**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 and grain formation.

*Drought - Detection and Solutions*

grain yield (**Table 3**).

canopies at midday.

Triticale and barley

Wheat Positive association (*r*

Chickpea Positive association (*r*

Common bean Positive association (*r*

Groundnut Positive association (*r*

Sorghum Positive association (*r*

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

*P* ≤ 0.001)

*P* ≤ 0.001)

*P* ≤ 0.001)

*P* ≤ 0.001)

*P* ≤ 0.001)

*P* ≤ 0.001)

Positive association (*r*

**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

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

**Crop Trait relationship with yield References**

2

2 = 0.40;

2

2 = 0.44;

2 = 0.19;

2 = 0.76;

= 0.45–0.89;

= 0.11–0.32;

Amani et al. [60]; Fischer et al. [37]; Balota

Asfaw et al. [25] and Polania et al. [28]

et al. [33]

Singh et al. [67]

Mutawa [58]

Roohi et al. [68]

Purushothaman et al. [59]

**82**

**Table 3.**

Our studies in beans and cowpea have also revealed that CTD measurements can be used to build a crop ideotype for water stress response. In our studies with beans and cowpea, we found that CTD values across stages decreased progressively on account of rapid depletion of moisture (**Figure 2**). The genotypes could be grouped into water savers and water spenders using the sign of CTD values (**Figures 3** and **4**). The water spenders have higher stomatal conductance and lose water through transpiration, whereas water savers have conservative water use on account of lower stomatal conductance or early closure of stomata and as such have hotter canopies. Under irrigated conditions also, we found a linear relationship with genotypes having higher CTD values showing better yields, whereas under water-stressed conditions, high-yielding genotypes could be found in both groups.

Canopy temperature can be related to the genetic potential of the root's capacity to explore soil moisture [32, 56] and as such can be used as effective surrogate trait for the analysis of root development and biomass partitioning under drought stress [57]. Cool canopies (+CTD) are reported to be associated with enhanced plant access to water by virtue of deeper roots (Lopes and Reynolds 2010), and the genotypes with cooler canopies have been reported to yield 30% more, with a concomitant increase of 40% in root dry weight. CTD has been reported to be correlated with yield under both drought stress [32, 35, 59] and hot irrigated conditions [32]. Drought-susceptible genotypes which suffered relatively greater yield loss under drought stress tended to have warmer canopies at midday. Our studies have revealed that 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].

In recent years, with the availability of high-throughput phenotyping platforms, canopy temperature depression has been widely used to study genotypic response to drought. Blum et al. [62] used canopy temperatures of drought stress wheat genotypes to characterize yield stability under various moisture conditions. In most of the studies using CTD, a positive correlation has been found between

**85**

**Figure 3.**

**Figure 4.**

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

*Variation for CTD averaged over 3 stages in 20 genotypes of cowpea under irrigated conditions.*

drought susceptibility index and canopy temperature in stressed environments. Drought-susceptible genotypes which suffered relatively greater yield loss under stress tended to have warmer canopies at midday. Under well-watered conditions also, CTD provides a fair indication of potential yield performance during drought and could effectively be used as a technique to assess genotypic response to drought. Rashid et al. [35] reported that significant correlation between canopy temperature and yield under moisture-stress conditions and stress susceptibility index values indicated the potential for screening wheat genotypes for drought response. Canopy temperature depression is positive when the canopy is cooler than the air (CTD = Ta – Tc). It has been used in various practical applications including evaluation of plant response to environmental stress [66] and irrigation scheduling [69], to evaluate cultivars for water use [70], tolerance to heat [71], and drought [35, 62]. In general, CTD has been used to assess plant water status because it represents an overall, integrated

*Variation for CTD averaged over 3 stages in 20 genotypes of cowpea under drought stress.*

*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*

#### **Figure 3.**

*Drought - Detection and Solutions*

genotypes could be found in both groups.

Our studies in beans and cowpea have also revealed that CTD measurements can be used to build a crop ideotype for water stress response. In our studies with beans and cowpea, we found that CTD values across stages decreased progressively on account of rapid depletion of moisture (**Figure 2**). The genotypes could be grouped into water savers and water spenders using the sign of CTD values (**Figures 3** and **4**). The water spenders have higher stomatal conductance and lose water through transpiration, whereas water savers have conservative water use on account of lower stomatal conductance or early closure of stomata and as such have hotter canopies. Under irrigated conditions also, we found a linear relationship with genotypes having higher CTD values showing better yields, whereas under water-stressed conditions, high-yielding

Canopy temperature can be related to the genetic potential of the root's capacity to explore soil moisture [32, 56] and as such can be used as effective surrogate trait for the analysis of root development and biomass partitioning under drought stress [57]. Cool canopies (+CTD) are reported to be associated with enhanced plant access to water by virtue of deeper roots (Lopes and Reynolds 2010), and the genotypes with cooler canopies have been reported to yield 30% more, with a concomitant increase of 40% in root dry weight. CTD has been reported to be correlated with yield under both drought stress [32, 35, 59] and hot irrigated conditions [32]. Drought-susceptible genotypes which suffered relatively greater yield loss under drought stress tended to have warmer canopies at midday. Our studies have revealed that 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]. In recent years, with the availability of high-throughput phenotyping platforms, canopy temperature depression has been widely used to study genotypic response to drought. Blum et al. [62] used canopy temperatures of drought stress wheat genotypes to characterize yield stability under various moisture conditions. In most of the studies using CTD, a positive correlation has been found between

