Managing Vapor Pressure Deficit in Greenhouses
Vapor Pressure Deficit
Vapor pressure deficit (VPD) offers a more accurate characteristic for describing water saturation of the air than relative humidity because VPD is not temperature dependent. Vapor pressure can be thought of as the concentration, or level of saturation of water existing as a gas, in the air. As warm air can hold more water vapor than cool air, so the vapor pressure of water in warm air can reach higher values than in cool air. There is a natural movement from areas of high concentration of water vapor to areas of low concentration of water vapor. Just as heat naturally flows from warm areas to cool areas, so does water vapor move from areas of high vapor pressure, or high concentration, to areas of low vapor pressure, or low concentration. This is true for any given air temperature.
Vapor Pressure Deficit and Plant Transpiration
A key point when considering the concept of VPD as it applies to controlling plant transpiration is the vapor pressure of water vapor is always higher inside the leaf than outside the leaf. Meaning the concentration of water vapor is always greater within the leaf than in the greenhouse environment, with the possible exception of having a very undesirable 100 percent relative humidity in the greenhouse environment. This means the natural tendency of movement of water vapor is from within the leaf into the greenhouse environment. The rate of movement of water from within the leaf into the greenhouse air, or transpiration, is governed largely by the difference in the vapor pressure of water in the greenhouse air and the vapor pressure within the leaf.
Vapor Pressure Deficit to Monitor Plant Stress
Vapor pressure deficit is a good indicator of plant stress brought about by either excessive transpiration (high VPD values) or the inability to transpire adequately (low VPD values). When the VPD is too high (humidity too low) the rate of evaporation from the leaves can exceed the supply of water into the roots. This in turn will cause the stomata to close, and photosynthesis to slow or stop. Once the stomata close, the leaves are at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate.
Optimum Vapor Pressure Deficit Values
Since the principles of VPD can be used to control the transpiration rate, there is a range of optimum VPDs corresponding to optimum transpiration rates for maximum sustained yield. Because both RH and VPD are related to temperature, VPD indexes can be obtained from handy reference charts when two of the three values are known. Table 10.1 presents the temperature—relative humidity combinations required to maintain the range of optimal VPD in the greenhouse environment. It is important to remember that this table only displays the temperature and humidity targets to obtain the range of optimum VPDs, it does not consider the temperature targets that are optimal for specific crops. There is a range of optimal growing temperatures for each crop that will determine a narrower band of temperature and humidity targets for optimizing VPD.
Measuring Vapor Pressure Deficit
To measure the actual leaf VPD, one would need to accurately measure the temperature of the leaf tissue. While not impossible, this is seldom practical, since leaf temperatures can vary wildly throughout a crop as some leaves are in shade and others in full sunlight making it difficult to accurately measure leaf VPD. For stress indication and humidity control purposes, the point is not to measure the actual leaf VPD to within strict tolerances, but to gain an insight into how the current temperature and humidity surrounding the crop is affecting the plants. The air temperature and humidity near the leaves, as measured by a properly positioned aspirated sensor module suspended within or as close as possible to the crop canopy is usually sufficient to provide a good indication of the actual leaf VPD.
Vapor Pressure Deficit Control
Although VPD can indicate whether the current evaporation stress on the plants appears to be too high or low, it doesn't indicate the reason, since the effect is a combination of temperature and humidity. However, one can still use VPD to influence climate management strategies, and as the basis for operating humidification equipment such as fog and mist systems.
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