**4. CTD as an effective surrogate trait for drought screening**

Canopy temperature is one of the many physiological traits that may help to identify drought-tolerant cultivars. Canopy temperature depression is the difference between air temperature and plant canopy temperature [51]. Under drought conditions, stomatal conductance decreases when soil moisture is not adequate to keep up with evaporative demands; and this, in turn, increases canopy temperature [52]. Plant morphological trait such as canopy architecture also influences canopy temperature not only through the angle of leaves to the light source but also through the degree of mutual shading in the canopy . Canopy temperature can provide plant-based information on the water status of the crop [53]. Under both greenhouse and field conditions, genotypes with a cooler canopy temperature (higher CTD) under drought stress use more available soil moisture to cool the canopy by transpiration to avoid excessive dehydration [54, 55]. In a large number of experiments in diverse crops, CTD has been found to have significant correlation with grain yield (**Table 3**).

Canopy temperature is also related directly to the genetic potential of the root's capacity to explore soil moisture [32, 56]. Canopy temperature depression can be used as effective proxy traits for the analysis of root development and biomass partitioning under drought stress [57]. Cool canopy temperatures are reported to be associated with enhanced plant access to water by virtue of deeper roots [49], and the common bean genotypes with cooler canopy temperatures reported 30% more yield associated with an increase of 40% in root dry weight at 60–120 cm. Canopy temperature depression has been shown to be correlated with yield under drought stress ([32, 35, 58, 59]; **Table 3**) and hot irrigated conditions [32, 60]. Canopy temperatures under well-watered conditions also indicate potential yield performance during drought and could effectively be used as a technique to assess genotypic response to drought [61]. Blum et al. [62] used canopy temperatures of drought stress wheat genotypes to characterize yield stability under various moisture conditions. A positive correlation was found between a drought susceptibility index and canopy temperature in stressed environments. Drought-susceptible genotypes which suffered relatively greater yield loss under drought stress tended to have warmer canopies at midday.


**83**

and grain formation.

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

**5. Toward a crop ideotype based on canopy temperature depression**

Blum has proposed ideotypes of crop plants based on canopy temperature depression for use in plant breeding as per the drought types such as the isohydric ("water saving") model and the anisohydric ("water spending") model. The water saving model has a distinct advantage in the harsher environments, whereas the water spending model is expected to perform relatively better under more moderate/mild drought situations. Polania et al. [28] have proposed that the water spender genotypes can be used for cultivation in areas exposed to intermittent drought stress with soils that can store greater amount of available water deep in the soil profile. However, water savers can be more suitable in semiarid to dry environments dominated by the terminal drought stress. The water savers or isohydric genotypes are characterized by a shallow root system with intermediate root growth and penetration ability and thin roots. Such genotypes are early and have high water use efficiency, reduced transpiration and limited leaf area and canopy biomass development, reduced sink strength, and superior photosynthate remobilization to pod and grain formation. Contrary to this, water spenders or anisohydric genotypes have a vigorous and deep rooting system with rapid root growth rate and penetration ability and a thicker root system. Such genotypes are early and have highly effective water use, moderate transpiration and fast leaf area and canopy biomass development, moderate sink strength, and superior photosynthate remobilization to pod

CTD can be affected by biological and environmental factors like water status of soil, wind, evapotranspiration, cloudiness, conduction systems, plant metabolism, air temperature, relative humidity, and continuous radiation [63] and has preferably been measured in high air temperature and low relative humidity because of high vapor pressure deficit conditions [60]. At the end of the 1980s, CIMMYT began CTD measurements on different irrigated experiments in Northwest Mexico. Phenotypic correlations of CTD with grain yield were occasionally positive [37]. CTD has been used as selection criteria for tolerance to drought and high temperature stress in wheat breeding, and the used breeding method is generally mass selection in early generations like F3. According to this method, firstly, bulks which show high CTD value (have cool canopy) are selected in F3 generation. Later, single plants which show high stomata conductance (g) among bulks also show cool canopy at the same selection generation; thus, both of these traits are used at the same breeding program [63]. CTD can be a reliable indicator of crop performance under both irrigated and drought stress conditions. Under irrigated conditions there was a linear trend of higher yield with CTD; however, under drought stress, both negative CTD and positive CTD could be identified, and in both classes, high-yielding genotypes were identified. The water savers probably could sense drought stress in early phases of growth and could trigger conservative water use that could be used in later stages of growth [30]. However, the reduction in water use is generally achieved by plant traits and environmental responses that could also reduce yield potential [64]. Under optimum experimental conditions provided that data are collected when the canopy is sufficiently expanded to cover the soil, CTD can be a good predictor of crop yield (*r* = 0.6–0.85; [65]). In wheat, yield progress was found to be associated with cooler canopies [37], and significant genetic gains in yield have been reported in response to direct selection for CTD [55, 65]. Reynolds et al. have made a comparative analysis of aerial and handheld IR thermometers and found that correlation of CTD with grain yield was comparable (r = 0.68\*\* and 0.73\*\*, respectively).

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

#### **Table 3.**

*Correlation of CTD with grain yield in various crops.*

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

CTD can be affected by biological and environmental factors like water status of soil, wind, evapotranspiration, cloudiness, conduction systems, plant metabolism, air temperature, relative humidity, and continuous radiation [63] and has preferably been measured in high air temperature and low relative humidity because of high vapor pressure deficit conditions [60]. At the end of the 1980s, CIMMYT began CTD measurements on different irrigated experiments in Northwest Mexico. Phenotypic correlations of CTD with grain yield were occasionally positive [37]. CTD has been used as selection criteria for tolerance to drought and high temperature stress in wheat breeding, and the used breeding method is generally mass selection in early generations like F3. According to this method, firstly, bulks which show high CTD value (have cool canopy) are selected in F3 generation. Later, single plants which show high stomata conductance (g) among bulks also show cool canopy at the same selection generation; thus, both of these traits are used at the same breeding program [63].

CTD can be a reliable indicator of crop performance under both irrigated and drought stress conditions. Under irrigated conditions there was a linear trend of higher yield with CTD; however, under drought stress, both negative CTD and positive CTD could be identified, and in both classes, high-yielding genotypes were identified. The water savers probably could sense drought stress in early phases of growth and could trigger conservative water use that could be used in later stages of growth [30]. However, the reduction in water use is generally achieved by plant traits and environmental responses that could also reduce yield potential [64]. Under optimum experimental conditions provided that data are collected when the canopy is sufficiently expanded to cover the soil, CTD can be a good predictor of crop yield (*r* = 0.6–0.85; [65]). In wheat, yield progress was found to be associated with cooler canopies [37], and significant genetic gains in yield have been reported in response to direct selection for CTD [55, 65]. Reynolds et al. have made a comparative analysis of aerial and handheld IR thermometers and found that correlation of CTD with grain yield was comparable (r = 0.68\*\* and 0.73\*\*, respectively).
