When the sun goes down, the absence of light signals a major shift from energy production to maintenance and development. The daytime period, called the photophase, is dominated by photosynthesis, which uses light to build sugars. The scotophase, or dark period, is when the plant utilizes those stored sugars for essential activities across its entire structure. Plant life is a constant, dynamic process, not a simple on/off switch based on sunlight.
Energy Consumption Through Cellular Respiration
When photosynthesis ceases, cellular respiration becomes the sole source of metabolic energy for the plant. This process occurs continuously, day and night, within the mitochondria, breaking down carbohydrates created during the day. During the scotophase, the plant consumes oxygen and releases carbon dioxide, effectively reversing the net gas exchange of the day.
Respiration converts stored glucose and oxygen into usable energy in the form of adenosine triphosphate (ATP). ATP is the energy currency needed to power all living processes, including nutrient transport, DNA replication, and new cell growth. The process begins with glycolysis in the cytoplasm, where glucose is split before entering the mitochondria to complete the energy conversion.
The rate of respiration may decrease by an average of 25% over eight hours of darkness in some species. This reduction reflects a shift from growth-related energy use to simple maintenance functions as the night progresses. This continuous breakdown of stored sugars is necessary to keep the plant running until sunrise.
Plants that use Crassulacean Acid Metabolism (CAM), such as succulents, open their stomata at night. They take in carbon dioxide during the dark hours and store it as an organic acid, using it for photosynthesis during the day when stomata are closed. This strategy conserves water, making the dark period processes important for these desert-adapted species.
Sensing Time and Regulating Development
The dark period serves as a precise environmental signal used to regulate development and seasonal timing. Plants possess an internal biological clock, the circadian rhythm, which creates cycles of activity lasting approximately 24 hours, even without external light cues. This clock is temperature-compensated, meaning it remains accurate across a range of temperatures, providing a reliable internal sense of time.
The internal clock synchronizes the plant’s physiological processes, such as gene expression and enzyme activity, with the external day-night cycle. Synchronization is achieved through light and temperature receptors, which adjust the clock’s timing. The clock then regulates gene expression to control when specific activities occur.
A primary use of darkness is photoperiodism, the physiological response to the relative lengths of the light and dark periods. Plants use specialized photoreceptors, such as phytochrome and cryptochrome, to measure the length of the scotoperiod. This measurement dictates seasonal events like flowering, leaf shedding in autumn, and bud dormancy.
Photoperiodism determines flowering, categorizing plants based on their required dark duration. Short-day plants, such as chrysanthemums, only flower when the night period is longer than a certain threshold. Long-day plants, like spinach, require a night period shorter than a particular length to initiate flowering.
Water Uptake and Nutrient Redistribution
During the day, water movement is driven by transpiration, the evaporation of water vapor through stomata. When plants close their stomata at night to conserve water, the transpirational pull ceases. This cessation allows another force, root pressure, to become prominent.
Root pressure is generated by the active transport of mineral ions from the soil into the root’s vascular tissue, the xylem. This accumulation of solutes lowers the water potential inside the root, causing water to diffuse into the xylem via osmosis. The resulting influx creates a positive pressure that pushes water and dissolved minerals upward through the stem.
This pressure is measurable and, in smaller plants, can cause guttation, where droplets of xylem sap are exuded from specialized pores on the leaves. This process ensures that water and minerals are continuously supplied, even when the daytime mechanism of transpiration is inactive. While water moves upward, the phloem tissue transports and redistributes energy reserves.
Carbohydrates manufactured in the leaves, primarily sucrose, are actively loaded into the phloem for transport to non-photosynthetic areas. This process, known as phloem loading, often accelerates in the dark, moving energy from “source” leaves to “sink” areas like growing roots or storage organs. The rapid transport of these sugars ensures energy is delivered where it is needed for maintenance and growth overnight.