**84**

**Figure 2.**

*Mean CTD across genotypes at the second, third, and fourth week of stress imposition.*

*Variation for CTD averaged over 3 stages in 20 genotypes of cowpea under irrigated conditions.*

#### **Figure 4.**

*Variation for CTD averaged over 3 stages in 20 genotypes of cowpea under drought stress.*

drought susceptibility index and canopy temperature in stressed environments. Drought-susceptible genotypes which suffered relatively greater yield loss under stress tended to have warmer canopies at midday. Under well-watered conditions also, CTD provides a fair indication of potential yield performance during drought and could effectively be used as a technique to assess genotypic response to drought. Rashid et al. [35] reported that significant correlation between canopy temperature and yield under moisture-stress conditions and stress susceptibility index values indicated the potential for screening wheat genotypes for drought response. Canopy temperature depression is positive when the canopy is cooler than the air (CTD = Ta – Tc). It has been used in various practical applications including evaluation of plant response to environmental stress [66] and irrigation scheduling [69], to evaluate cultivars for water use [70], tolerance to heat [71], and drought [35, 62]. In general, CTD has been used to assess plant water status because it represents an overall, integrated

physiological response to drought and high temperature [60]. Overall, the existing literature suggests that dominant mechanisms that increase CTD vary with environment and crop species .

Canopy temperature is a useful indicator of crop water status [43] and has the potential as a tool for indirect selection of genotypes tolerant to drought and heat-stressed environments [55]. For field experiments in wheat, CT data is most commonly measured on a whole-plot basis using a handheld infrared thermometer [71], although more rapid assessment using thermal imaging [72] is growing in popularity. CT is influenced by a number of environmental factors including the amount of solar radiation hitting the canopy, soil moisture, wind speed, temperature, and relative humidity [73]. Genetic differences in CT result from variation in the plant's ability to move water through the vascular system, differences in stomata aperture driving transpiration, root biomass and depth, metabolism, and source sink balance [74]. As such, CT has been shown to correlate with these physiological traits under field conditions and integrates them into a single low-cost diagnostic measurement that has a potential for selection of tolerant parental genotypes or early generation breeding lines [55]. CT has moderate heritability across environments in both diverse sets of germplasm [49] and in related material such as recombinant inbred populations [73]. Lopes and Reynolds [49] found similar broad-sense heritability for a diverse set of 294 spring wheat lines (H2 = 0.38) and a set of 169 sister lines (H2 = 0.34) across well-watered, drought-stressed, and heat-stressed environments in Northwest Mexico. Genetically, CT is a quantitative trait. Pierre et al. [74] determined the gene action for CT to be mainly additive by additive in five wheat populations with some dominant effects. Genetic mapping shows CT to be controlled mostly by small effect loci that are pleiotropic with variation in other traits, such as days to heading and plant height [20]. The correlation between CT and yield is consistently negative in the literature in both drought and heat environments such that a cooler canopy provides a yield benefit under stress [73]. Exceptions have been shown in both bread wheat [75], where CT measurements taken in Mexico were positively correlated with yield at international sites, and in durum wheat [76], where CT was found to increase with date of cultivar release and increasing yield. Experiments investigating CT are often conducted with sets of lines preselected for variation in canopy temperature or other tolerance traits [49], international trials of elite drought and heat tolerant lines [45], or using historical germplasm [9, 19, 21] and may not be representative of variation present in the early stages of yield testing in a breeding program. Reynolds et al. [55] demonstrated that advanced lines derived from "physiological crosses" targeted at one or more adaptive traits had a definite yield advantage over "conventional crosses" where physiological traits including CTD were not considered in parental selection. However, there is a need to investigate the ability of CT to select high-yielding lines within the germplasm flow of a breeding program where very little preselection for stress tolerance per se has been done.

#### **6. Conclusion**

Both empirical breeding and analytical approaches are used for improving crop performance under changing climate (drought, high temperature, etc.). However, there is a strong argument evolving in support of the analytical approaches based on indirect selection approaches using efficient surrogate traits to enhance the scale and reliability of phenotyping. Infrared thermometry can detect small differences in leaf temperature in both field and greenhouse conditions, measurements are fast and nondestructive, and the trait has a moderate to high heritability and

**87**

**Author details**

provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Parvaze Ahmad Sofi\*, Asmat Ara, Musharib Gull and Khalid Rehman Dry Land Agricultural Research Station, SKUAST-Kashmir, India

\*Address all correspondence to: parvazesofi@gmail.com

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

The facilities provided by the Faculty of Agriculture are gratefully

shows positive correlation with yield [44]. Measurements should however be made well before the crop maturity and due consideration should be given to biological and environmental factors such as water status of soil, wind, evapotranspiration, cloudiness, conduction systems, plant metabolism, air temperature, relative humidity, and continuous radiation [55]. In light of substantial experimental evidence that a fairly positive relationship exists between yield and CTD under both stressed and nonstressed conditions, it is essential to incorporate CTD as effective complementary trait in selection programs aimed at developing climate resilient varieties.

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

**Acknowledgements**

acknowledged.

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

shows positive correlation with yield [44]. Measurements should however be made well before the crop maturity and due consideration should be given to biological and environmental factors such as water status of soil, wind, evapotranspiration, cloudiness, conduction systems, plant metabolism, air temperature, relative humidity, and continuous radiation [55]. In light of substantial experimental evidence that a fairly positive relationship exists between yield and CTD under both stressed and nonstressed conditions, it is essential to incorporate CTD as effective complementary trait in selection programs aimed at developing climate resilient varieties.
