We don’t know why certain flowers tuck themselves in at night, or why some leaves fold themselves together as the sun sets, but we know how they do it. These openings and closings of petals and leaves is called nyctinasty [nik-TIN-as-tee]. Nyctinastic movements are also called sleeping movements.
Options in plant movement
While plants are not free to get up and walk around, they do have options when it comes to movement. Some plants follow the sun’s movements across the sky, in a behavior called phototropism. There are also rare individuals who exhibit skototropism by moving away from sunlight. Note that both of those words end with -tropism. Movement is tropic [TRO-pic] if it is in reaction to a source of stimulus. Tropic movement is nearly always growth related and it is dependent on the direction of the stimulus. If a plant’s reaction is independent of the stimuli’s position, it is called a nastic movement. Nastic movements may or may not be growth related. If you see a plant behavior word that ends with -nasty, you will now know that it is a nastic behavior. Nyctinasty is one of those words.
Latin bed times
We all have our bedtime routines. Some plants do, too. These routine behaviors are called nyctinastic. You just learned that -nastic means movement independent of a stimuli’s position. When it happens because of nighttime, we add the Latin prefix nyct-, which means ‘at night’. Put the Latin for non-directional reaction to nighttime together and you get nyctinasty.
Why do flowers close at night?
We don’t know. Charles Darwin thought that nyctinasty was used to protect against freezing. Some scientists theorize that it has to do with pollination and reducing competition, or protecting nectar from bacteria and fungal spores. Other possibilities include saving up aroma molecules for when they will be most effective, energy conservation, or as a means to prevent pollen from getting wet. Wet pollen is heavy and insects are less likely to carry as much, potentially reducing pollination rates. Yet another theory is that nyctinastic flowers and leaves close up shop to prevent being eaten by nighttime herbivores. The truth is, we don’t know why. We do know, from laboratory tests, removing the gene that causes nyctinasty results in plants with smaller leaf areas and reduced biomass. We may not know why they do it, but we do know how they do it.
How flowers and leaves open and close
Several different flowers, such as tulips, dandelions, crocuses, and daisies, and the leaves of many legumes species, open each morning and close each night. Some flowers, particularly the Kalanchoe genus, grow new or longer cells each morning, on the inside of the flower, to open it, and on the outside of the flower each evening, to close it. Other flowers, and most nyctinastic leaves, rearrange fluids within the plant to cause these movements. This movement of fluids is a reaction to changing temperatures and light frequencies.
Movements of liquid and light
Nyctinasty is triggered within a plant in response to changes in external light, temperature, and humidity, and an internal circadian clock. As the sun sets, light frequencies change and temperatures drop. The shorter wavelength and higher deflectability of blue light gives way to longer, less readily deflected red wavelengths, and lower temperatures. The reduction of blue light triggers blue-green pigments (phytochromes) to rearrange potassium ions within the plant. This rearrangement of ions pulls water along with it, causing turgor. Turgor refers to rigidity that is normally caused by the presence of fluids.
So, as dawn arrives and temperatures start to rise, interior cells grow faster, or are inflated with water, to push open your flowers. At days end, outer cells grow longer and faster, or internal cells are deflated, and the flowers close for the night. This is nyctinasty.
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