Nyctinasty is a rhythmic movement of plants that responds to the onset of darkness, or a "sleeping" plant. Hibiscus, a nyctinasty plant, has a circadian cycle as they open their tracts during the day, and close them at night. The movement of the hibiscus flower is accomplished by a change in electrolyte concentration causing fluid movement and changes in turgor pressure throughout the plant.
The initial stimulus such as a lack of light in photoreceptors causes an electrical signal to be transmitted to neighboring cells in a plant. This causes a change in the turgor pressure of certain cells in the pulvinus to allow the leaf to bend at the top. On top of stimulus calcium-permeable anion channels open to allow the flow of calcium ion to the cytoplasm of the cell causing it to slow down. This electrical signal is distributed down the phloem to neighboring cells as successive calcium channels open up. In response to changes in the potential voltage of the membrane, potassium channels and digested chlorine open up causing ions to leak out. The increasing concentration of ions outside the cell creates an electrochemical gradient that releases water from the cell through osmosis. Aquaporins and hydrogen ion ATPase also help with the movement of water molecules. This causes a change in turgor pressure as fluid flows from the flexor cells in the pulvinus and into the extensor cells to allow the leaves to bend to close the flower. The hibiscus nyctinasty method is an example of a plant movement to improve durability.
Not all types of plants show nyctinasty, some are only seen in the movement of leaves while others are in bloom. Nyctinasty in flowers alone can be divided into several stages: flowering of the day vs. Night bloom, unity vs. a repeated flowering cycle, or a combination of two. In Genus Hibiscus we see a lot of one-day flowering flowers with certain hybrids that can reach repetitive cycles. It is believed that the specific flowering cycle is a form of protection and reproduction, with many species in cold climates closing their flowers at night to protect them from frost while some desert species have flowering plants at night to prevent further water loss. Predatory animals and pollinators are also major factors that contribute to flowering cycles; some flowers are closed at night to prevent predators from other night owls as opposed to flowering flowers at night that rely on nocturnal pollinators.
Darwin's experiments examined foliar nyctinasty to show that it is an evolutionary process for improving fitness. His experiments suggest that FN can affect the leaves' ability to moderate their radiation exposure by reducing leaf exposure in the cold night sky and increasing the exposure of other plants to extra heat to avoid frost damage and to stay warm. He performed experiments when he attached Oxalis and Trisodium leaves horizontally to the ground compared to straight leaves to show that the leaves were horizontal, showing greater damage to snow than their warm, vertical counterparts.
Nyctinasty in hibiscus plants is a way to protect against bad conditions as cold temperatures can be dangerous. In the absence of light stimulation and circadian rhythms the plant is able to initiate ion movements to allow flowering to close.
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