LEAF SURFACE TEMPERATURE IN HYDROPONIC GROWING
For most growers monitoring the air temperature and RH around the crop is sufficient for creating a reasonably optimised grow environment. However, for more advanced growers and growers who use VPD (Vapour Pressure Deficit) an infrared thermometer is an extremely handy piece of equipment.
An infrared thermometer is a thermometer that enables growers to measure the temperature of the leaf surface. This provides a more precise way to monitor what is occurring a cellular level in the plant.
For example, the leaf surface temperature (LST) under ideal conditions should be somewhat lower (approx 1 – 2 oC) than the ambient air temperature. This temperature differential occurs because as a plant transpires water through the leaves it acts as a cooling system for those leaves. A higher relative humidity will slow the rate of transpiration; as a result, the surface temperature of the leaf will rise while low relative humidity can increase the rate of transpiration reducing LST. Significant variations between ambient air temperature and LST, or an equal or higher LST than ambient air temperature typically indicates crop stress and less than optimal growth.
Therefore, infrared thermometers are a very handy tool for measuring crop stress.
Additionally, an infrared thermometer allows growers to take leaf temperature into account which is an advantage because it is the leaf temperature, rather than the surrounding air temperature that is more important in plant growth. I.e. from a scientific perspective, extremely important biochemical processes take place in the tissue of the plant’s leaves. Many of these biochemical processes are temperature dependent. For example, the enzyme rubisco carries out the photosynthetic fixation of carbon dioxide in the plant’s chloroplast and the chloroplasts are located in the cells of the plant’s leaves. The efficiency of rubisco to fix carbon is temperature dependent; therefore, leaf surface temperature gives us a better idea of chloroplast temperature which better indicates what is occurring at a cellular level in the plant.
When considering air temperature versus LST, think of things this way. Measuring the ambient air temperature in the grow room is like measuring the air in our living environment; it is important for us to have a proper temperature to live in. By comparison, measuring LST is like taking a person’s temperature with a thermometer under their tongue. It tells us what’s going on inside the person and gives us a good indication of that person’s health. By carefully monitoring LST inline with ambient air temperatures, this gives growers a better indication of what is occurring in their crop.
It is important to note that various environmental factors influence LST; factors such as humidity. I.e. higher relative humidity will likely increase leaf surface temperature, given the same ambient air temperature under lower relative humidity, as evaporative cooling of the leaf through transpiration will not occur as readily. Conversely, lower humidity will increase the rate of transpiration and reduce LST. Plants being too close to the lights with insufficient airflow/cooling and the amount of unused light absorbed by the leaf also significantly effects LST. Additionally, light colour spectrum effects LST. This one comes back to “unused light absorbed by the leaf also significantly effects LST”. A less-efficient light spectrum, where a lot of the light cannot be used in photosynthesis, will tend to heat the leaf more, while a more-efficient spectrum will heat the leaf less, as more of the original light energy is converted directly to chemical energy for photosynthesis instead of heat. What this basically means is that under lighting that has an ideal spectrum and, as result, a high degree of this light is converted into chemical energy for photosynthesis, higher ambient air temperatures may be required to achieve optimum LST when compared to lighting with a less ideal spectrum. So, for example, one commercial study concluded that under LED lighting higher ambient air temperatures were required to achieve optimum LST than were required under HPS and MH, where a higher degree of the light isn’t absorbed by the plant, which results in a higher degree of unused light and a higher LST (given the same ambient air temperature). 
It’s complex stuff. LST is influenced by a range of factors; however, through the use of an infrared thermometer growers are able to better understand what is going on in the body of the plant to better optimise ambient air temperatures that best promote optimum LST.
A 2017 study found that the rate of photosynthesis in cannabis (hemp) was optimal at an LST range of 25–35 °C. However, just keep in mind that optimum temperature is genetic dependent and this applies both to ambient air and leaf temperatures. As a tip, based on my own experiences, with most high THC genetics I have found that optimum growth occurs at between an LST of 25 – 28oC (77 – 82.4oF) LST where humidity is in the range of 65 – 75%.
Keep in mind that genetics/cultivar will play some role in determining the optimum LST. For this reason, some experimentation within these temperature ranges is recommended.
Note that the leaf surface temperature is affected by, but not equivalent to the ambient air temperature in the growing environment. Leaves can be cooled through evaporation occurring in open pores in the leaf/stomata (stomatal conductance) that allow gas exchange, and are warmed by absorbed but unused light, whether from artificial or natural sources. Other factors that influence LST are CO2 levels (higher/elevated CO2 reduces stomatal conductance, increasing LST) and artificial light type etc. Leaf surface temperature is almost always different than ambient air temperature.
For more information on Infrared thermometers and LST speak to your hydroponic retailer.
 Black Dog LED. Effects of Different Artificial Grow Lighting Technologies on Leaf Surface Temperature. Study available at https://www.blackdogled.com/lst
 Kailei Tang, Paul C. Struik, Stefano Amaducci, Tjeerd-Jan Stomph and Xinyou Yin (2017). Hemp (Cannabis sativa L.) leaf photosynthesis in relation to nitrogen content and temperature: implications for hemp as a bio‐economically sustainable crop