For anyone interested
WHEN THE LIGHTS GO OUT
By Keith Roberto
and Brandon Mathews
Everyone knows that plants need light for photosynthesis. What they don’t know is that plants need darkness, too! But why? Are they trying to get a restful sleep for a busy day of photosynthesis? Not many people try to grow plants in continuous light. It seems we all have a hunch that the dark cycle is an important part of a plant’s life, but what are they really doing? This article will shed some light on the mysterious and often misunderstood dark cycle.
All plants have complex energy generating systems that function both in sunlight and in the dark. However, these reactions are coupled and rely on the products and intermediates produced by each biochemical process, day or night. In short, plants use light energy, water and CO2 during photosynthesis to generate sugar and oxygen that is later metabolized by the dark reactions to generate cellular CO2 and energy. Carbon dioxide generated in the dark cycle is used as the carbon source for maintenance molecules and some is even expelled by the plant. There are many common misconceptions regarding the role of CO2 in the dark, but it will soon become clear what plants do without their beloved sunlight.
We must keep in mind that plants are pre-historic and have developed complex metabolic systems to adapt to an ever changing environment. Plants used to enjoy an atmosphere of highly concentrated carbon dioxide before they did us a favor and converted it to oxygen. As the globe varies greatly in temperature, humidity, and light conditions, plants have diversified to cope with their geographic neighborhood. Forced to adapt to modern times, plants now have specialized systems to utilize the relatively low concentration of atmospheric CO2, around 0.036% or 360ppm. To best provide for any plant species, an artificial environment should closely resemble their natural conditions. Once these conditions are understood, further steps can be taken to enhance plants’ metabolic activity.
When the sun goes down, a greenhouse environment undergoes a few fundamental changes such as a shift in light wavelength and a decrease in temperature. As the sun sets, the wavelength of light generated by the sun shifts from blue to red. During the day, photosynthesis is most efficiently propelled by blue light (450nm) because it is a shorter wavelength and thus carries more photon energy. At sunset, red light (650nm) initiates a sequence of chemical responses that trigger essential metabolic processes to begin. Similar to humans, plants spend the day gathering energy (money) and generating (buying) food. In the evening they metabolize this food to provide their cells with the energy they need to form new cells, repair damaged cells, produce important enzymes and proteins, and prepare themselves for sunrise and photosynthesis. Essentially, they carry out cyclic processes known as a circadian rhythm, from Latin meaning “approximately a day.”
All cellular events require metabolic energy, primarily in the form of ATP or NADH. These high energy molecules are manufactured by many biochemical processes, as plants have evolved to scavenge energy at all periods of the day. Photosynthesis is the process by which a plant uses light energy to break apart water, generating O2, protons and electrons. Oxygen is the magical energy transporter in all forms of aerobic respiration, and is used to transfer electrons in the production of the energy rich molecules ATP and NADH. Coupled to the products of photosynthesis, the Calvin Cycle fixates CO2 to generate 3-Carbon sugars during the light cycle. These sugars are later converted into 6-Carbon sugars like glucose and fructose, the primary substrates used to make cellular carbon and the bulk of ATP and NADH during aerobic respiration of the dark cycle.
As fragile as plants appear to be, they are dedicated survivors and thrive in a wide range of light and temperature conditions. Temperature is as important a variable as light because it directly affects humidity, dissolved gas concentrations, water stress, and influences the ratio of water loss to carbon fixation. Changes in the leaf are most prevalent because they are the primary site of light absorption, sugar formation, and gas exchange. During the night, stomates in the leaf are nearly closed as the need for gas exchange is small and to prevent unnecessary water loss. During the day when photosynthesis is in full swing, the demand for CO2 uptake is great and stomata are wide open. Unfortunately, high temperatures increase water loss through the same stomatal openings that are trying to uptake CO2. Therefore, photosynthesis is both temperature and light dependent as an increase in temperature reduces the amount of carbon that is fixed, or carboxylated, into sugar by the Calvin Cycle. Photosynthesis reaches a maximum rate at a temperature of 30°C (85ºF) and remains efficient ± 5°C (75-95ºF).
