**5. Breeding for heat tolerance: a next-generation breeding approach**

The emphasis of crop improvement has been primarily on improving the economic yield in majority of the crop plants. This targeted breeding for economically desirable traits in crops has resulted in reduced genetic variability in the commercial varieties/hybrids to reach homogeneity in appearance. Hence, other essential genes that enable growth and reproduction in adverse environments can be absent from modern cultivars owing to their exclusion or loss during domestication or subsequent germplasm improvement, and linkage to non-beneficial loci or drag on productivity in optimal environments [1]. Useful loci and allelic variants often correspond to the downregulation or disruption of genes in susceptible genotypes. Hence, continued and broader mining of germplasm could be advantageous. Toward breeding for heat tolerance in crops, the initial search for tolerant sources should begin among the modern cultivars/advanced breeding lines and landraces of the species. The further search should be shifted to primary and secondary gene pool in sequence. For efficient utilization of the identified sources, we need to understand the underlying component traits, their inheritance including genes/QTLs and also association among important traits. All this information generated would facilitate targeted breeding for heat tolerance in crops.

## **5.1. Germplasm as sources for heat tolerance breeding**

Wide variation for heat tolerance has been noted in both cultivated and related wild species among different crop plants (**Table 2**). Landraces are the varieties preferably handled by local farmers which are adapted to their native environment and could be the potential sources of HT. Significant variability for HT has been noted among wheat landraces, wherein tolerant ones tend to have higher leaf chlorophyll contents [64] and higher stomatal conductance, which can be utilized in breeding programs. Early maturity under high-temperature conditions is closely correlated with lesser yield losses in many crop plants. In wheat, early heading varieties performed better than later-heading varieties because they (i) produced fewer leaves per tiller and retained more green leaves, (ii) had longer grain-filling periods, and (iii) completed grain filling earlier in the season when air temperatures were lower [107].

The early maturity-led escape mechanism enabled addressing heat stress in wheat in Eastern Gangetic Plains and various South Asian locations [108, 109]. Selection for early flowering and maturity has also enabled to escape heat stress in spring-sown chickpea in Mediterranean


have been developed/proposed by different researchers are briefly discussed above, however, the primary field screening methods also include seedling thermo-tolerance index (STI) [73], seed to seedling thermo-tolerance index (SSTI) in pearl millet [74], and heat tolerance index (HTI) as growth recovery after heat exposure in sorghum [75]. Thermo-tolerance screening at germination and early vegetative stage is found effective for pearl millet and maize [76]. These field techniques would help in preliminary identification of heat tolerance lines and thus proceed with minimum number of lines for further screening and validation. At the same time, breeder should ensure the quality of individual line data by comparing with tolerant check at all the times. This will facilitate the more reliable way of advancing the heat tolerance

**5. Breeding for heat tolerance: a next-generation breeding approach**

erated would facilitate targeted breeding for heat tolerance in crops.

**5.1. Germplasm as sources for heat tolerance breeding**

The emphasis of crop improvement has been primarily on improving the economic yield in majority of the crop plants. This targeted breeding for economically desirable traits in crops has resulted in reduced genetic variability in the commercial varieties/hybrids to reach homogeneity in appearance. Hence, other essential genes that enable growth and reproduction in adverse environments can be absent from modern cultivars owing to their exclusion or loss during domestication or subsequent germplasm improvement, and linkage to non-beneficial loci or drag on productivity in optimal environments [1]. Useful loci and allelic variants often correspond to the downregulation or disruption of genes in susceptible genotypes. Hence, continued and broader mining of germplasm could be advantageous. Toward breeding for heat tolerance in crops, the initial search for tolerant sources should begin among the modern cultivars/advanced breeding lines and landraces of the species. The further search should be shifted to primary and secondary gene pool in sequence. For efficient utilization of the identified sources, we need to understand the underlying component traits, their inheritance including genes/QTLs and also association among important traits. All this information gen-

Wide variation for heat tolerance has been noted in both cultivated and related wild species among different crop plants (**Table 2**). Landraces are the varieties preferably handled by local farmers which are adapted to their native environment and could be the potential sources of HT. Significant variability for HT has been noted among wheat landraces, wherein tolerant ones tend to have higher leaf chlorophyll contents [64] and higher stomatal conductance, which can be utilized in breeding programs. Early maturity under high-temperature conditions is closely correlated with lesser yield losses in many crop plants. In wheat, early heading varieties performed better than later-heading varieties because they (i) produced fewer leaves per tiller and retained more green leaves, (ii) had longer grain-filling periods, and (iii) com-

