**2. Breeding for drought tolerance**

Breeding for drought tolerance is a sustainable option to reduce the risk of crop failure by improving the ability of crop plants to extract water from the deeper soil strata through better root architecture, by decreasing the amount of crop water demands (improving water use efficiency), or by improving a crop's ability to survive longer periods without water, thereby ultimately increasing yields in rainfed environments. However, breeding for drought tolerance is complex because


#### **Table 1.**

*Proportion of the cultivated area affected by drought in different regions and for different crops (source: [12]).*

**79**

**Table 2.**

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

**Crop Yield reduction References** Barley 49–57% [13] Chickpea 78% [14] Groundnut 55–72% [15] Maize 43–80% [16] Oat 79% [17] Potato 89% [17] Rice 42–66% [18] Brassica 39% [17] Rye 52% [17] Bread wheat 57% [17] Durum wheat 74% [19] Pigeon pea 42% [20] Green gram 71% [20] Black gram 74% [20]

it involves quantitative inheritance and environmental influence [21]. Efforts to breed for drought tolerance are invariably hampered by the amount of time required to phenotype a large number of individuals and poor or inconsistent correlation between a phenotype and yield under drought conditions due, in part, to multiple mechanisms involved. Various authors have investigated the genetic basis of drought tolerance in common bean and reported that both additive and nonadditive gene actions are involved in drought tolerance [22, 23]. Schneider et al. [24] reported a strong genotype x environment interaction in the expression of identified quantitative trait locus (QTL), such that potential for marker-assisted selection in breeding for drought tolerance was also inconclusive. Selection based solely on yield performance confounds the complexity of breeding for drought as yield is a highly complex trait with low heritability especially under stress conditions. Therefore, it is imperative to identify less complex traits related to the drought that will improve upon selection for drought tolerance and separate these traits into major components which may help further understanding of the genetic basis.

A better understanding of the relationship between below- and aboveground traits will contribute to improved productivity under drought stress. Root traits including structure and their spatial distribution of root system in different soil horizons are essential for yield improvement because of its high heritability under drought stress [25–27] and high correlation with yield traits [28]. However, extensive use of roots as the target traits for developing climate resilience suffers from the difficulties associated with studying roots, especially under field conditions. The shoot traits are easy to measure and quantify; however, it has to be linked with root traits with the perspective of improving drought tolerance. In the following sections, we discuss some of the potential aboveground traits that have been shown to be correlated with improved drought tolerance as well as better grain yield under stress. Currently, there is a huge shopping list of relatively unranked traits that have been proposed to be used as surrogates for drought tolerance response. Canopy temperature depression has emerged as a potential surrogate in view of substantial natural variation in crops as well as its

correlation with yield under both stress and nonstress conditions [29].

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

*Impact of drought stress on yield reduction in different crops.*


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

#### **Table 2.**

*Drought - Detection and Solutions*

reported in various crops.

**2. Breeding for drought tolerance**

1–3°C by the mid-twenty-first century and by about 2–5°C by the late twenty-first century, depending on the region and emission scenario [3]. Based on historical data collected in Africa on more than 20,000 trials between 1999 and 2007, each "degree day" spent above 30° is likely to reduce crop yields by 1% under optimal conditions and that penalty is going to increase up to 1.7% under water-limited conditions [4]. The impact of a climate change is not only about the projected increase in temperature, but it also affects the magnitude and distribution of rainfall, as well as availability of water at critical times of the crop growth [5]. While as the total amount of rain has recorded an increase in Africa over the last few years, the erratic and unpredictable nature of the drought and floods cycle has also increased [6]. Globally, rainfed agriculture is practiced in 80% of the total agricultural area and generates 62% of the world's staple food (FAOSTAT, 2011). In view of the current global water scarcity scenarios, climate change implications, and increases in demand for nonagricultural water use, the expansion of the area under irrigation, especially in developing countries, does not seem to be a realistic proposition to address food security challenges. Drought is one of the major production constraints in agriculture worldwide. It principally affects crops cultivated under rainfed conditions, which represent 80% of the total cultivated area worldwide. It is estimated that cultivation on the earth is only possible on 16% of the potentially arable area due to limited availability of water [7, 8]. Africa is strongly affected by drought almost every 12 years, but drought intensified during the years 2009– 2011, during which, the wheat yields reduced by 45% in Kenya [9]. Similar trends have also been reported from Australia where drought reduced wheat yields by 46% in 2006 [10]. Around 17% of the global cultivated area was affected by drought during the period 1980–2006 [11]. **Tables 1** and **2** depict the proportion of cultivated areas implicated by drought stress and estimated yield reductions

Breeding for drought tolerance is a sustainable option to reduce the risk of crop failure by improving the ability of crop plants to extract water from the deeper soil strata through better root architecture, by decreasing the amount of crop water demands (improving water use efficiency), or by improving a crop's ability to survive longer periods without water, thereby ultimately increasing yields in rainfed environments. However, breeding for drought tolerance is complex because

**Region Crop species Proportion of the cultivated area** 

*Proportion of the cultivated area affected by drought in different regions and for different crops (source: [12]).*

Africa Wheat 80% Eastern Asia Maize 50% Europe Maize 60% North America Wheat 47% Oceania Barley 70% South America Maize 50% South Asia Wheat 65% Southeast Asia Rice 65%

**affected by drought**

**78**

**Table 1.**

*Impact of drought stress on yield reduction in different crops.*

it involves quantitative inheritance and environmental influence [21]. Efforts to breed for drought tolerance are invariably hampered by the amount of time required to phenotype a large number of individuals and poor or inconsistent correlation between a phenotype and yield under drought conditions due, in part, to multiple mechanisms involved. Various authors have investigated the genetic basis of drought tolerance in common bean and reported that both additive and nonadditive gene actions are involved in drought tolerance [22, 23]. Schneider et al. [24] reported a strong genotype x environment interaction in the expression of identified quantitative trait locus (QTL), such that potential for marker-assisted selection in breeding for drought tolerance was also inconclusive. Selection based solely on yield performance confounds the complexity of breeding for drought as yield is a highly complex trait with low heritability especially under stress conditions. Therefore, it is imperative to identify less complex traits related to the drought that will improve upon selection for drought tolerance and separate these traits into major components which may help further understanding of the genetic basis.

A better understanding of the relationship between below- and aboveground traits will contribute to improved productivity under drought stress. Root traits including structure and their spatial distribution of root system in different soil horizons are essential for yield improvement because of its high heritability under drought stress [25–27] and high correlation with yield traits [28]. However, extensive use of roots as the target traits for developing climate resilience suffers from the difficulties associated with studying roots, especially under field conditions. The shoot traits are easy to measure and quantify; however, it has to be linked with root traits with the perspective of improving drought tolerance. In the following sections, we discuss some of the potential aboveground traits that have been shown to be correlated with improved drought tolerance as well as better grain yield under stress. Currently, there is a huge shopping list of relatively unranked traits that have been proposed to be used as surrogates for drought tolerance response. Canopy temperature depression has emerged as a potential surrogate in view of substantial natural variation in crops as well as its correlation with yield under both stress and nonstress conditions [29].
