day, June 21st, the summer solstice. Then the days begin to shorten and by the winter solstice, December 21st, they are as short as they will get. The length of the days and nights varies according to latitude. If you compare the longest and shortest days of the year near the equator and the difference will be in minutes. At temperate latitudes, which covers most the United States, the longest day could be 15 hours and the shortest 9 hours. At very high latitudes in the Arctic, days could last for a season or more. These are the facts of life for plants, and since they need the sunlight to live, they must adapt to these changes and make the most of them.

The most dramatic evidence of photoperiodism in many species of seed-bearing plants is shown in the timing of flowering. Plants flower at the most opportune times of the year in response to the changing light-dark ratios caused by the yearly cycle. There are very good reasons why plants need to time their flowering periods perfectly. Many flowering plants have co-evolved with insects that are needed to move pollen from one plant to another so that reproduction can occur. Plants have done this by making their flowers attractive to the insects, but also by having their flowers open at the same time of day that the insects are active. There are many examples of this, the most dramatic being the night-blooming cereus plant which blooms for only one night and is pollinated by one type of moth.

Plants that have photoperiodic rhythms are of two basic types. Short-day plants time their flowering by registering long dark periods, that is long nights. Long-day plants register long day lengths which then induces them to flower. Of course, the length of day and night during the year is related to latitude and plants of the same species that have a large north to south range will compensate for this. Plants will also measure the range of temperatures during the year and this can be another factor in the timing of flowering. There is also a third category of plants which are said to be day-neutral and utilize several signals in the environment to induce flowering.

The actual mechanism in the plant that allows it to read day-length is located in the circadian clock. Here’s a simplified explanation of how it works. The circadian clock itself is located in special cells where certain proteins are made at a fixed rate. As these proteins reach a certain level, protein making shuts down. When there is not enough of this protein, the process starts again. It’s a bit like a thermostat that keeps a house within a certain temperature range. The genes that drive the cycle are affected by light, and they become entrained to the light-dark cycle of the day. When light hits them earlier or later, depending on the species, the cycle adjusts itself. This continues throughout the year and results in a good adaptation to the natural cycles of Earth and Sun. When a critical day or night length is reached, the plant knows that it is time to flower.

Because different plants flower at different times of the year, gardeners have placed plants such that there are continuous blooms in the garden. And since different plants flower at different times of the day, gardeners have placed plants to provide continuous blooms during a single day. When arranged carefully, a gardener can place different types of plants in a circle to create a flower clock. The flower clock has been credited to Carl von Linné or Carl Linnaeus (1701-1778) of Sweden, one of the great names in biology.

Linnaeus was a passionate collector and organizer of nature. He is the person who created the modern system of classification for plants and animals in which each organism has a genus name and a species name, like Homo sapiens – us people. He walked through Europe studying nature naming and describing over 12,000 species of plants and animals. While it is not known to be a fact, it is thought that Linnaeus made a floral clock that allowed one to tell the time of day. From his careful observations he came to know at what hour a particular plant would flower and he placed that plant in a clock-like circle at its appropriate time. For his clock, Linnaeus selected local flowers that would bloom even on cloudy days. Linnaeus also noticed that some flowers would open and close according to the weather. He noticed others that opened and closed according to the length of the day – plants that were responding to photoperiodism. A third type changed opening and closing times according to the time of day – and these are the ones he used in his flower clock. Some of these which are listed below with their approximate flowering time. As you can see not all hours are covered and for the most exact reading of this clock, the timing of the closing of certain species must be used.

AM

5:00 Morning glories, wild roses

6:00 Spotted cat’s ear, catmint

7:00 African marigold, orange hawkweed, dandelions

8:00 Mouse-ear hawkweed, African daisies

9:00 Field marigold, gentians

10:00 Helichrysum, Californium poppy

11:00 Star of Bethlehem

12:00 Passion flower, goatsbeard

PM

16:00 ‘Four o’clock’ plant opens

18:00 Evening primrose, moonflower

21:00 Flowering tobacco

22:00 Night blooming cereus

Some common plants that can be used to make a functional, though not precise, flower clock include morning glories and wild roses that open just after dawn. These are followed by dandelions, gentians and California poppies which open near noon. In the afternoon the morning glories close and then later the four o’clocks open, followed by evening primroses and moonflowers. In making a garden flower clock, you should know when each type of flower closes as that can also be a way of telling time.

Serious gardeners will grow plants that bloom at a wide range of times during the growing season. The flowering of daffodils in spring, day-flowers in summer and chrysanthemums in fall are all set by the plants’ responses to changes in the length of day. Even varieties of plants within the same species have been developed that will flower at different times of the year as well. A goal of a good gardener and landscaper is to have a constant series of blooms throughout the growing season, a constant stream of color from plants – a kind of seasonal clock. And all of this is possible due to the complex biological clocks in plants that allow them to read the sky and bloom when the time is right. Plants have evolved to be closely in tune with the natural astronomical cycles that are such a fundamental part of our environment – and we couldn’t exist without them.

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