The early maturity-led escape mechanism enabled addressing heat stress in wheat in Eastern Gangetic Plains and various South Asian locations [108, 109]. Selection for early flowering and maturity has also enabled to escape heat stress in spring-sown chickpea in Mediterranean

pleted grain filling earlier in the season when air temperatures were lower [107].

genotypes in any afore-discussed screening tools.

52 Next Generation Plant Breeding


before the onset of heat stress. In durum wheat, this property has been utilized in development of early maturing genotypes such as Waha-1, Omrabi-5, and Massara-1 [119]. In rice, introgression breeding has facilitated the transfer of HT from "N22" to "Xieqingzao B" line

N22 cross has offered HT as well as high yield [121]. In wheat, *Aegilops tauschii* was successfully used as a donor for incorporating HT-relevant component traits such as cell membrane stability and chlorophyll retention into cv. PBW550 through backcrossing [122]. More recently, attempts were made in wheat aiming at introgression of wheat-*Leymus racemosus* chromosome to cv. Chinese spring to enhance HT and better adaptation under heat stress [123]. Impressive accomplishments were achieved in harnessing the natural genetic variation for HT, and additional efforts are underway to introduce the heat tolerance QTLs/genes into different genetic backgrounds [88, 120]. In cotton, through pollen selection under HS, heat tolerance genes were transferred from a donor line "7456" (*G. barbadense* L.) to a heat-sensitive genotype "Paymaster 404" through backcrossing [113]. In sunflower, by using temperature induction response (TIR) technique, adequate genetic variability was observed for thermotolerance among the parental lines of the hybrid KBSH-1, viz. CMS234A, CMS234B, and 6D-1 [124]. The availability of potential donors for HT would encourage plant breeders not only to deploy these novel sources directly in breeding schemes but also to excavate the resilient

The efficiency of direct selection for yield improvement under stressed conditions is hindered by low heritability and a complex network of major and minor QTLs governing them [125, 126]. Breeding for high-yielding and heat tolerance lines is limited by the influence of environmental factors, poor understanding of genetic inheritance of HT, and less availability of validated QTLs/cloned gene(s) for HT in plants [127]. Physiological trait-based breeding would be an ideal strategy for incorporating gene(s)/QTLs that determine heat tolerance. Such an approach has been adopted in wheat at CIMMYT to develop heat tolerance varieties [128]. Toward breeding for heat tolerance, physiological traits that need to be considered include those related to canopy structure, delayed senescence, photosynthesis efficiency, less respiration rates, reproductive traits, and harvest index [127, 129]. Genetic variability has been assessed in several crop plants under HS for several physiological traits and suitable tolerant

Genetic variability existing for the plant phenologies conferring HT need to be exploited. Alternatively, selection for morphophysiological traits involved in heat stress adaptation, and also indirectly associated with yield, can be utilized for enhancing HT in crop plants that has been explored in wheat [127]. Substantial genetic variability for photosynthetic rate under HS has been noted in wheat and rice, which would serve as a potential indicator of HS tolerance [130, 131]. While screening over 1000 wheat genotypes, the chlorophyll fluorescence was established as an important physiological parameter [132]. Canopy temperature depression (CTD) has been found to act as heat escape mechanism in cotton [133], while for HT in wheat [81, 134]. The cooler canopy temperature (CT) under HS caused higher yield in wheat [109, 135]. Under HS, the CTD, flag leaf stomatal conductance and photosynthetic rate

lines [120]. Additionally, the advanced line derived from Gayabyeo/

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by developing BC1

F8

alleles that underlie tolerance.

**5.3. Physiological traits-based breeding**

sources and associated traits have been identified (**Table 2**).

together are found to be positively correlated with yield in wheat [62].

