The Relationship between Light, CO2, Temperature and Relative Humidity in Photosynthesis
Photosynthesis defines the process by which plants manufacture glucose which becomes the building blocks for growth. Scientists summarize the process as follows: using light, carbon dioxide + water = glucose + oxygen. The process occurs within special structures called chloroplasts located in the cells of leaves. Optimum photosynthetic rates lead to the removal of greater amounts of carbon dioxide from the local atmosphere, producing greater amounts of glucose.
While photosynthesis involves a series of complex biological and chemical processes, put simply, the rate of photosynthesis determines the rate of growth. A plant which is photosynthesizing at optimum will grow and produce at optimum levels while a plant photosynthesizing at below optimum will grow and produce at below optimum.
The key factors affecting the rate of photosynthesis are light intensity and colour, carbon dioxide concentration and temperature. In any given situation any one of these may become a limiting factor; in other words a deficit of CO2 and/or light, and/or excessive or too low air temperatures directly affects the rate at which photosynthesis can take place.
Light and rate of photosynthesis
Light is the energy of photosynthesis; it powers all aspects of plant growth. Without light there would be no green plant life on earth.
Energy = Energy
Light is primarily responsible for creating chemical energy (in the form of glucose) in plants. Glucose then becomes the fuel for other agents of plant growth.
This process is called photosynthesis. Photosynthesis is the fundamental food making process in all green plants.
Light is the energy source for the complex chemistry of photosynthesis.
In a nutshell: The plant takes in water and nutrients from the growing media and carbon dioxide (gas) from the air, and in the presence of chlorophyll it makes the simple sugar glucose, which is now the store for light energy. In this process oxygen is separated from the water and passes out into the air.
Glucose is the building block for several other sugars and complex carbohydrates that the plant makes (e.g. fructose, sucrose, starch). The glucose is stored in the leaf tissue. In addition to this, it is the leaves that catch and trap the light energy. Therefore, leaves receive light and store it as chemical energy.
The light energy required by plants is confined almost entirely to the visible spectrum of light (400nm – 700nm).
While there are key points within this spectrum (435nm and 675nm etc), growth is optimized under the entire range of the spectrum. This is because different color wavelengths stimulate different biochemical reactions within the plant. As a result of this, different physiological functions are activated and energized, which, in turn, determine plant growth rates and formation (morphological) characteristics.
Photosynthesis depends on the energy created by a combination of both light intensity and color.
Photosynthesis is a cumulative process. During the daylight hours a plant traps and stores light energy. This process can be likened to water dripping into a bucket. Over several hours (let’s say 18 hrs) the bucket slowly accumulates water. When the bucket is full, the water which continues to drip into the bucket can no longer be captured and goes to waste. However, if the tap is dripping too slowly the bucket doesn’t become full within our 18 hour timeframe.
The accumulation of light energy is similar to this. If the plant receives adequate levels of light it is able to fill itself with chemical energy. In a situation where there is not enough light the plant will only become partly filled with chemical energy. If the plant receives more light than it is able to use, the excess light goes to waste.
Most plants experience optimum growth in high light intensities. However, a single leaf is light saturated at a much lower level than is required to saturate an entire plant. The higher intensities of light are needed to compensate for light shading (light shading refers to areas of the plant that only receive low levels of light, or no light, due to foliage growth inhibiting the access of light). The plant draws on the excess light in order to fulfill the needs of the entire plant. That is, the plant is able to draw in higher levels of light than is required by the immediate leaf area and then distribute the light energy throughout areas of the plant that are shaded by foliage growth.
The parts of the plant that are open to light, therefore, can be described as receivers for energy and conductors of energy to the larger body of the plant. Too much shading or not enough light will greatly limit this process.
Again, let’s consider our bucket example: here we should think of our bucket as not a single bucket but, instead, many smaller buckets which we wish to fill. This time our dripping tap can quickly fill a single small bucket. However, in order to fill all of the smaller buckets our dripping tap still has to fill the equivalent volume of the larger (single) bucket.
