Written by Peter van Weel, one of the three authors of the book: Plant Empowerment, the Basic Principles.
Stomata play an important role
Plants require exchange of gasses and energy to their environment to grow and to survive. Air movement and stomata play a key-role in that process. Stomata are located under the leaves, connecting an air space inside the leaf filled with water vapor and CO2 gas with the air around the leaf.
CO2 is the building block for plant growth and water evaporation is required for cooling the plant and to transport calcium to the plant cells, which is required to obtain strong walls that protect the plant against fungal diseases. The transfer of both gasses is determined by the opening status of the stomata and the vapor pressure difference of the gas between both sides of the stomata.
The vapor pressure is determined by the concentration of the gas and its temperature. The water vapor inside the air space has an RH of 100% and its temperature equals the leaf temperature. The CO2 concentration inside the vacuole is always lower than in the surrounding air since the plant consumes CO2. The stomata can also prevent wilting of the plant by closing, but in a commercial greenhouse this must be prevented. The wider the stomata can be kept open without getting a water stress situation, the more CO2 can be absorbed and transferred into sugars by the photosynthesis process.
The air surrounding the leaf is usually split into three different layers. Directly surrounding the leaf is a so called ‘Microclimate Layer’. This layer contains still air (no air movement) and can vary in thickness depending on the amount of airflow available in that layer. The still air causes that this layer has a higher concentration of water vapor (blue dots in the drawing) than the air in the greenhouse. On the other hand, the CO2 concentration (red dots) in the microclimate layer tends to be lower than in the greenhouse air.
Inside the canopy, certainly in dense leaf packages, the concentration of both gases can also be different from the greenhouse air because of little or no air movement in that layer.
Breaking the limitations of the microclimate layer
A leaf can receive energy in two ways, by means of radiation coming from light or heating pipes and by means of convection when the surrounding air is warmer than the leaf. Only 1 % of this energy is used for growth, the remaining 99% increases the leaf temperature. In nature, two objects with a different energy content will start to exchange energy from the warm to the cold object. This can happen in three ways. The first is that the warm plant starts to lose energy by radiation to a cold ‘Sky’. Air movement has no direct influence on this exchange and when the sun, overhead heating pipes or lamps are on, this loss plays no role. The second exchange method is that the water in the leaf is warmed up and this increases the vapor pressure of that water. When the stomata open, water vapor will flow to the surrounding air with a lower vapor pressure. The second exchange method is that the warmer leaf exchanges energy with the surrounding colder air by convection. Since the leaf area is usually around 3 m2 per m2 greenhouse and exchanges heat on both sides, even a small difference in temperature between the leaf and the surrounding air will result in an energy exchange by convection.
Imagine what happens when the air surrounding the leaf would not move. That layer of still air starts to act as an insulation, thus reducing the energy exchange by convection. Then the leaf temperature will rise.
This temperature rise will increase the water vapor pressure inside the leaf and result in an increase in transpiration. After a while the layer of still air around the leaf will become more saturated with water. A gas (water vapor), can freely move through space, this process is called ‘diffusion’. What is important here is that the amount of water molecules that can be transferred between two volumes of air depends on the difference in vapor pressure and the distance that the molecules have to move. Let’s compare two situations for the layer conditions. Inside the leaf, RH is 100%. In the microclimate zone a 5 cm thick layer of still air around the leaf the RH is 95%. The temperature of the leaf is 21 0C. The surrounding air is 20,5 0C. The transfer of water vapor over the 5 cm distance will then be 2,6 grams per m2 leaf per hour. Be aware that in a dense crop the 5 cm thickness of the still layer can easily become thicker. The transpiration can be improved by introducing air movement in the canopy layer. When dry air from the greenhouse area above the plants is blown between the canopy two things will happen. The microclimate layer becomes thinner and the RH of the canopy layer comes down. Assuming that the RH in the canopy can be lowered to 90% and the layer of still air in the microclimate layer reduces to 5mm, the diffusion will increase to 31,6 g/m2/hour. A tenfold improvement, and enough for optimal calcium transport to the plant cells, that requires a transpiration of 10-20 g/m2/h in the dark.
When lights are on under a closed screen, transpiration will quickly rise to a level of 80 g/m2/h. And under a closed screen still air can easily occur because warm air rises and is trapped under the screen while cold air inside the canopy tends to drop. Both layers will not mix, so between the canopy we have a layer of colder, still air. That is why many growers prefer to use a heating pipe below the canopy as soon as the lights are on. This makes the air move upwards and improve the conditions inside the canopy. But this is a very inefficient method, since it further increases transpiration and energy consumption. It also increases the amount of heat trapped under the screen, resulting in a rising temperature of the greenhouse.
To understand why the introduction of air movement inside the canopy is a much better idea than bringing extra heat under the plants, it is necessary to have a closer look at the effects of moving air inside or near the canopy. First of all, it will reduce the layer of still air to a few millimeters, depending on the roughness of the leaf surface. Secondly, it will lower the moisture concentration of the air surrounding the leaf since fresh air with a lower moisture content is brought in. Assuming an RH of 85% of the supplied greenhouse air and a 2 mm layer of still air, 120 gram/m2/hour can be transported to the boundary of the microclimate layer by diffusion, so that will work for the situation that the lights are on under a closed screen.
