**3. Canopy temperature (CT) and canopy temperature depression (CTD)**

Plant water balance is a direct measure of drought response of crops. In fact, the transpiration is the main cause of changes in leaf temperature, and there is a direct relationship between leaf temperature, transpiration rate, leaf porosity, and stomatal conductance [30]. As long as the plants continue to transpire through open stomata, the canopy temperatures could be maintained at metabolically comfortable range; otherwise, higher temperature would slow or retard the vital enzymatic activities and consequently the overall metabolism. The closure of stomata for a considerable period of time, especially during the periods of higher evaporative demands driven by high temperature and vapor pressure deficit, is known to increase the leaf temperature [31] and hamper plant's ability to maintain a relatively cooler canopy during grain filling period as an important physiological adaptation for stress [22]. Canopy temperature differences have been shown to correlate well with the transpiration status in rice, potatoes, wheat, and sugar beet. Deviation of temperature of plant canopies from the ambient temperature, also known as canopy temperature depression = air temperature (Ta) − canopy temperature, has been recognized as an indicator of overall plant water status [33] and facilitates in evaluation of plant response to stresses like high temperature [34] and drought [35, 36]. CTD is positive when the canopy is cooler than the air, and this value has been associated with yield increase in different crops [37, 38]. The thermal imagery system is a powerful tool as it can capture the temperature difference of plant canopies quite rapidly.

Thermal infrared imaging and infrared thermography (IRT), to measure the canopy or leaf temperature, are the twin approaches that measure the extent of evaporative cooling occurring in a crop canopy and allow a remote sensing of the plant water balance. Between these two approaches, thermal infrared imaging through an infrared camera offers several benefits compared with temperature sensors, most importantly the facility for spatial resolution and the ability to sample larger area. Most infrared cameras currently have arrays of 320 × 240 sensor elements, which mean that >75,000 individual temperature readings are recorded in a single image. This allows more precise measurements in a fraction of the time needed to perform several replicate readings per plot, which is also prone to error due to changing environmental conditions between measurements. Canopy temperature is one such integrative trait that reflects the plant water status or the resultant equilibrium between root water uptake and shoot transpiration [39]. Canopy temperature has been used successfully as selection criteria in breeding for drought-prone environments [33, 37, 40].

At ambient temperature, all objects emit far-infrared light of approximately 10 μm wavelength [41]. Detectors sensitive in the 8–14 μm wavelength bands convert this radiation into a temperature reading. Such detectors are the basis of non-imaging infrared thermometers, which yield an average temperature measurement of all objects within the field of view. Applications of these simple and affordable instruments include forest canopy studies and irrigation scheduling in field crops [42]. There are yet other thermometers based on infrared imaging that can capture images by adding a scanning system, and each point of measurement is a temperature value based on a pseudo-color value that depends on the radiation captured. The radiation is converted to visual pseudo-color images representing different temperature levels. Both the nonimaging and scanning image thermometers are now being routinely used to measure the temperatures of leaves or canopy in controlled and field conditions. In case of greenhouse or growth chamber experiments where only one or two plants are used per replication, leaf temperatures are used, whereas in case of field experiments where comparatively larger plots are used, canopy temperature is mostly used. Nowadays, unmanned aerial vehicles (UAVs) or robotic equipments fitted with sensors and cameras can be used for monitoring stress advancement in greenhouses and field trials.

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**Figure 1.**

*Infrared camera images of bean leaves (source: P. A. Sofi).*

(**Figure 1**).

*Canopy Temperature Depression as an Effective Physiological Trait for Drought Screening*

soil-borne disease that affect root growth may confound the relationship.

Grant et al. [50] investigated the robustness and sensitivity of thermal imaging for detecting changes in stomatal conductance and leaf water status in a range of plant species (grapevine, bean, and lupin) under greenhouse or controlled environment conditions. In particular, they compared absolute leaf temperatures and thermal indices of plant stress with stomatal conductance and water potential. Thermal imaging is successfully distinguished between irrigated and nonirrigated plants of different species, with strong correlations between thermal indices and stomatal conductance as measured with a leaf pyrometer. Factors such as leaf angle are important and should be given due consideration when using thermal imaging for indirect measurement of the level of drought stress of the tested materials

Infrared thermometry was first used for scheduling crop irrigation in the 1970s [43], while the use of canopy temperature in drought screening began in the early 1980s [44]. The use of canopy temperature in Centro Internacional de Mejoramiento de Maíz y Trigo or International Maize and Wheat Improvement Center (CIMMYT) breeding research began in the early 1990s for hot, irrigated environments [45] and has also been used as a selection criterion for isolating drought-tolerant parental lines for initiating strategic crossing as well as for early generation selection under drought (i.e., from F3 generation onward). Canopy temperature measured by non-imaging IR thermometer can markedly accelerate selection of drought-tolerant genotypes given on high operational speed (≈10 seconds per plot), simplicity, and relatively economically friendly measurements. It is also integrative of the whole canopy due to scoring many plants at once, thus reducing error associated with plant-to-plant variation [46]. In addition, measurements of CT on plants do not interfere with the sensitive stomata, in comparison with other methods that estimate leaf conductance such as porometry and other gas exchange approaches. These may include accurate estimation of the temperatures of different organs of a single plant or the simultaneous capture of CT of all plots in a large trial [47, 48]. Besides, canopy temperature may be related directly to the genetic potential of the root's capacity to explore soil moisture [49]; however, factors such as microelement deficiency or

