Monitoring stomatal opening: sufficient opening under LED exposure?

Stomata play a crucial role in the assimilation, moisture and energy balance of a crop. Too limited an opening inhibits growth. However, if stomata are open further than necessary, the extra evaporation leads to unnecessary energy consumption. Stomata thus act between the guardrails of necessity and excess. Live insight into the opening position of stomata is therefore highly desirable.

The objective of this study was twofold:

• Development and validation of the stomata sensor for 'live measurement' of stomatal behavior in the greenhouse. This work package was carried out on example crop anthurium. •
• Knowledge development on stomata behavior under LED lighting in the example crop sweet bell pepper. What is the minimum energy supply so that stomata opening does not limit photosynthesis? This question applies in a broad sense when growing under full-LED illumination.

For Anthurium, it has been found that the stomata sensor fed by a net radiation meter gives estimates of stomatal conductivity that are in the same order of magnitude as those measured by accurate photosynthesis measuring devices. A major concern remains that the reliability of the stomata sensor depends on the reliability and representativeness of the measurement position of all the sensors used. This requires attention from the grower.

Dissipation of energy

For sweet bell pepper, a test setup showed that it was not the supply of energy that was leading in stomatal conductivity, but the removal of energy: on the one hand through convection, on the other hand through evaporation. In practice, therefore, in the situation of LED lighting and stomata restriction for photosynthesis, the radiation (i.e. dissipation of energy) will first have to be minimized and the convective transfer of energy reduced, so that evaporation becomes the primary dissipation of energy. A higher percentage of blue light also stimulated stomatal opening, and offers a solution if sufficient evaporation cannot be achieved by limiting the dissipation of energy.

With the knowledge developed, a "guardrail" can be established for stomatal opening. This allows light, CO2 and heat to be utilized more efficiently. A photosynthesis model can be used to visualize 'live' whether stomata are sufficiently open. On the other hand, heat is lost unnecessarily when stomata are open further than necessary, because more (latent) heat needs to be extracted (dehumidification) or an unnecessary amount of spray is applied.