Humidity is a common issue in agriculture, whether in an indoor growing facility or greenhouse. As any parameter in such an environment, it affects, and is affected by numerous factors.

In this article we will discuss an often-overlooked piece of the puzzle – how humidity is affected by radiation.

Relative Humidity & Transpiration

In order to understand transpiration, radiation, and their effect on humidity, it is first crucial to understand relative humidity (RH).

Relative humidity measures the saturation of water vapor in the air, so when RH is at 100%, the air cannot contain additional vapor, and transpiration slows. To fully understand the interplay in a greenhouse, it is important to note that RH is directly affected by the temperature. As warmer air has a larger water capacity, heating causes the RH to drop, and vice versa.

Plants are constantly performing transpiration, releasing water vapor through stomatal pores located on the leaves. At the most basic level, transpiration rate is dictated by the amount of water absorbed by the roots, which determines the amount of water in the plant; and the air’s relative humidity, which dictates the amount of water which may physically evaporate.

For a better understanding of relative humidity check out “Humidity – It’s All About the Dew Point”

Direct Influence of Radiation on Transpiration

Opening of the stomata is done in order to allow an exchange of gases between the plant and the atmosphere. Most notably, to balance CO2 uptake, which is a crucial part of photosynthesis. But when the pores are open, the water contained in the leaf is exposed, causing it to evaporate. This is the main reason radiation has such a direct effect on the transpiration.

It has been well established, during the past century, that radiation directly affects the opening of stomatal pores, and thus transpiration. Trials conducted found that stomata respond to visible light, being affected similarly by red and blue broadband spectrums. But more recent studies found that IR (infra-red light), which is not visible, also increases stomata activity, thus increasing transpiration.

These findings have since been challenged, but they reveal the fact that there are still many unknowns when it comes to the direct effect of radiation on stomatal activity and transpiration.

Indirect Influence of Radiation on Transpiration

So, radiation by itself affects transpiration in a linear fashion. But radiation does not arrive without heat. Even LED lights, which produce a fraction of the heat, still have an impact when used intensively, such as in a grow room.

This has an opposite effect on humidity. As the space heats up, the relative humidity declines and transpiration rate increases. But, every plant has an optimal temperature range, so heat cannot be used unchecked as a method of humidity reduction.

Better Understanding, Better Control

Humidity is a bit of a tricky parameter to fully understand and control. But having a better grasp on the processes that affect it, and how, may lead to more effective humidity control methods, tailored correctly to the space, and operating as a part of a whole climate control scheme.

Not all aspects of this relationship are currently clear, especially with modern growers experimenting with different spectrums and lighting regiments, in order to achieve different results. But radiation must be taken into consideration as a factor that not only affects the physiology and growth of a crop, but also the entire climate and energy balance of the operation.