**Table 2.** Heat tolerance sources and associated traits/indices of important food crops.

region and south India [32, 110]. The genetic variability for HT in rice could be exploited to screen germplasm and select cultivars that open flowers earlier in the morning or that maintain a high number of spikelets per panicle in warm environments [111]. A positive correlation between canopy temperature depression (CTD) and membrane stability with grain yield have been noticed and recommended as useful traits in selecting high-temperature tolerant genotypes in wheat [80, 112].

Generally, there is a strong correlation between pollen production and viability, anther dehiscence, and seed set. The anthers of heat tolerance rice cultivars dehisce more easily than those of susceptible cultivars under high-temperature conditions [90, 93]. Higher pollen grain fertility under HS may serve as an important criterion for measuring HT [113]. Similarly, gametic selection has been proposed as a viable option for addressing HS in maize [71]. In wheat, maintaining grain weight under heat stress during grain filling is a measure of HT [114, 115]. Hence, it has been proposed that high grain-filling rate and high potential grain weight can be useful selection criteria for improving HT [116]. Stay-green character has also been suggested for mass screening of wheat genotypes for HT [117]. However, this trait may be disadvantageous as it is associated with the tendency to retain the stem reserves [118].

The sources identified for heat tolerance using suitable screening method have to be confirmed for their level of tolerance across different temperature regimes, and breeding approaches have to be outlined to incorporate the tolerance into desirable agronomic background. Transfer of tolerance from cultivated germplasm could be easy but the chances of finding the sources of tolerance are quite less. The crop wild relatives may have higher level of tolerance, but their incorporation into cultivated background needs a perfect prebreeding/backcross program. The availability of molecular markers in crop plants of economic importance could rather be put to use by forward and background selection for introgression of desirable genomic regions associated with heat tolerance.

## **5.2. Conventional breeding: a traditional approach**

The conventional breeding efforts toward development of heat tolerance cultivars are comparatively less among different crops. However, the emphasis has been quite recent and some efforts are being made in few important food crops such as wheat, rice, maize, tomato, potato, etc. Heat escape is an alternative mechanism through which plant completes its life cycle before the onset of heat stress. In durum wheat, this property has been utilized in development of early maturing genotypes such as Waha-1, Omrabi-5, and Massara-1 [119]. In rice, introgression breeding has facilitated the transfer of HT from "N22" to "Xieqingzao B" line by developing BC1 F8 lines [120]. Additionally, the advanced line derived from Gayabyeo/ N22 cross has offered HT as well as high yield [121]. In wheat, *Aegilops tauschii* was successfully used as a donor for incorporating HT-relevant component traits such as cell membrane stability and chlorophyll retention into cv. PBW550 through backcrossing [122]. More recently, attempts were made in wheat aiming at introgression of wheat-*Leymus racemosus* chromosome to cv. Chinese spring to enhance HT and better adaptation under heat stress [123]. Impressive accomplishments were achieved in harnessing the natural genetic variation for HT, and additional efforts are underway to introduce the heat tolerance QTLs/genes into different genetic backgrounds [88, 120]. In cotton, through pollen selection under HS, heat tolerance genes were transferred from a donor line "7456" (*G. barbadense* L.) to a heat-sensitive genotype "Paymaster 404" through backcrossing [113]. In sunflower, by using temperature induction response (TIR) technique, adequate genetic variability was observed for thermotolerance among the parental lines of the hybrid KBSH-1, viz. CMS234A, CMS234B, and 6D-1 [124]. The availability of potential donors for HT would encourage plant breeders not only to deploy these novel sources directly in breeding schemes but also to excavate the resilient alleles that underlie tolerance.

## **5.3. Physiological traits-based breeding**

region and south India [32, 110]. The genetic variability for HT in rice could be exploited to screen germplasm and select cultivars that open flowers earlier in the morning or that maintain a high number of spikelets per panicle in warm environments [111]. A positive correlation between canopy temperature depression (CTD) and membrane stability with grain yield have been noticed and recommended as useful traits in selecting high-temperature tolerant

**Crop Heat tolerance sources\* HT associated trait/index References**

B89-200, TN88-63 Seed yield [103]

susceptibility index (HSI)

Heat tolerance index (HTI), and heat

[101]

[104]

Sorghum DeKalb 28E Pollen viability, seed set, seed yield, and harvest index

> SRC-1-12-1-182; SRC-1-12-1-48; 98020-3-1-7-2; 98012-3-1-2-1

Cowpea California Blackeye 27 (CB27) Flower production and pod set [102]

Chickpea ICCV 92944; several genotypes Seed set [105, 106]

Heat tolerance sources include germplasm, breeding lines, populations, varieties/hybrids, wild species, etc.

