ICARDA scientists use the infrared camera to see how drought affects plant physiology.

Infrared photography ‘measures’ — accurately and in real time — how plants are responding to water stress.

The one thing dryland farmers fear, above all else, is drought. In many parts of the world, droughts are becoming more frequent, and more severe, as a result of climate change. ICARDA scientists are using a new tool — the infrared camera — to study how plant physiology is affected by drought. The results are helping to better understand the mechanics of drought tolerance, and develop new drought-tolerant varieties that could transform the lives of millions of farmers worldwide.

Masanori Inagaki has studied drought tolerance for many years. He is a scientist with the Japan International Research Center for Agricultural Sciences, and a visiting scientist at ICARDA. Dr Inagaki explains how the camera works. “The technique is known as infrared thermography. We use a specialized camera to measure the temperature of plant foliage. The hotter the foliage, the greater the level of water stress the plant is facing.”
 

Why does water stress affect temperature?

IIt’s all about evaporation, or the lack of it. During the process of photosynthesis, plants transpire by excreting water through tiny pores in their leaves. One by-product of transpiration is cooling. Transpiration reduces the temperature of the leaf surface, just as sweating cools down your skin. When soil moisture levels drop, so does transpiration — so leaf temperature rises.

Lower leaf temperatures generally mean the plant is better able to survive high temperatures and acute water shortages; it gives higher grain yields under a given level of water stress.
 

How do you photograph temperature?

Any hot object emits energy in the form of infra-red waves. Detecting and measuring these ‘heat waves’ gives an accurate, instant, non-destructive measurement of the surface temperature of the object. Infrared cameras can detect thermal radiation emitted from individual leaves and show the temperature distribution over the whole plant. Dr Inagaki uses a camera which produces an image consisting of different colored pixels, each representing a different temperature. A single ‘photograph’ contains more than 70,000 pixels. The camera is highly sensitive, able to detect temperature differences of less than one-tenth of a degree.
 

How do crop scientists use this information?

Infrared thermograph of three wheat plants under water-stressed conditions. Leaf temperature measurements can help identify drought-tolerant genotypes.

The infrared camera adds a new dimension to ICARDA’s drought tolerance research. “We now have a way to measure drought stress almost in real time,” says Dr Inagaki. “By analyzing ‘photographs’ of foliage temperature, taken every day or every hour, we can continuously monitor the levels of water stress, and better understand the physiology of drought tolerance.”

The team conducted a series of experiments on wheat plants grown under different conditions: in pots in growth chambers where moisture and temperature were carefully controlled, and in field plots under more natural conditions, under different levels of irrigation (i.e. different levels of moisture stress). “We have detailed numbers on leaf and canopy temperatures under changing moisture and solar radiation levels, and a better understanding of how plants respond to drought and high temperatures,” says Dr Inagaki. “Experimental methods for wheat have been standardized, so that they can be used by researchers everywhere. A number of drought-tolerant genotypes have been identified, that will form the basis for future plant breeding programs. In the next stage, thermography measurements will also be used to develop optimal irrigation schedules that alleviate plant stress at critical growth stages, without using excessive amounts of water.”

Experimental wheat plots, irrigated (left) and non-irrigated. Canopy temperatures were 7°C lower, and yields 150% higher, in the irrigated plot.

Any problems with thermography?

Thermography is a great tool, but it’s not perfect. Dr Inagaki and his team are working to address several technical problems, before sharing the technology more widely with national research centers.

First: developing better statistical tools to distinguish between genotype effects and weather effects. Hot weather means hot foliage in all genotypes, although drought-tolerant genotypes will be slightly cooler than susceptible ones. ICARDA biometricians are developing statistical tools to distinguish between the two effects: overall temperature rise caused by high air temperature, versus the moderating effect of drought tolerance.

Second: more experiments at higher stress levels. Infrared thermography has been trialed under moderate water stress, with encouraging but not conclusive results. “Genotypic differences in stress-temperature and temperature-yield relationships will probably become more apparent under severe water stress,” Dr Inagaki explains. The new experiments, starting next season, will help identify highly drought-tolerant genotypes that can thrive in hot, dry environments.

Third: getting rid of ‘noise’. Severe water stress leads to sparse plant growth and larger bare areas of soil. These bare areas reflect solar radiation, increasing the temperatures measured by the camera. Successful application of infrared thermography will require a technical arrangement to exclude this ‘background effect’.

For more information, contact Dr Masanori Inagaki (M.Inagaki@cgiar.org)