The leaf is a very complex organ. Stomates are surface pores on the underside of the leaf that are regulated by guard cells that vary the size of the pore in response to environmental cues. Water and CO2 cannot be simultaneously transported through the narrow stomata. Fortunately, during the day when water is readily available, many stomata are dedicated to CO2 uptake rather than water transpiration. This factor is known as the Transpiration Ratio. In a typical C-3 plant, approximately 500 molecules of water are lost for each single molecule of CO2 fixated by a leaf. The most abundant protein in the leaf, around 40%, is the one responsible for CO2 fixation, known as ribulose bisphosphate carboxylase/ oxygenase, commonly called rubisco and abbreviated RuBP. As the chemical name suggests, this protein is capable of accepting both CO2 and O2. This is a competitive reaction, but fortunately, RuBP has a much higher affinity for carbon dioxide than oxygen. Throughout a typical day, carboxylation occurs three times more than oxygenation of RuBP.
There are a few barriers to CO2 uptake in a leaf. The first is boundary layer resistance where a thin, unstirred layer of air on the under surface of the leaf reduces CO2 diffusion. This resistance decreases with leaf size and wind speed. The second is intercellular air space resistance which hinders the diffusion of CO2 between layers in the leaf. The third, and major contributing factor, is stomatal resistance, which is a direct regulation by the stomata to gas exchange.
Temperature has a direct affect on the transpiration ratio. Not only does heat induce water loss through stomata, an increase in temperature also reduces the concentration of dissolved CO2 in air, thus favoring oxygenation of RuBP rather than carboxylation. This negative effect is known as photorespiration, the use of oxygen instead of carbon dioxide. Be careful not to confuse this term with aerobic respiration which is the process of glycolysis, the breakdown of sugar to generate metabolic energy which will be discussed later. Shade plants have more chlorophyll per unit area and also have very low photorespiratory rates. Sun plants have more rubisco per unit area and can handle a higher photosynthetic load.
It is always a good idea to supplement a greenhouse with CO2 during the light cycle when stomata are open and gas exchange is readily occurring. Simply doubling the ambient concentration to 700ppm will increase the photosynthetic rate by 30-60%. At optimum light and temperature conditions with supplemental CO2, photosynthesis is only limited by the ability of the Calvin Cycle to regenerate the first sugar acceptor molecule, ribulose-1,5-bisphosphate. On the other hand, in low CO2 concentrations more carbon dioxide is given off during aerobic respiration at night than diffuses into the leaf during the day. This ratio is known as the CO2 compensation point.
Why would the rubisco protein have evolved to use both CO2 and O2? Plants are highly adaptable and need to be able to thrive in tropical conditions of great light intensity and high nighttime temperatures that favor water loss and low ambient CO2 concentration. Even a typical environment can have extreme conditions out of the average range. In addition to the Calvin Cycle to fixate carbon dioxide, plants have a backup mechanism that recovers lost potential when oxygen associates within the active site of RuBP. The Photorespiratory Carbon Oxidation cycle (PCO) is a minor process that converts oxygenated RuBP into a small amount of cellular CO2 by rearrangement of the amino acids glycine and serine.
In fact, there are a few mechanisms by which plants concentrate intracellular CO2. The previous information is primarily regarding a typical tomato plant or flower, the C-3 class of plants in which photosynthesis produces a 3-Carbon sugar. Other classes of carbon fixation include C-4 and CAM processes of desert and grasslike plants that live in the hottest and driest conditions. The stomata of these plants are closed during the day and open at night to make the most efficient use of water. Because there is little to no photosynthesis occurring in the dark, the uptake of CO2 is low, and these C-4 and CAM mechanisms concentrate carbon dioxide to be used by the Calvin Cycle.
