**3. Heat tolerance mechanisms in food crops**

temperature of 10–15°C above ambient is generally considered as heat shock or heat stress (HS). However, HS is a complex function of intensity (temperature in degrees), duration, and rate of increase in temperature [6]. Some researchers believe night temperatures are major limiting factor, while the others argue that day and night temperatures do not affect the plant independently. Hence, diurnal temperature is a better predictor of plants response to high temperature with day temperature having a secondary role [7]. At high temperatures, severe cellular injury and even cell death may occur within minutes, which could be attributed to catastrophic collapse of cellular organization [8]. At moderately high temperatures, injuries or death may occur only after long exposure. Direct injuries due to high temperatures include protein denaturation and aggregation, and increased fluidity of membrane lipids. Indirect or slower heat injuries include inactivation of enzymes in chloroplast and mitochondria, inhibition of protein synthesis, protein degradation and loss of membrane integrity [9]. These injuries eventually lead to starvation, inhibition of growth, reduced ion flux, production of

Plants can experience wide range of HS on daily or seasonal basis. Temperature plays an important role in all stages of crops such as seedling emergence, vegetative stage, flowering/ reproductive, and grain filling stages. Optimal temperature for growth and development differs for different plant species and genotypes within species. Exposure to temperature outside optimal range though not necessary be lethal, but can be stressful. The observed effects depend on species and genotype, with abundant inter- and intra-specific variations [12, 13]. Under elevated temperatures, various physiological injuries have been observed such as scorching of leaves and stems, leaf abscission and senescence, shoot and root growth inhibition, or fruit damage that leads to decreased plant productivity [14]. Heat stress induces changes in respiration and photosynthesis and thus leads to a shortened life cycle and diminished plant productivity [12]. In many crop species, the effects of high-temperature stress are more prominent on reproductive development than on vegetative growth, and the sudden decline in yield with temperature is mainly associated with pollen infertility [15, 16]. The effect of high temperature among different crop plants during germination, vegetative growth, reproductive growth, and different physiological processes such as photosynthesis, membrane fluidity, respiration, water balance, oxidative stress, and antioxidant defense have

A wide range of plant developmental and physiological processes are negatively affected by HS. Sexual reproduction and flowering in particular have been recognized as extremely sensitive to HS that often results in reduced crop productivity [18]. High temperature is found most deleterious at flower bud initiation stage, with sensitivity being maintained for 10–15 days [18, 19]. Many legumes and cereals show a high sensitivity to HS, during flowering, and cause severe reductions in seed set probably due to reduced water and nutrient transport during reproductive development [15]. Generally, the male gametophyte is found sensitive to high temperatures at all stages of development, while the pistil and female gametophyte are more tolerant [20]. But in pearl millet, female reproductive parts was found more heat sensitive than male [21] as the stigma protrudes out of the florets. The HS often accelerates rather than delays the onset of anthesis that means the reproductive phase will be initiated prior to the accumulation of sufficient resources [16]. Shorter developmental phases for field crops have relatively negative effects on final grain weight and yield [22, 23]. Male sterility due to HS is widely observed among many sensitive crop plants, wherein the impairment of pollen

toxic compounds and reactive oxygen species (ROS) [10, 11].

48 Next Generation Plant Breeding

been discussed in detail elsewhere [17].

Heat tolerance (HT) is generally defined as the ability of the plant to grow and produce economic yield under high temperatures [6]. For surviving under HS, crop plants could manifest short-term (avoidance) and long-term (evolutionary changes) strategies. Short-term avoidance or acclimation mechanisms include changing leaf orientation, transcriptional cooling, altering membrane lipid composition, reflecting solar radiation, leaf shading of tissues that are sensitive to sunburn, and extensive rooting [30, 31]. Early maturation is found closely related to smaller yield losses in many crop plants [32], which is mainly due to escape mechanism. For example, tolerant wheat genotypes are defined by maintenance of photosynthesis, chlorophyll content, and stomatal conductance under heat stress, while the yield of these genotypes is maintained through higher seed set, grain weight, and extended grain filling duration (GFD) even at elevated temperatures [33]. At supraoptimal temperatures, heat tolerance grass species and cultivars exhibit higher activity in the photosynthetic apparatus [34, 35] and higher carbon allocation and nitrogen uptake rates [36]. Plants also utilize various mechanisms against HS such as ion transport, osmoprotectants, free-radical scavengers, and late embryogenesis abundant (LEA) proteins, wherein factors ubiquitin and dehydrin involved in signaling cascades and transcriptional control are essentially significant to counteract stress effects [37].