Transport of moisture out of the canopy
To make sure that the volume of moisture escaping from the microclimate layer will be transferred in the same rate out of the canopy, air that contains a lower amount of moisture per m3 than the air at the border of the microclimate layer must be blown through the canopy. It is a mistake to think that air with a lower Relative Humidity or a higher Vapor Deficit will do that job. Both factors are related to the temperature of the air. For example, warm air with a low RH is not necessarily more dry than colder air with a higher RH. In numbers: air of 20 0C and 80 % RH contains 14 g/m3 of moisture. Air of 19 0C and 85% RH or air of 18 0C and 90% RH contain exactly the same amount of moisture. The amount of dehumidification by air movement can be calculated by multiplying the volume of air with the difference in Absolute Humidity content of the two types of air. If air of 20 0C and 100 % RH with an AH of 17,5 g/m3 is replaced by air of 20 0C and 80% RH with an AH of 14 g/m3, every m3 of air movement will remove 17,5-14= 3,5 g/m3 water. To remove 20 g/m2/hour transpiration in the dark period, we need to make sure that within and around the canopy 20/3,5 = 5,7 m3/m2/hour of dry greenhouse air is moved. When lights are on under a blackout screen, we need to move 80/3,5 = 22,8 m3/m2/hour.
This does not mean that without air movement there will be no transpiration. What happens then is that the leaves get warmer, because of a lower transpiration. That warmer leaf will introduce air movement because warm air is lighter than colder air and starts rising, thus breaking the still air. However, the total transpiration is reduced. So, air movement keeps the leaf temperature lower and increases transpiration.
Air movement for cooling
During the day, an interesting phenomenon can take place. Greenhouse air can get warmer than the evaporating leaf. Since, evaporation depends on the amount of energy absorbed by the leaf, this can increase transpiration since extra energy is absorbed from the surrounding air. Air movement will increase that. In the end stomata may have to close to reduce the risk of wilting. This means that photosynthesis is reduced, because less CO2 can pass the stomata. The best method to prevent this problem is to reduce transpiration by increasing the humidity in the greenhouse. This can be done by reducing the window opening or by misting.
The reverse is also possible, that the air is colder than the leaf. This is especially true for plants with less stomata. Many green plants are like that. In that case the leaf can warm up so quickly under intense radiation that problems of overheating can occur. To reduce that problem usually a shading screen is closed. But this reduces photosynthesis. As long as the air is colder than the leaf, it makes sense to start moving the air around the leaf, because it will cool the leaf. This is also true for non-transpiring plant parts such as flowers, fruits and shoots. Air movement will make it possible to keep the shading screen open more often, allowing all the PAR coming in without overheating the crop.
Air movement for uniform temperature distribution
During the dark period the top of the canopy will often become colder than the surrounding air. This is caused by radiation loss to the cold roof. Even when a screen is closed, the bottom of the screen material will be colder than the greenhouse since the air above the screen is cold. Traditionally the heating pipes were situated above the canopy, thus preventing the cooling of the leaves. But, with increasing insulation values of the screens, the need for warm pipes is gone and also pipes are more placed between and below the canopy. Air movement in a vertical direction can compensate the cooling of the top of the canopy by moving warm air along it.
Usually this is a very energy-efficient way to make temperatures in the greenhouse more uniform without adding extra heat. Adding heat would increase transpiration to a level that is not needed and this will require to open the ventilation windows extra. Although introducing vertical air movement is a positive action, two warnings must be given. First, for crops that are grown at relatively low temperatures (<10 0C), air circulation must be switched off when the temperature of the bottom of the screen drops more than 2 0C below the greenhouse air temperature. Moving air down would bring the canopy below dewpoint, resulting in botrytis. A leaf temperature measurement with an infrared camera can prevent this. The second warning is that vertical air movement will never be able to compensate for temperature unevenness in the greenhouse caused by independent horizontal airflows. Actually, horizontally blowing fans will also not be able to solve that problem continuously because it is caused by cold air that drops through a porous screen. Also, the position of the cold spot in the greenhouse is related to the direction as well as the strength of the wind. The only real solution to horizontal temperature differences is one or more airtight screens.
Bring CO2 to the stomata
Another interesting effect of air movement is the transport of CO2. A developing crop will consume so much CO2, that still air in the microclimate layer again can limit the supply because the speed of diffusion is too low. By increasing the concentration of CO2 concentration in the greenhouse air, through CO2 supply via a tube, this problem can be reduced, but this will also increase the losses through the ventilation windows. Air circulation can improve the distribution, especially within the canopy and also break the still air in the microclimate layer.
Without CO2 supply, the concentration in the canopy can drop to values below 350 ppm when the ventilation windows or a screen are (partially) closed, thus reducing photosynthesis. Nowadays a concentration of 400 ppm or more can be found outside. Ventilation in combination with air circulation can solve this limitation.
Summarized effects of air movement
Air movement within the canopy is good for moisture removal to keep transpiration going and to prevent fungal diseases, for cooling when the plant receives too much radiation. It will allow to use less shading. In the dark, air movement can prevent that the top of the canopy gets too cold without driving the energy consumption up. It is also good to bring the CO2 to the stomata. Vertical temperature distribution can be improved by vertical air movement under a closed screen, especially when the lights are on. Improvement of the horizontal temperature distribution with a fan cannot be expected, this requires an airtight screen.