In this example only some of the buckets can catch water from the dripping tap. However, the buckets that are able to catch water can also release water into the buckets that aren’t catching water from the dripping tap. This means that some of the buckets have the role of ensuring that all of the buckets are filled.
If sufficient numbers of the buckets are able to catch water they will slowly fill all of the buckets (within the given timeframe). If only a few of the buckets can catch water then all of the buckets will only be partially filled.
The cumulative process of plants trapping light and storing light as chemical energy is similar to this. If you like, a plant’s leaves are a series of buckets that we wish to fill with chemical energy within a given timeframe; the leaves that receive the light distribute chemical energy throughout the plant for storage. This means that the more leaves that are able to trap light, the better the photosynthetic potential. Therefore: Light color + light intensity + surface area of plant exposed to light = chemical energy potential.
We’ll be covering a great deal more information on light later in the book so let’s leave that one there for now.
Carbon dioxide and rate of photosynthesis
Light provides the energy for photosynthetic pigments to convert carbon dioxide (CO2) and water into sugars and oxygen. As light intensity increases, up to a point where the machinery of photosynthesis – the chloroplast – can no longer convert the light into chemical energy, the amount of sugar increases and thus, more energy is available for plant growth and maintenance. However, the concentration of CO2 also influences photosynthesis in a dramatic way.
To simplify things somewhat, plants consume CO2 and release oxygen during the day as part of the process of photosynthesis (carbon dioxide + water → sugar + oxygen). At night they consume oxygen but don’t release oxygen. Instead they release CO2 as part of the process of respiration (sugar + oxygen → carbon dioxide + water). Therefore, plants require CO2 during the day for photosynthesis and oxygen at night for respiration.
During photosynthesis CO2 enters the plant through small openings in the leaves called stomata. It is then captured or ‘fixed’ by photosynthetic enzyme Rubisco and is then converted into carbohydrates. When atmospheric CO2 concentration goes up, more CO2 will enter the leaves of plants (photosynthetic/growth rates will increase) because of the increased CO2 gradient between the leaf and the air.
CO2 plays the most important role in the biomass production of plants because more than 90% of dry matter of living plants is derived from photosynthetic CO2 assimilation. Plants use the carbon from CO2 and convert it into carbon compounds such as glucose, carbohydrates, lignin, and cellulose which is what becomes the biomass of the plant.
An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction, and so the rate of photosynthesis generally increases until limited by another factor or until the point at which critical mass is achieved and additional CO2 cannot be used by the plant. I.e. think of the plant as a growth factory. The factory can only operate so efficiently. The machinery of photosynthesis (the chloroplast) has its limits. There comes a point where more CO2 simply goes to waste.
Air Temperature and rate of photosynthesis
During photosynthesis CO2 enters the plant through small openings in the leaves called stomata where it is then captured or ‘fixed’ by photosynthetic enzymes and is then converted into carbohydrates. When air temperatures become excessively warm a plant may close its stomata to reduce water losses. When ambient conditions are excessively warm for a plant and it closes its stomata for too long in an effort to conserve water it has no way to move carbon dioxide and oxygen molecules resulting in less than optimal photosynthesis.
Additionally, photosynthesis takes place in two stages which are light-dependent reactions and the light-independent reactions of the Calvin cycle. The light-independent reactions of photosynthesis are dependent on temperature. They are reactions catalysed by photosynthetic enzymes.
Enzymes are protein molecules used by living organisms to carry out biochemical reactions. The proteins are folded into a very particular shape, and this allows them to bind efficiently to the molecules of interest. At lower than optimal temperatures, the enzymes that carry out photosynthesis do not work efficiently (enzymatic activity decreases), and this decreases the rate of photosynthesis.
As temperature increases the enzymes approach their optimum temperatures and the overall rate of photosynthesis increases until a threshold at which maximum photosynthesis occurs. As temperature rises above optimum, enzymatic activity decreases until such a point where temperatures become so high that the enzymes are denatured (destroyed).
Thus, at below or above optimum temperatures the rate of photosynthesis decreases.
