The newest generation of powerful LED grow lights have changed the way that growers maximize profits and crop quality. Growers often ask what adjustments need to be made to their growing environment in order to optimize their facility for LED lighting. Dr. Matt Wheatley, a cannabis consultant working with LumiGrow, explains:

“New and experienced growers that want to reap the benefits of an LED strategy should familiarize themselves with the relationships between environmental factors that affect the efficiency of photosynthesis; leaf temperature, humidity, carbon dioxide concentration, and light intensity. In this article, we’ll discuss the effect of temperature on photosynthetic rate; how temperature and CO₂ concentration are intimately linked to plant growth at higher temperatures; and compare the differences between ambient temperatures in HPS and LED grow rooms,” explains Dr. Matt Wheatley.

When scientists discuss photosynthesis and temperature, they typically reference leaf temperature, not the ambient room temperature; this makes sense given that the biochemistry of photosynthesis takes place inside plant’s leaves. In contrast, when grow room designers discuss temperature, they usually reference the ambient room temperature. In most growing environments, the leaf temperature will be higher than the ambient air temperature surrounding the plant. This is especially true for plants grown under high-pressure sodium (HPS) lights, which emit infrared radiation that is absorbed as heat by the plant.

RuBisCO is the plant enzyme responsible for the chemical reaction that is the first step of carbon fixation. This chemical reaction is seen as the conversion of CO₂ and water into simple sugars during photosynthesis. The chemical reaction that RuBisCO performs is temperature dependent.

With full sunlight and ambient CO₂ concentrations of about 300ppm, as well as a temperature range of 5°C-27°C, the rate at which CO₂ is absorbed by the plant and converted to sucrose increases as the temperature increases (Figure 1 below), leading to increasing gains in net photosynthesis.

If the internal leaf temperature rises above 27°C, RuBisCO enzymes begin to perform the reverse reaction, with some of the RuBisCO enzyme converting sucrose and oxygen into CO₂ and water in a process known as photorespiration. As leaf temperatures approach 40°C, net photosynthesis will become negative as the plant burns more carbon than it gains. So, under normal ambient CO₂ levels, a grower will achieve the greatest growth with leaf temperatures just below 27°C.

Fortunately, an indoor grower can adjust their environment to achieve optimal growing conditions. Controlled environments allow growers to maintain optimal temperatures, carbon dioxide concentrations, light intensity and relative humidity. So, let’s explore how a grower can adjust the growing environment to take advantage of high rates of growth that occur at high temperatures.

Increasing CO₂ concentrations will extend the temperature range in which RuBisCO may fix CO₂ into sugar. With increased CO₂ we see that as temperature increases, so does the rate of the chemical reaction that RuBisCO performs. This works because an increase in CO₂ concentration means that the ratio of chemical substrates to products is being increased. If the CO₂ concentrations are increased from ambient 300ppm to 1500ppm, the change in the ratio of reactants to products will allow plants to continue to fix CO₂ into sucrose at leaf temperatures well above 27°C, all the way up to about 36°C! (see Figure 1)

Figure 1 - Changes in photosynthesis as a function of temperature at CO₂ concentrations that saturate photosynthetic CO₂ assimilation (A) and at normal atmospheric CO₂ concentrations (B). Photosynthesis depends strongly on temperature at saturating CO₂ concentrations. Note the significantly higher photosynthetic rates at saturating CO₂ concentrations. (Taiz and Zeiger, Plant Physiology 3rd edition)

And as the temperature increases, so does the rate of carbon fixation and plant growth. This means that if growers are careful with their environmental controls, they may achieve very high rates of carbon fixation and plant growth at leaf temperatures well above 30°C.

[Pro Tip]: Consider adjusting the vapor pressure deficits (VPD) in your grow spaces for the various phases of plant growth to see optimal crop performance.

Related Article: VPD for Cannabis Cultivation

HPS bulbs emit a large infrared peak between 800nm and 900nm. This infrared peak significantly increases leaf temperatures at the top of the canopy, where most of the infrared light is absorbed. When examining the differences between leaf temperatures of plants grown in the same room under either HPS or LED lights, we will see significant temperature differences that infrared light causes. Figure 2 shows the difference in leaf temperature for leaves 3’ below either LED or HPS lamps in the same room with the ambient temperature set at 25.7˚C.

Figure 2 - The effect of Infra-Red light on internal leaf temperature. (Graph courtesy of LumiGrow Plant Research Group)

In one of our studies, cannabis consultant Dr. Matt Wheatley measured the photosynthetic activity and internal leaf temperature of leaves at different distances from the light source. The internal leaf temperature measurements were very clear. The leaves of plants under LEDs did not show an increase for internal leaf temperature significantly above the ambient room temp at any distance between 2’ and 4’ from the lamp. In contrast, the leaves of plants under the HPS lamps showed a wide range of internal temperatures; the highest temps were apparent at the top of the canopy and lowest internal leaf temperatures were at the bottom of the canopy. This partially explains why HPS lamps produce top-heavy crops while LED lighting creates a more uniform canopy.

Since the rate of carbon fixation by RuBisCO is affected by leaf temperature and CO₂ concentration, increasing the ambient temperature in LED-lit rooms will increase the rate of photosynthesis and plant growth.

Based on scientific study and experimentation, LumiGrow has found that an increase in the ambient room temperature of 5-7°C in LED-lit rooms relative to temperatures in HPS grow rooms is necessary to achieve similar internal leaf temperatures and plant growth rates as those experienced by plants in HPS-lit rooms. Keep in mind that as the ambient room temperature increases, the relative humidity decreases, and proportionate adjustments should be made to the relative humidity to adjust your Vapor Pressure Deficits.

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Want to get in touch with LumiGrow? They can be reached via the following methods:

1. Website: https://www.lumigrow.com/
2. Phone: 800-514-0487
3. Email: info@lumigrow.com