The stomata manage important plant processes: evaporation and CO2 absorption. The opening (or closing) of the stomata determines the CO2 absorption and the evaporation of the plant. The evaporation process is important because it allows the plant to keep itself cool. The moment water shortage threatens the plant, and therefore stress is experienced, the stomata close and it becomes more difficult for the plant to get water vapour out and CO2 in. In short; as long as the stomata are open, you know the process is going well. With the GPE stomata conductivity module you can determine the state of the stomata and thus their conductivity.

The GPE stomata conductivity module is constructed from various measurements and calculations. You can find the content of the module in the table below.

All available calculations in the GPE stomata conductivity module and explanations thereof, are presented below.

Plant: PAR µmol/ m²/s

It is possible to connect two PAR sensors. If two PAR sensors are connected, the highest plant PAR measurement is shown. As a result, the measurement is less influenced by surrounding factors.

Plant: VPD – kPa

The Vapour Pressure Difference (VPD) is the difference between the vapour pressure of the leaf and the vapour pressure of the greenhouse air. The target value of the VPD is between 0.2 and 1.5 kPa. It is generally stated that the somata close at a VPD above 1.5 kPa.

Calculated crop evaporation - g/m².h

This calculation gives you an indication of the instantaneous crop evaporation.

This calculation gives you an indication of the cumulative crop evaporation per 24 hours.

Calculated stomata conductivity kg/kPa/m².s

Calculates the average opening of the stomata in kg/kPa /m².s.

Water absorption l/m²

The GPE stomata conductivity module also calculates the water absorption in l/m². For this, the watering l/m² and the drain (l/m² and/or %) must be connected.

If the calculated water absorption and the cumulative crop evaporation deviate too much, a correction is automatically made.

To view the standard graphs, first click on Modules in the left navigation menu (outlined in yellow), then click GPE stomata conductivity, which allows you to select a department. Clicking on a department will take you to the screen pictured below. On this screen, you will find graphs (outlined in purple) and your manual data entry (Data entry, outlined in red). You can set up the module by clicking Data entry. If you have not set up the module yet, please read on for the step-by-step process. The main graph is the first one called 1. Stomata Conductivity. In this graph, greenhouse temperature, plant temperature, RH, radiation, VPD and the stomata conductivity are visible.

For the GPE stomata conductivity module, several parameters must be entered to make a good estimate of the heat transfer through convection between the crop and the greenhouse air. If one of the parameters changes, you can also make adjustments by following the steps described below. Go to Modules (outlined in yellow) in the left navigation menu, then click GPE stomata conductivity and select the correct department. You will enter in the screen below.

When you click on Data entry, you will arrive at the screen below. Here, you will work with parameters. A parameter is a set value used for continued calculations until the model encounters a new number. Therefore, you can also add future values.

To start, the values must be entered on the module’s start date. To find the start date, simply click on the week selection drop-down menu (outlined in red), select the first week and enter your values. Click here to see which values can be entered. After entering your values, the value of the parameter is displayed behind the item (outlined in yellow). When you hover your cursor over this parameter value, you will also see the date of date of input.

This section contains both the meanings of the parameters can be found and a number of guidelines are given.

The number of Watts/m² per degree difference between the plant temperature and the greenhouse temperature. The convection energy can be calculated with this value.

At how many measured PAR µmol/m².s do you want the day to start?

Stomata conductivity night - kg/kPa.m².s

How much does the crop evaporate (per 1 kPa) at night? Example: with an entered value of 25 and a VPD of 1 kPa, the crop evaporates 25 g/m² hour at night.

Leaf Area Index of a fully grown crop.

Crop light interception -%:

What is the percentage of light that the crop intercepts from the total amount of light that enters your greenhouse? The light interception must also, for example, take possible paths into account. This influences the net percentage of light that the crop receives; which contributes to the crop evaporation

* The parameter values listed here are based on a fully grown crop. In the initial stage (cultivation), you should adapt your values to the state of your crop.