Humidity is a common issue in agriculture, whether in an indoor growing facility or greenhouse. As any parameter in such an environment, humidity relates to numerous others factors. In this article we will discuss an important piece of the puzzle – how radiation affects humidity.

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 temperature directly affects RH. As warmer air has a larger water capacity, heating causes the RH to drop, and vice versa.

Plants constantly perform transpiration, releasing water vapor through stomatal pores on their leaves. At the most basic level, transpiration rates are dictated both by the amount of water absorbed by the roots and the air’s relative humidity level. The water that a plant absorbs determines the amount of water vapor it may transpire. The relative humidity determines the amount of additional water vapor the air can hold. Together, these 2 parameters dictate the amount of water that will evaporate from the spores.

Direct Influence of Radiation on Transpiration

The stomate open 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 open, they expose the water in the leaf, causing it to evaporate. This is the main reason radiation has such a direct effect on the transpiration.

Radiation directly affects the opening of stomatal pores, and thus transpiration. According to trials, stomata respond to visible light, with red and blue broadband spectrums both affect it similarly. But more recent studies found that IR (infra-red light), which is not visible to humans, 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 rates increase. But, every plant has an optimal temperature range, so heating is a limited 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, may lead to more effective humidity control methods.

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