Transpiration is a mechanism of heat avoidance and serves as the primary mediator of energy dissipation. Generally, the rate of transpiration increases with increasing of canopy temperatures due to its effects on both vaporization and vapor pressure deficit (VPD). Crop transpiration is the most active and common method of cooling crop tissues (transpiration cooling effect), with plant cooling requirements increasing with temperature [10]. The ability to maintain high stomatal conductance at high temperatures promotes transpirational heat dissipation, as observed in heat tolerance bread and durum wheat genotypes [11] and various heat tolerance and sensitive chickpea genotypes [38]. Heat stress has been known to cause malfunction of photosystem (PS) II, reduced efficiency in electron transport, and increase in ROS production. Heat tolerance has been linked to increased tolerance of the photosynthetic apparatus [39]. ROS detoxification mechanisms are known to play important roles in protecting plants against HS [40, 41]. HT is closely correlated with increased capacity of scavenging and detoxifying of ROS. Induction of thermotolerance may be ascribed to maintenance of a better membrane thermostability and low ROS accumulation [36, 42] due to an improved antioxidant capacity [43]. HS responses among plants are mainly due to their inherent ability to survive and also to acquire thermotolerance to lethal temperatures. Genetic variability among crops for HT is mainly due to expression of different stress-responsive genes [44], acquisition of thermotolerance, and synthesis and accumulation of HSPs that are well correlated with the antioxidant defense system [45]. The maintenance of high membrane thermostability (MTS) is related to thermotolerance [46] and an important selection criterion which is determined by measuring the electrical conductivity. MTS has been successfully employed to assess thermotolerance in many food crops worldwide. The role of thermoprotectants such as HSPs, proline, glycine betaine, trehalose, brassinosteroids, salicylic acid, abscisic acid, polyamines, and nitric oxide in offering heat tolerance through endogenous synthesis or by exogenous application in different crops has been discussed in detail by Kaushal et al. [17]. Future pioneering studies in model plants can pave the way to identify key regulators as target for gene manipulation of stress tolerance in crop plants. It has also been envisaged that metabolic fingerprinting can be used as breeding tool for development of plants with the best potential to tolerate abiotic stresses.

as a function of temperature. It is simple, quick, and inexpensive and holds promise for the rapid screening in a large number of crops, e.g., wheat [58] and legumes (pigeon pea,

Breeding Cultivars for Heat Stress Tolerance in Staple Food Crops

http://dx.doi.org/10.5772/intechopen.76480

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**iii.** *Estimation of membrane lipid saturation*: A higher share of saturated fatty acids in membrane lipids increases the lipid melting temperature and prevents a heat-induced increase in the membrane fluidity. To maintain the membrane fluidity, plants increase the content of saturated and monounsaturated fatty acids, modulating their metabolism in response to increasing temperatures [60]. Thus, increasing the saturation level of fatty acids appears to be critical for maintaining the membrane stability and enhancing heat tolerance in

**iv.** *Canopy temperature depression (CTD)*: The surface temperature of the canopy is related to the amount of transpiration resulting in evaporative cooling. A hand-held infrared thermometer (IRT) allows canopy temperature (CT) to be measured directly and easily during afternoon (13:00 and 14:30 h) remotely and without interfering with the crop. The viewing angle should be around 40° to the horizontal line above the canopy so as to avoid the confounding effect of soil temperature. Studies have shown that CT is correlated with many physiological factors: stomatal conductance, transpiration rate, plant water status, water use, leaf area index, and crop yield. Genotypes with cooler canopy temperatures can be used to indicate a better hydration status. Under heat stress conditions, CTD is related to vascular capacity, cooling mechanism, and heat adaptation. CTD has been

proved to be a rapid and stable test that can be used for selection, e.g., wheat [62].

sterility, pollen viability, silk receptivity and some agronomic traits in maize [67].

as proof of cause-and-effect because weather variables are confounded [69].

maize [71], and ICC1205 and ICC15614 in chickpea [28, 72].

breeding for heat tolerance in various crop species.

**vi.** *Selection in hot production environments*: It has been effective in wheat [68] and maize [67]. Heat stress screening, one criteria for selection of site is high VPD area where low yield was found associated with high VPD during all the growing season, high maximum temperature during most of the growing season, and low photothermal quotient corrected by VPD in the critical period of grain set before flowering. The relationships found are agronomically robust and provide a guide for experimental research but cannot be taken

**vii.** *Pollen-based screening of genotypes*: Using this method, various heat tolerance accessions have been identified in different crop species, e.g., DG 5630RR in soybean [70], AZ100 in

All these techniques need to be validated for a large number of crops for their applicability in future. Regardless of the screening method, a key objective for plant breeders is to develop an effective set of thermotolerance markers which can be used for further implementation of

Identification of the superior germplasm for heat tolerance is essential for effective genetic manipulation through breeding process. However, identification of reliable and effective heat screening methods is a major challenge in conventional breeding to facilitate detection of heat tolerance lines [6]. Although a number of screening methods and selection criteria that

**v.** *Visual assessment methods/morphological methods*: Male sterility in cowpea [63]; pollen viability, stigma receptivity in maize [64]; grain sterility in rice [65]; asynchrony of male and female floral organ development in chickpea [66], leaf firing, tassel blasting, tassel

chickpea, groundnut, and soybean) [59].

creeping bentgrass (*Agrostis stolonifera*) [61].