Generally speaking, the optimum range for daytime temperatures for heavy flowering indoor crops tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties. Following is a graph that demonstrates the rate of photosynthesis in relation to leaf temperatures.
Source: Chandra, S. Lata, H , Khan, I. A. and Elsohly, M. A. (2008) Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions
Heat and Photorespiration
Rubisco is a key enzyme in photosynthesis catalyzing carbon dioxide fixation. Rubisco is ubiquitous for photosynthetic organisms and is regarded as the most abundant protein on earth
The simple view of photosynthesis is that it involves carbon dioxide (CO2) uptake and release of oxygen (O2). Unfortunately it is not quite this simple. Even during photosynthetic CO2 uptake (‘fixation’), some CO2 is simultaneously released by the process of photorespiration. Photorespiration is a consequence of the high oxygen content of the air, which leads to a competing oxidation reaction at the same site as carbon fixation, resulting in loss of carbon and energy from the plant. This is largely seen as a wasteful process where the rate of carbon fixation is reduced.
Any factor that reduces the availability of CO2 or increases the availability of O2 to rubisco will increase the levels of photorespiration.
One factor that increases photorespiration is high air temperatures.
The decrease in photosynthesis rate, or rise in photorespiration, as temperature increases is due to an increase in the affinity of rubisco and oxygen. Rubisco combines more with oxygen relative to carbon dioxide as temperature rises, which slows the rate of photosynthesis. In other words, rubisco acts mainly as a carboxylase (combining with carbon dioxide) at lower temperatures but acts more as an oxygenase (combining with oxygen) at higher temperatures.
Studies have consistently shown that the rate of photorespiration is decreased with cooler air temperatures.
In understanding the impact that temperature has on the rate of photorespiration you can perhaps see that there is a very narrow range in what would be considered optimum temperatures to promote optimum growth. Cool temperatures will decrease the rate of photorespiration; however, cool temperatures also decrease the rate of photosynthesis. As temperature increases the rate of photosynthesis increases but so too does the rate of photorespiration. At warmer than optimum temperatures the rate of photorespiration increases to a point where optimum rates of photosynthesis are compromised as the plant acts more to oxygenase, resulting in lower CO2 fixation and growth rates.
In understanding this it is also important to understand that different genetic variants of the same species of plant will have an optimum temperature which promotes optimum growth. For this reason, while we can generalize somewhat and say optimum growth is likely to occur between the temperature range of 26 – 28oC (78.8 – 82.4oF) it is important to understand that the strain you are working with may perform best at slightly higher or lower temperatures than those which are typically stated as optimum in literature. For example, some tropical varieties, which evolved in warmer climates, tend to perform best at 29 – 30oC (84.2 – 86oF) while some cooler climate, Northern Hemisphere varieties tend to perform best at 26 – 27oC (78.8 – 80.6oF). Therefore, it is recommended that you cautiously experiment with growroom temperatures to identity what is the sweet spot for promoting the highest level of growth with the particular genetics you are working with.
Heat Stress in Plants
High leaf temperatures reduce plant growth and limit crop yields. In outdoor grown crops, estimates range up to a 17% decrease in yield for each degree Celsius increase in average growing season temperature. Even moderate heat stress can reduce the photosynthetic rate to near zero in plants.
Damage to leaves (plant tissue necrosis) can be caused by reactive oxygen species (ROS). For example, rubisco can make hydrogen peroxide (H2O2) as a result of oxygenase side reactions. H2O2 production by rubisco was recently shown to increase substantially with temperature. Overproduction of reactive oxygen species (hydrogen peroxide, H2O2; superoxide, O⋅-2; hydroxyl radical, OH⋅ and singlet oxygen, 1O2) can cause oxidative damage to plant macromolecules and cell structures, leading to inhibition of plant growth, or even to plant death. 
Visual heat injury symptoms in plants include scalding and scorching of leaves and stems, sunburn on fruit and flowers, leaf drop, rapid leaf death and a reduction in growth.
Daytime versus Night Temperatures – Thermoperiod DIF
While not strictly relevant to photosynthesis per se, one cannot discuss optimum daytime (lights on) temperatures without also talking about optimum night (lights off) temperatures.