**Table 2.** Heat tolerance sources and associated traits/indices of important food crops.

Generally, there is a strong correlation between pollen production and viability, anther dehiscence, and seed set. The anthers of heat tolerance rice cultivars dehisce more easily than those of susceptible cultivars under high-temperature conditions [90, 93]. Higher pollen grain fertility under HS may serve as an important criterion for measuring HT [113]. Similarly, gametic selection has been proposed as a viable option for addressing HS in maize [71]. In wheat, maintaining grain weight under heat stress during grain filling is a measure of HT [114, 115]. Hence, it has been proposed that high grain-filling rate and high potential grain weight can be useful selection criteria for improving HT [116]. Stay-green character has also been suggested for mass screening of wheat genotypes for HT [117]. However, this trait may be disadvanta-

The sources identified for heat tolerance using suitable screening method have to be confirmed for their level of tolerance across different temperature regimes, and breeding approaches have to be outlined to incorporate the tolerance into desirable agronomic background. Transfer of tolerance from cultivated germplasm could be easy but the chances of finding the sources of tolerance are quite less. The crop wild relatives may have higher level of tolerance, but their incorporation into cultivated background needs a perfect prebreeding/backcross program. The availability of molecular markers in crop plants of economic importance could rather be put to use by forward and background selection for introgression of desirable genomic

The conventional breeding efforts toward development of heat tolerance cultivars are comparatively less among different crops. However, the emphasis has been quite recent and some efforts are being made in few important food crops such as wheat, rice, maize, tomato, potato, etc. Heat escape is an alternative mechanism through which plant completes its life cycle

geous as it is associated with the tendency to retain the stem reserves [118].

genotypes in wheat [80, 112].

Common bean

54 Next Generation Plant Breeding

\*

regions associated with heat tolerance.

**5.2. Conventional breeding: a traditional approach**

The efficiency of direct selection for yield improvement under stressed conditions is hindered by low heritability and a complex network of major and minor QTLs governing them [125, 126]. Breeding for high-yielding and heat tolerance lines is limited by the influence of environmental factors, poor understanding of genetic inheritance of HT, and less availability of validated QTLs/cloned gene(s) for HT in plants [127]. Physiological trait-based breeding would be an ideal strategy for incorporating gene(s)/QTLs that determine heat tolerance. Such an approach has been adopted in wheat at CIMMYT to develop heat tolerance varieties [128]. Toward breeding for heat tolerance, physiological traits that need to be considered include those related to canopy structure, delayed senescence, photosynthesis efficiency, less respiration rates, reproductive traits, and harvest index [127, 129]. Genetic variability has been assessed in several crop plants under HS for several physiological traits and suitable tolerant sources and associated traits have been identified (**Table 2**).

Genetic variability existing for the plant phenologies conferring HT need to be exploited. Alternatively, selection for morphophysiological traits involved in heat stress adaptation, and also indirectly associated with yield, can be utilized for enhancing HT in crop plants that has been explored in wheat [127]. Substantial genetic variability for photosynthetic rate under HS has been noted in wheat and rice, which would serve as a potential indicator of HS tolerance [130, 131]. While screening over 1000 wheat genotypes, the chlorophyll fluorescence was established as an important physiological parameter [132]. Canopy temperature depression (CTD) has been found to act as heat escape mechanism in cotton [133], while for HT in wheat [81, 134]. The cooler canopy temperature (CT) under HS caused higher yield in wheat [109, 135]. Under HS, the CTD, flag leaf stomatal conductance and photosynthetic rate together are found to be positively correlated with yield in wheat [62].

