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An infrared sensor is mounted inside a protective PVC casing to minimize temperature fluctuations on the sensor and to protect it from the elements. The sensor can be pointed in any desired direction.

An infrared sensor is mounted inside a protective PVC casing to minimize temperature fluctuations on the sensor and to protect it from the elements. The sensor can be pointed in any desired direction.

When a child approaches her mother and says, "I don’t feel good," the first thing the mother does is put her hand on the child’s forehead. She may even go get the thermometer. She’s checking to see if her child has a "temperature." If the child’s temperature exceeds 98.6°F, then something is not right.

Can we apply this same principle to growing fruit?

Trees and vines that are healthy and have all the water they need are actively transpiring water. Water is moved up from the soil through the plant and then released out of small openings in the leaves called stomata. Converting liquid water in the leaf to water vapor that exits through the stomata requires a lot of energy. This energy loss cools the plant’s leaves. If a tree or vine is stressed, it will close its stomata, stop transpiring as much water, and the leaf will be warmer than a healthy leaf. The bottom line: A healthy tree or vine will have cooler leaves than a water-stressed or unhealthy tree or vine. Therefore, it may be possible to determine a tree or vine’s health status by "taking its temperature."

The problem is that leaves are very thin and have very little thermal mass. If a thermometer was placed on a leaf, the thermometer would change the leaf temperature instead of the leaf changing the temperature of the mercury in the thermometer. But we can overcome this by taking the tree’s temperature remotely using infrared radiation sensors. Infrared radiation is similar to visible light and radio waves, yet it has a much longer wavelength. Infrared radiation is also appropriately called long-wave radiation. Everything that has a temperature above absolute zero (-460?F) emits infrared radiation. The amount of radiation that is emitted depends on the temperature of the object and some of that object’s surface properties. The hotter the object, the more radiation is emitted. Infrared radiation sensors have the ability to detect long-wave radiation and can therefore remotely take a plant’s temperature.

By taking a tree or vine’s temperature continually, it might be possible to detect stress in real time and possibly even respond to that automatically. We may be able to automate irrigation systems, turning the water on and off at just the right times to optimize tree health, fruit yield, and quality. With the same system, we may also be able to automate evaporative cooling sprinklers and/or give better information for frost protection. Dr. Matthew Whiting and I are studying whether this is possible at the Washington State University Irrigated Agriculture Research and Extension Center in Prosser, Washington.

However, there are many challenges to overcome. Changing air temperatures, sunlight exposure, and even humidity due to weather and normal daylight changes will cause a tree’s leaf temperature to fluctuate up and down no matter how healthy it is. The crop water-stress index and the crop stress index are both fairly complex algorithms that can be used to interpret a tree’s temperature in reference to these environmental conditions. However, both of these methods require air temperature and relative humidity measurements. The crop water-stress index also requires wind speed measurements. These additional sensors are expensive and introduce additional complications with coordinating the data collection and post-processing of this data.

Time-temperature threshold

An alternative way to measure canopy temperature is the time-temperature threshold. Time-temperature threshold is an irrigation scheduling method developed by researchers in Texas that uses just the canopy temperature without additional weather measurements. The time-temperature threshold method has recently been demonstrated to be useful for fully automating irrigation scheduling. In trials, this method was more responsive to plant stress and gave as good, or better yields, as the most accurate (although very capital and labor intensive) scientific irrigation scheduling techniques using neutron probe soil moisture measurements.

Row crops

In fact, the time-temperature threshold method was taken further and was used to fully automate a center pivot using canopy temperature sensors mounted on the pivot itself. Unfortunately, all of this work was done in southern climates on row crops such as cotton, corn, and soybeans. However, there is no reason to believe that this method won’t work just as well on fruit trees. Work has begun at the WSU-Prosser research station to determine how to apply the time-temperature threshold method of irrigation scheduling to perennial tree or vine crops where the permanent nature of the crops makes more sense, since the removal and reinstallation of instrumentation every season needed to accommodate farming equipment in row crops is unnecessary.

The coupling of orchard irrigation systems with infrared canopy temperature sensors holds considerable promise for the tree fruit industry if it proves to be successful. If results on fruit trees are similar to the previous results found on row crops, irrigation scheduling could be completely automated; and done so in a manner that maximizes yields and decreases water use.

Growers would not only see a decreased work load for irrigation management, but could also see lower pumping energy bills, less costly fertilizers and/or pesticides washed out of the root zone, and less mess with runoff. This is not to mention environmental benefits due to fewer nutrients and pesticides washed into streams and water bodies. Infrared temperature sensors may be able to automatically control irrigation systems for overhead cooling. In short, a system where infrared temperature sensors are coupled with the automatic control of water for irrigation and possibly cooling protection can help maximize yields of high quality fruit and protect the valuable investment that growers have in orchards, and do so in a way that lowers a grower’s work load.

All this from simply taking the tree’s temperature.