The plant is continuously seeking a balance between transpiration (loss of moisture) and photosynthesis. The functioning of the stomata plays an important role in this. Moderate climate conditions are best for achieving this.

Stomata can be compared to doors on the surface of the leaf. They allow air containing carbon dioxide, oxygen and water vapour to both enter and leave the leaf. Therefore they play a crucial role in the functioning of the plant. Transpiration, cooling and uptake of CO2 for photosynthesis are all dependent on the state of the stomata.

Over the short term the extent to which the stomata are open or closed is important. Over the long term the number of stomata that are formed is important. Both the functioning and the formation of the stomata are dependent on environmental factors.

In principle, on a sunny day the stomata should be wide open to allow in sufficient CO2 yet sufficiently closed to limit moisture loss. These are of course contradictory desires. Therefore the plant is continuously seeking the balance between letting in CO2 and limiting moisture loss. The balance is dependent on the moisture level in the plant, air humidity, light intensity, light colour and CO2-concentration. This multitude of factors doesn’t make it easy for the grower to specifically manage the functioning of the stomata.

Relative humidity (RH) is especially important
But there are a number of general principles that the grower can take into account. Insight into this has been growing recently due to the work by researchers of Plant Dynamics and Plant Lighting who have been studying potted plants in particular. This is a group of plants that can benefit greatly if they could grow under higher light levels. The functioning of the stomata is directly related to this.

In general, a higher light level stimulates the stomata to open. Also the colour of the light is important. Blue light encourages them to open, green light to close. However, it is always a question of graduation and the effects of colour are not the same throughout the day. In addition other light colours also play a role, although this is sometimes still not understood. Trials give varying results so that using light colour to steer the opening of the stomata is still too ambitious at this time. The positive effect of blue light on the formation of the stomata is much clearer.

The research on potted plants shows that the relative humidity (RH) is the most important factor. A higher RH leads to wider opening as well as increased formation of the stomata in the early development stage. These principles also apply to vegetables and ornamentals. A very high CO2-level stimulates their closure.

Coatings improve plant functioning
These insights support the use of coatings that ensure both greenhouse temperature and plant temperature don’t become too high. The grower is able to ventilate less and the RH remains at the right level. Also, the CO2 level drops less quickly. Also the CO2 level falls less quickly.

The higher RH ensures that the stomata open wider and stay open longer so the crop can take up more CO2 and therefore produce better. Over the long term the higher RH promotes the formation of more stomata.

Because the crop does not get stressed as quick transpiration can continue longer which has a positive effect on the uptake and distribution of calcium.

The improved greenhouse climate under coatings makes it possible to allow in more light, which certainly with potted plants can significantly boost production. Diffuse light, made possible with ReduFuse (IR), provides extra advantages. There are no longer sunspots in the greenhouse. In these sunspots the plant quickly becomes stressed, the stomata close and as a result photosynthesis decreases. In addition, diffuse light results in a more uniform climate in the greenhouse. That is beneficial in keeping the vapour pressure deficit (VPD) within the limits. This also keeps the stomata open for longer so CO2 can enter longer.

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