An often overlooked environmental factor that can greatly impact yields is the DIF, or the day/night temperature differential which is also referred to as the thermoperiod or thermoperiod DIF. DIF is the difference in the highest daytime (lights on) temperature and the lowest nighttime (lights off) temperature which is calculated by subtracting the nighttime temperature from the daytime temperature. So, if our daytime temperature was 28oC (82.4oF) and our nighttime temperature was 18oC (64.4oF), DIF would be 10oC (50oF). In this case because we have a higher day than night temperature we would have a positive DIF of 10oC (50oF) or DIF = + 10oC (+50oF). If we were to reverse this situation and have a warmer night temperature than day we would have a negative DIF or DIF = – 10oC (- 50oF). If night and day temperatures were the same this would be expressed as equal DIF.
Generally speaking, optimum growth rates with heavy flowering indoor crops will be achieved when daytime temperatures are about 6-10oC (42.8-50oF) above nighttime temperatures (positive DIF). This allows the plant to photosynthesize (build up) and respire (break down) during an optimum warmer daytime temperature and to curtail the rate of respiration during a cooler night. Temperatures higher than needed at night cause increased respiration, sometimes above the rate of photosynthesis.
Thus, optimum range for daytime temperatures for heavy flowering indoor crops, as previously noted, tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties, while optimum nighttime temperatures should be 6-10oC (42.8-50oF) lower at 18 – 22o (64.4 – 71.6oF).
There is one proviso to this, which we’ll cover later when discussing reducing plant stem elongation (stretch) through running an equal or negative DIF during the ‘stretch’ phase of the crop cycle.
Relative Humidity and Photosynthesis
Excessive relative humidity (RH) prevents plants from properly taking in CO2 and moving nutrients and water, resulting in a reduction in photosynthesis.
Relative humidity refers to the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a certain temperature. If the relative humidity level is 75 percent this means that every kilogram of the air in the respective space contains 75 percent of the maximum amount of water that it can hold for a given temperature.
Relative humidity levels affect when and how plants open the stomata on the undersides of their leaves. Plants use stomata to transpire, or “breathe.” Transpiration is the evaporation of water from the surface of leaf cells in actively growing plants. The process of transpiration provides the plant with evaporative cooling, nutrients, carbon dioxide entry and water.
Land plants can transpire passively by evaporation because the difference between the humidity of the gas in the stomata and the surrounding air causes the water in the stomata to diffuse outward.
A hydrated leaf would have a RH near 100%. Any reduction in water in the atmosphere below this creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and the greater the driving force for transpiration. When RH is too high, the atmosphere contains more moisture, reducing the driving force for transpiration.
A reduction in transpiration reduces CO2 intake, resulting in less than optimal photosynthesis.
Low Humidity Also Reduces Photosynthesis
If humidity is very low the rate of transpiration becomes too high. As a result, the plant closes its stomatal openings to minimize water loss and wilting. Unfortunately, this also means photosynthesis is slowed and subsequently, so too is plant growth.
Optimum relative humidity levels that ensure high rates of photosynthesis are typically expressed at between 45 – 75%. However, it is important to note that higher levels of humidity can promote leaf and flower fungal infections (e.g. botrytis and powdery mildew) and, therefore, the lower end of the humidity range (45 – 50% RH) is recommended once flowers begin forming.
 Zelitch I (1975). Improving the efficiency of photosynthesis. Science, 188: 626-633.
 Lobell D.B. & Asner G.P. (2003) Climate and management contributions to recent trends in U.S. agricultural yields. Science 299, 1032
 Sharkey, T.D. Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene. Plant, Cell and Environment (2005) 28, 269–277
 Hossain MA, Bhattacharjee S, Armin SM, Qian P, Xin W, Li H-Y, Burritt DJ, Fujita M, Tran LSP (2015) Hydrogen peroxide-priming modulates abiotic oxidative stress tolerance: insights from ROS detoxification and scavenging. Front Plant Sci 6:420