Mississippi soybean producers are always looking for improved soybean farming tools. Soybean producers who irrigate are especially mindful of new technology that may be used to increase yields and/or improve water use efficiency.

The following items underline the interest in canopy temperature as a possible tool for predicting/estimating water deficit stress in soybeans, and how that might be translated into irrigation scheduling.

• A Feb. 22 2012 Delta Farm Press article entitled “Canopy temperature more reliable indicator of crop stress?” Notice the question mark in the title. Two statements in the article are worth special attention.

“If we can monitor canopy temperature continuously, it gives us a very sensitive measure of water leaving the plant.”

“Right now, we don’t know how to translate that into terms that are important for agricultural management.”

• A Feb. 21 2012 article entitled “A new tool for mapping water use and drought” posted on the Crop Management website. In this article, the ALEXI (Atmosphere-Land Exchange Inverse) model that has been developed by USDA-ARS scientists is described.

This model “uses thermal infrared imagery from satellites and calculates soil and plant temperatures that can be used to create maps of ET (evapotranspiration) rates of plants growing in cultivated areas....” With help from new satellite imagery, the researchers hope to be able to move toward routine mapping at the “field scale” level.

• A producer query about the use of canopy temperature as a scheduling tool in irrigation research projects.

First, a little background information on why canopy temperature can be used as an indicator of plant moisture deficit stress and its subsequent effects on plant processes.

The flow of water from soil through the plant accomplishes three major functions:

• Provides vital minerals for plant growth;


• Provides the water that is necessary to keep cells turgid, and subsequently keep their shape and allow for cell expansion and growth; and


• Provides cooling of the plant due to evaporation of water from leaf surfaces through stomata.

The water potential gradient or demand for water of dry air outside the leaf pulls water from the soil through the plant and into the outside air through the stomata. It is this movement of water through the plant to the outside air that keeps leaves cool.

As soil dries and less moisture is available to the plant, water potential within the plant is reduced (held with greater tension) and the stomata in the leaves close. This process reduces the amount of water that moves through the plant and thus results in less cooling of the leaves. Put another way, water deficit stress elevates leaf temperatures because less water being pumped through the plant into the surrounding air results is less plant cooling.

Photosynthesis, transpiration, and leaf growth are major plant processes that are related to water potential or status of water in the plant. Thus, crop growth and productivity are greatly dependent on a plant’s ability to extract water from soil. Leaf temperature can be used as a gauge of how these plant processes may be affected as soil water becomes insufficient to supply the water demanded by dry air.
Since transpiration functions mainly to cool leaves, canopy temperature relative to that of the surrounding air is an indication of the level of moisture deficit stress a plant is encountering. This relationship between canopy temperature, air temperature, and transpiration involves atmospheric conditions, available soil water, and plant parameters such as canopy size and structure, and leaf adjustments (wilted or turgid).

Canopy temperature can be measured remotely by an infrared thermometer. Proper use of the thermometer and interpretation of obtained data are critical for meaningful results. Some factors to consider are:

• Measurements of canopy temperature should be taken during the time of day when moisture deficit stress (even in well-watered plants) is greatest. This occurs in the 1-3 hours past solar noon.


• Measurements over time at an individual site should be taken at the same time of each day.


• Multiple measurements of the canopy temperature at a site of interest should be conducted roughly within two hours from the start of the measurement period.


• The target area must consist entirely of leaves of the target plant; i.e., no plant flower parts or weeds.


• Object(s) (such as soil) other than the canopy leaves in the target area will result in erroneous readings because it/they will possess properties that are different from those of the target plant canopy. Therefore, canopy temperature measurements should only be taken when there is a full canopy of the target crop in the thermometer viewing area.


• The distance between the canopy and the thermometer, and the angle with respect to the target area, must be maintained during individual and over-time measurement periods.


• Atmospheric conditions during measurements should be relatively stable. Cloudy or windy conditions should be avoided. Transient cloudiness is especially problematic since it has an almost immediate impact (~ < 2 minutes) on leaf temperature. Results from a Stoneville MS study provide a method for adjusting canopy temperature measurements to account for the intermittent cloud cover that frequently occurs in the Midsouth during the growing season.


• Canopy temperatures likely will differ among soybean varieties in a given environment.


• Soybeans growing on soils with differing textures will differ in their overnight speed of recovery from daytime water deficits. Since the amount of plant growth at night depends on this speed of recovery from daytime moisture deficit stress, canopy minus air temperature values should be indexed to soil texture.

Now to the question posed in the title.

There is no doubt that increasing canopy temperature over a period of time indicates that plants are experiencing increasing moisture deficit stress. However, there must be an established, crop-specific baseline canopy temperature deficit (relative to the surrounding air) that can be used to assess the cumulative effect of increasing canopy temperatures under conditions of decreasing soil moisture over time in order to use canopy temperature as a trigger for irrigation.

The second quote in the first bullet point of this article seems to be a proper answer for Midsouth soybean farmers when all things stated above are considered. Hopefully, this will change in the near future; for now, there is not enough base knowledge to use this methodology for scheduling irrigation of soybeans on a field-scale level.
The information presented here is not meant to disparage canopy temperature as an indicator of crop water deficit stress which in turn might be used for scheduling irrigation. Rather, it is meant to provide background information for the continued assessment and development of canopy temperature measurement as one of the soybean farming tools available for irrigators.

 

larryheatherly@bellsouth.net