During the dark cycle, plants undergo aerobic respiration. Respiration is divided into three parts: Glycolysis, the Kreb or Citric Acid cycle (TCA), and the Electron Transport Chain. Glycolysis is the breakdown of sugars to shuttle smaller sugar molecules and intermediates to the Kreb Cycle. The Kreb Cycle then generates cellular CO2 and energy rich molecules like ATP, NADH and FADH. These energy carriers are then incorporated into the electron transport chain, coupled to the protons and electrons produced during photosynthesis to establish a proton gradient across the chloroplast thylakoid membrane, similar to a battery. The Kreb Cycle generates on average 34 molecules of ATP per 6-Carbon sugar. This represents a net ATP gain as many more molecules are produced than consumed in all other metabolic processes.
Red light plays an important role in the regulation of the dark cycle. Red light is the color of the rising and setting sun. Plants temporally govern most biochemical processes by a circadian rhythm, a type of internal biological clock. In a natural environment this rhythm is set to a 24 hour cycle, although a plant can be trained to operate on however many hours a light and dark cycle add up to. Interestingly enough, it is rhythmic because even in constant darkness the biological functions persist in a cyclic fashion, although if left in complete darkness over time the rhythm does fade away. Such processes include leaf movement, flowering and ripening response, and the regulation of enzymes and hormones. The main protein responsible for this response is known as phytochrome.
Phytochrome, abbreviated Pr , is converted to its active form, Pfr , upon irradiance by red light (650nm). Conversely, it can also be reconverted and deactivated by irradiance of far-red light (720nm). The activity of phytochrome is not solely dependant on its active form, but rather on the ratio of Pfr to the total phytochrome concentration. In this way, plants can sense the movement of the sun and the length of day. In addition to absorbing in the red light region, phytochrome also shows a slight response in the blue-light region (450nm). In combination with other blue light photoreceptors, this response is responsible for solar tracking of leaves as the sun moves through the sky.
The flowering response has been determined to be a result of the length of darkness a plant receives. Inversely, a plant that flowers with short nights are termed Long Day Plants (LDP). A typical vegetable plant that matures in early Fall, when nights become longer, are termed Short Day Plants (SDP). Because plants are adapted to absorbing whatever photons they can, whenever they can, as in a shady forest, interrupting the dark cycle with light can dramatically alter its circadian rhythm. SDP are more sensitive to this response than LDP. Just a five minute irradiance can have an affect, whereas a LDP would need about one hour of light interruption to take affect.
Regulation of a plant’s energy metabolizing systems function on many levels. A biochemical pathway can only proceed as fast as the rate limiting enzyme or substrate. The primary source of regulation is genetic. Chloroplasts and mitochondria have their own genetic code that produce the enzymes needed for their respective process. The only way to up-regulate genetic expression is either through genetic engineering or producing more of these genes by making sure the plant has all its required nutrients to produce more new cells. Another mode of regulation is through the limiting pathway intermediate, as mentioned regarding CO2 supplementation where the limiting factor becomes the regeneration of ribulose-1,5-bisphosphate. Unfortunately, the regeneration of this substrate is also regulated by the electron transport chain. Sometimes a limiting reactant can be artificially added to increase metabolic activity, as in the addition of amino acids, hormones and cofactors like trace vitamins and minerals. Ultimately, the major mode of regulation is environmental. Changes in water properties, nutrient availability, temperature, light duration and strength, humidity, and dissolved gas concentrations are big obstacles that need to be orchestrated to achieve maximal metabolic activity.
As one can see, plants are definitely not getting a restful sleep at night. To keep up with our demand for their products and beauty they need to work around the clock. Plants have concrete biochemical processes and care should be taken to provide the proper environment. One cannot expect a plant to flourish as if by magic. After all, we all have our own personal needs and your plants do too!