Membrane thermostability MTS is considered as a useful component for measuring HT, while assessing genetic variability in different crops [49]. Selection for MTS during anthesis stage under HS led to significant yield improvements in wheat [50]. Various physiological traits and their relative contributions to HT in wheat have been discussed in detail earlier [129]. Screening against HS based on parameters such as electrolyte leakage from cell membrane and chlorophyll fluorescence revealed negative association of membrane injury with specific leaf weight in some legume species including groundnut and soybean [59]. The combination of the two selection parameters, viz. high chlorophyll content and MTS, was implicated to carry out selections in Brassica and wheat [34, 136]. The relative cell injury level (RCIL) under HS could also be taken as a reliable index in determining HS tolerance in cotton [137].

locus *OsHTAS* (*Oryza sativa* heat tolerance at seedling stage) was identified from the genotype HT54, which contributed high-temperature tolerance at 48°C especially during seedling and grain-filling stages [161]. The QTLs identified using molecular markers in different crops provide a way to transfer the causative heat tolerance gene(s)/QTLs to elite cultivars. In parallel, the fine mapping accompanied by cloning of candidate QTL will help the breeders to commence marker-assisted breeding for incorporating HT in various important crops in near future.

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For transfer of quantitative traits such as HT, molecular markers would enable the recovery of desirable genotypes in a precise and time-saving fashion [162]. Molecular markers have been useful in identifying heat sensitive advanced generation introgression lines in rice [120, 149]. The near isogenic lines (NILs) created by introducing desirable allele into the heat sensitive cultivar showed considerable reduction in the incidence of heat-induced injuries such as whiteback kernels [163]. Recently in rice, a 1.5-Mb chromosomal region harboring a robust QTL controlling better grain quality under HS has been transferred from "Kokoromachi" to "Tohoku 168" using marker-assisted backcrossing [162]. The resultant NILs had improved grain quality

DNA markers related to various HT/component traits have been identified in different crops such as rice [148, 161, 163–165], wheat [143–145, 150, 152, 166, 167], and cowpea [159, 168]. Once the markers associated with QTLs have been identified, the candidate QTLs can further be introgressed into elite lines through marker-assisted selection (MAS) strategies. One of the difficulties of developing superior genotypes for heat stress is that these traits are generally controlled by small effect QTLs or several epistatic QTLs [3]. To overcome this problem, approaches that can be employed are pyramiding several QTLs in the same genetic background using large populations through marker-assisted recurrent selection (MARS) or

MAS programs for complex traits such as heat tolerance are not effective mainly due to the genotype × environment and gene-gene (i.e., epistasis) interactions, which frequently result in a low breeding efficiency [170]. In contrast to MAS strategies which use markers for which a significant association with a trait has been identified, the GS method predicts breeding values using data derived from a vast number of molecular markers with a high coverage of the genome. Its novelty is that it uses all marker data as predictors of performance and subsequently delivers more accurate predictions [3]. Simulation studies indicated that GS may increase the correlations between predicted and true breeding value over several generations, without the need to re-phenotype. Thus, GS may result in lower analysis costs and increased

QTLs often do not translate well across genetic backgrounds and often produce smaller than expected adaptation effects. Thus, improving crop abiotic stress tolerance by exploiting the segregation of natural alleles rather seems challenging for such an adaptive QTL strategy [170]. When quantitative hereditary characteristics such as heat stress tolerance are involved, recurrent selection seems to be one of the most efficient methods in plant breeding. In multiple crosses, the probability is very small of obtaining superior genotypes that reunite all the favorable alleles. However, in this circumstance, a large segregating population is required, aspect that becomes unfeasible in practice [3]. The alternative is to adopt recurrent selection to gradually accumulate, by recombination cycles, the desirable and available alleles in different

over the susceptible parent.

genomic selection (GS) [169].

rates of genetic gain [171, 172].

The adaptation of root respiratory carbon metabolism can offer tolerance to soil temperature by managing the ion uptake load as reported in *Agrostis* species [138]. The efficient carbon and protein metabolism is known to confer higher thermotolerance to roots at 45°C in *Agrostis scabra* (a C3 perennial grass species) [139]. In wheat, stay-green trait associated with CTD has been indicated as a strong indicator of HT [140]. However, stay-green trait is less important in the context of yield on account of disability in translocation of stem reserves to grain under HS [118]. Whereas, the conditions encompassing heat stress alone, as well as heat stress and drought, stay-green trait is measured as normalized difference vegetation index (NDVI) at physiological maturity exhibiting a positive correlation with the yield [141]. Therefore, physiological trait-based breeding remains a promising improvement strategy to develop heat tolerance genotypes without causing yield penalty.
