Light Effect on Growth
Plants are the primary producers of energy in any ecosystem, meaning that they bring in new energy to it which supports the life that lives there. For plants the produce their own energy, they require sunlight to conduct photosynthesis.
Critical Period
In today's diversified culture of plants' varying species require different amounts of light in a day. The amount of time that a plant requires is called the critical period. Some plants, for instance, require more than 14 hours of light, and without this amount of light on a regular basis, their stalks become etiolated and the leaves of the plant fail to grow to their potential.
This is partly because their is not enough light available for photosynthesis and therefore not enough ATP available for cellular processes and growth.
Auxin also plays a part, as light destroys auxin, plants that are immersed in light have cells that do not become as elongated producing a weak stem. Plants who require more than 12 hours of light are deemed 'long day short night plants' because of their light dependant nature.
On the other side of the coin, some plants require longer periods of darkness, these being 'short day long night' plants. These type of plants only flower when the level of sunlight is below 12 hours, or more precise, there is more darkness than sunlight.
Photoperiodism (the length of light that each plant will react to) is stimulated by a pigment called phytochrome. Various forms of phytochrome react to these varying wavelengths, depending on which wavelength of light is absorbed by the plant. There is a constant reaction involving phytochrome in plants, which is the plants' indicator of whether it is daytime or night time. Phytochrome 660 is in abundance in the plant during the day, and converts itself to phytochrome 730 on absorbing light. This unstable form of phytochrome reverts to its original form during the day and once again the cycle starts all over again. This instance can control the degree of photosynthesis that occurs in the plant, while phytochrome is visibly one of the ways that plants react to sunlight. Phytochrome therefore has a regulating control on how and when photosynthesis occurs, and phytochrome levels during different times of the day will react variously within different species of plant.
When looking at Darwin's outlook on speciation and adaptive radiation, it is more plausible now to see why plants possess differing characteristics suited to their external environment and their needs. The latitude of where a plant exists also has great bearing on how they will cope in their environment, because, for instance, in the extreme latitudes, there is not sunlight for 6 months, and the following six months their is constant light. Seasons also play a part, and considering both seasons and latitude into account, it is evident why different climates make homes for different varieties of plants.
Plants have a balancing act between flowering, growing and producing energy for the mass already present on the plant. These chemicals, i.e. indole acetic acid, gibberellin and phytochrome all play their part internally, while reacting to the outside environment. With this in hand, all these reactions originate from one thing, the genetic coding that made the plant operate the way it does.
http://www.biology-online.org/3/9_effect_light.htm
There are no strains of cannibas that come from any where that there is not a period of darkness per each 24 hour period of time. While the areas where Ruderalis originate have very long photo periods during the summer even they do not grow in areas where there is 24 hours of sun shine out of each 24 hour period. This means they have never evolved to deal with 24 hours of darkness. They have evolved to deal with long hours of light and to short frost free growing seasons but not to 24 hours of light.
Also: (Assorted online article statements.)
Yes, plants do need darkness to grow.
First, in the photosynthesis process itself, there is a reaction known as 'dark reaction' pathway or lately known as 'carbon reaction' pathway where the free energy of ATP and reducing power of NADPH, are used to fix and reduce CO2 to form carbohydrate. This is very important process to release Oxygen into the air. This happened in the dark or at night.
Most plants seem to require a dark period, some scientists believe, to turn the products of photosynthesis into usable forms of energy.
IMHO 24 hours of light is often used to try to force (stress) flowering plants back into the vegative cycle. That in it self means gross hormonal and chemical changes must be caused by the 24 hours per day of light so as to force the plants to change. Why force (stress) a plant to do something it need not do especially when it slows growth while doing so.
Will pot plants survive under 24 hours of light? Sure they will put up with a lot of poor treatment and adverse growing conditions such as no period of daily darkness, but that does not mean they will thrive.
Plants must carry out respiration proportional to growth.
Cellular respiration: food molecules are broken down to release the energy in their chemical bonds.
Even though glucose is a source of energy for living things, Adenosine triphosphate, ATP, is the energy molecule used by most organisms. Energy is released when ATP is changed to ADP. This energy is then available for cells to carry on their life processes.
Only a very small amount of respiration occurs while plants are receiving light. The majority of repiration occurs at night. Chemically, cellular respiration is the exact opposite of photosynthesis.
While photosynthesis occurs only in some cells, respiration occurs in all cells. Limit respiration and you limit growth. Period.