*DOI: http://dx.doi.org/10.5772/intechopen.85966*

### *Canopy Temperature Depression as an Effective Physiological Trait for Drought Screening DOI: http://dx.doi.org/10.5772/intechopen.85966*

Infrared thermometry was first used for scheduling crop irrigation in the 1970s [43], while the use of canopy temperature in drought screening began in the early 1980s [44]. The use of canopy temperature in Centro Internacional de Mejoramiento de Maíz y Trigo or International Maize and Wheat Improvement Center (CIMMYT) breeding research began in the early 1990s for hot, irrigated environments [45] and has also been used as a selection criterion for isolating drought-tolerant parental lines for initiating strategic crossing as well as for early generation selection under drought (i.e., from F3 generation onward). Canopy temperature measured by non-imaging IR thermometer can markedly accelerate selection of drought-tolerant genotypes given on high operational speed (≈10 seconds per plot), simplicity, and relatively economically friendly measurements. It is also integrative of the whole canopy due to scoring many plants at once, thus reducing error associated with plant-to-plant variation [46]. In addition, measurements of CT on plants do not interfere with the sensitive stomata, in comparison with other methods that estimate leaf conductance such as porometry and other gas exchange approaches. These may include accurate estimation of the temperatures of different organs of a single plant or the simultaneous capture of CT of all plots in a large trial [47, 48]. Besides, canopy temperature may be related directly to the genetic potential of the root's capacity to explore soil moisture [49]; however, factors such as microelement deficiency or soil-borne disease that affect root growth may confound the relationship.

Grant et al. [50] investigated the robustness and sensitivity of thermal imaging for detecting changes in stomatal conductance and leaf water status in a range of plant species (grapevine, bean, and lupin) under greenhouse or controlled environment conditions. In particular, they compared absolute leaf temperatures and thermal indices of plant stress with stomatal conductance and water potential. Thermal imaging is successfully distinguished between irrigated and nonirrigated plants of different species, with strong correlations between thermal indices and stomatal conductance as measured with a leaf pyrometer. Factors such as leaf angle are important and should be given due consideration when using thermal imaging for indirect measurement of the level of drought stress of the tested materials (**Figure 1**).

**Figure 1.** *Infrared camera images of bean leaves (source: P. A. Sofi).*

*Drought - Detection and Solutions*

**3. Canopy temperature (CT) and canopy temperature depression (CTD)**

it can capture the temperature difference of plant canopies quite rapidly.

tion criteria in breeding for drought-prone environments [33, 37, 40].

Thermal infrared imaging and infrared thermography (IRT), to measure the canopy or leaf temperature, are the twin approaches that measure the extent of evaporative cooling occurring in a crop canopy and allow a remote sensing of the plant water balance. Between these two approaches, thermal infrared imaging through an infrared camera offers several benefits compared with temperature sensors, most importantly the facility for spatial resolution and the ability to sample larger area. Most infrared cameras currently have arrays of 320 × 240 sensor elements, which mean that >75,000 individual temperature readings are recorded in a single image. This allows more precise measurements in a fraction of the time needed to perform several replicate readings per plot, which is also prone to error due to changing environmental conditions between measurements. Canopy temperature is one such integrative trait that reflects the plant water status or the resultant equilibrium between root water uptake and shoot transpiration [39]. Canopy temperature has been used successfully as selec-

At ambient temperature, all objects emit far-infrared light of approximately 10 μm wavelength [41]. Detectors sensitive in the 8–14 μm wavelength bands convert this radiation into a temperature reading. Such detectors are the basis of non-imaging infrared thermometers, which yield an average temperature measurement of all objects within the field of view. Applications of these simple and affordable instruments include forest canopy studies and irrigation scheduling in field crops [42]. There are yet other thermometers based on infrared imaging that can capture images by adding a scanning system, and each point of measurement is a temperature value based on a pseudo-color value that depends on the radiation captured. The radiation is converted to visual pseudo-color images representing different temperature levels. Both the nonimaging and scanning image thermometers are now being routinely used to measure the temperatures of leaves or canopy in controlled and field conditions. In case of greenhouse or growth chamber experiments where only one or two plants are used per replication, leaf temperatures are used, whereas in case of field experiments where comparatively larger plots are used, canopy temperature is mostly used. Nowadays, unmanned aerial vehicles (UAVs) or robotic equipments fitted with sensors and cameras can be used for monitoring stress advancement in greenhouses and field trials.

Plant water balance is a direct measure of drought response of crops. In fact, the transpiration is the main cause of changes in leaf temperature, and there is a direct relationship between leaf temperature, transpiration rate, leaf porosity, and stomatal conductance [30]. As long as the plants continue to transpire through open stomata, the canopy temperatures could be maintained at metabolically comfortable range; otherwise, higher temperature would slow or retard the vital enzymatic activities and consequently the overall metabolism. The closure of stomata for a considerable period of time, especially during the periods of higher evaporative demands driven by high temperature and vapor pressure deficit, is known to increase the leaf temperature [31] and hamper plant's ability to maintain a relatively cooler canopy during grain filling period as an important physiological adaptation for stress [22]. Canopy temperature differences have been shown to correlate well with the transpiration status in rice, potatoes, wheat, and sugar beet. Deviation of temperature of plant canopies from the ambient temperature, also known as canopy temperature depression = air temperature (Ta) − canopy temperature, has been recognized as an indicator of overall plant water status [33] and facilitates in evaluation of plant response to stresses like high temperature [34] and drought [35, 36]. CTD is positive when the canopy is cooler than the air, and this value has been associated with yield increase in different crops [37, 38]. The thermal imagery system is a powerful tool as

**80**
