Best Photoperiod for Growing Plants

Colin Clark, Technical Horticulturist
Cannabis Greenhouse

There seems to be some debate about whether plants will grow better under 24hrs of light, and today we're breaking down our thoughts. To better understand this topic we need to know some basics about photosynthesis. As many know photosynthesis is the process of converting light energy into chemical energy for plants and other organisms. In basic terms light, carbon dioxide and water are used to create sugar (glucose) and oxygen. The equation is below:

Photosynthesis Equation

So presumptuously one could conclude that the more CO2, H2O and light you throw into the equation, the more sugar and oxygen is created which could promote more growth. This is partially true, yes more sugar and oxygen are created, but photosynthesis is both a pretty complex and relatively inefficient system. There are a lot of factors that come into play that determine plant growth and everything has a fine balance. If you dive deeper into photosynthesis you’ll find two main reactions at work, Light Dependency and Light Independency.

These concepts are where many people get confused partly because these terms have also been referred to as Light and Dark reactions. Light and Dark reactions are  inaccurate descriptions. A very important key to remember here is that BOTH of these reactions can occur in the presence of light. This is why plants CAN survive under 24hrs of light, however it’s not beneficial.

Plant respiration is the key process in converting all of the sugars created during photosynthesis into chemical energy such as ATP that plants use to power all functions required to survive and grow. You can think of respiration as the opposite of photosynthesis, and it follows the equation below;

C6H12O6 + 6O2 → 6CO2 + 6H2O + 32 ATP (energy)

Furthermore there are two types of respiration; dark respiration and photo respiration. Dark respiration can occur in both the presence and absence of light, while photo respiration ONLY occurs in the presence of light. It's also worth noting that photo respiration only occurs in C3 plants, not in C4 plants. Keep in mind though that over 90% of green plants on this planet are C3 plants, and some of the more notable C4 plants are Corn, Sorghum and Sugar Cane. Without going into the drawbacks of RuBisCo production just know that photo respiration is not a good thing for plants. It is considered a wasteful process and competes directly with the Calvin Cycle.

So, we now understand that plants need light for photosynthesis to create sugars, and we know that plants need respiration to convert those sugars into usable plant energy. We also know that plants CAN respire in the presence of light and dark and we also now know that respiration in the presence of light is detrimental to overall plant efficiency. So this alone should answer our question as to why plants do not perform better under 24hrs of light, however it’s much more complicated than that.

Darkness promotes so many more metabolic activities that are vital to proper growth of plants. For starters, plants utilize photoperiodism to regulate their flower functions and maintain proper circadian rhythms, the latter has been connected to many important cellular functions within plants. Plants that undergo 24hrs of light are in a constant process of creating sugars, this excess of sugars can not be properly utilized and eventually the plant will undergo stress to deal with the unprocessed sugars. Remember photosynthesis is not very efficient, and somewhere around 5% or less of sunlight energy is converted into actual biomass. So just throwing more light at a plant isn’t going to cause this process to become more efficient, in fact the opposite will occur, with photorespiration becoming the predominant form of respiration. For simplicity sake I will not go into the many other more complicated hormonal activities that benefit plants from dark periods, but let’s just say there is plenty of science that backs up the confirmation that plants DO indeed require a period of darkness to reach their full potential.

“I’ve been growing plants under lights 24/7 for years and they do fine!” I hear this and many other anecdotes quite often and my response is always, “Yes, they will do fine, but they will always do better when given proper dark periods.” For example some annuals such as cannabis can live for less than 12 weeks so they can avoid some of the long term damage that other perennials can experience from lack of a dark period. Cannabis that endures stress can also result in higher accumulation of Cannabinoids. So while it is possible to grow highly stressed plants under 24 hrs of light and see slightly higher Cannabinoid content, these plants will not be healthy and will be more prone to disease, deficiency, etc. It is never a suggested method for cannabis cultivation. There are easier methods of utilizing stress induction to increase Cannabinoids than running lights 24/7. Another huge factor to keep in mind especially at a commercial scale, is the amount of wasted energy and wear on your lighting and equipment. Running lights 24/7 means constant electrical draw, constant cooling, more water consumption, more nutrient usage, etc. All of this adds up very quickly and will easily overshadow ANY possibilities of increased growth/yield, which we’ve already shown here to not even be a possible benefit.

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