**2. Heat stress and associated effects on food productions**

A threshold temperature (TT) refers to a value of daily mean temperature at which a measurable reduction in growth begins. This is the range wherein changes in the photosynthetic capacity are irreversible, but other characters such as growth, flowering, etc., are reversible. The upper and lower developmental threshold temperatures are the ones at which growth and development ceases and they differ based on the plant species and genotypes. Cool season and temperate crops often have lower threshold temperature values compared to tropical crops. Every crop plants have threshold temperatures for different developmental stages (**Table 1**); upon exceeding this, crop experiences the stress.

High-temperature sensitivity is particularly important in tropical and subtropical climates as heat stress may become a major limiting factor for field crop production [6]. Heat stress (HS) is often defined as the rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development. A transient increase in


water and increasingly variable weather patterns that are associated with climate change [1]. Abiotic stresses such as drought, salt, cold, and high temperature continue to affect the crops individually or in combination. Climate change has increased the intensity of heat stress that adversely affects both agricultural and horticultural crops resulting in serious economic losses, particularly in agricultural dependent countries. Global climate change risks are expected to be as high as global mean temperature increase of ≥4°C would pose large risks to global and regional food security [2]. The combination of high temperature and humidity would be compromising the current production of the major food crops such as wheat, rice, and maize in tropical and temperate regions. The climate change without adaptation is projected to negatively impact production for local temperature increases of ≥2°C above the

Extreme climates including very high temperatures are predicted to have a general negative effect on crop growth and development leading to catastrophic loss of crop productivity and also widespread famine in future [3]. The increase of temperature by 3–4°C is expected to reduce crop yields by 15–35% in Africa and Asia, whereas by 25–35% in the Middle East [4]. Hence, adopting the mitigation strategies such as reforestation, water harvesting in field

methane emission, etc., are essential, but as they are dependent on government policies, it is difficult to achieve them in short term. For long term, the adoption of extinct native crops and its diversity in individual farm is highly required to meet not only the food and nutritional security but also the feed security for farm livestock. The development of heat stress tolerant cultivars would be an ideal solution for sustainable food production for which research is still in preliminary stage and needs donor investment to progress competitively to deliver climate-smart cultivars to farmers. A recent study has shown the climatic shift in >25% of its geographical area in India [5] and also significant increase of aridity in several parts of the country. Therefore, government needs to re-standardize the climate zones with respect to aridity and temperature while planning for any developmental and agricultural intervention.

A threshold temperature (TT) refers to a value of daily mean temperature at which a measurable reduction in growth begins. This is the range wherein changes in the photosynthetic capacity are irreversible, but other characters such as growth, flowering, etc., are reversible. The upper and lower developmental threshold temperatures are the ones at which growth and development ceases and they differ based on the plant species and genotypes. Cool season and temperate crops often have lower threshold temperature values compared to tropical crops. Every crop plants have threshold temperatures for different developmental stages

High-temperature sensitivity is particularly important in tropical and subtropical climates as heat stress may become a major limiting factor for field crop production [6]. Heat stress (HS) is often defined as the rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development. A transient increase in

**2. Heat stress and associated effects on food productions**

(**Table 1**); upon exceeding this, crop experiences the stress.

emitting devices and reducing wetland crops to avoid

late twentieth-century levels [2].

46 Next Generation Plant Breeding

and households, optimal use of CO2

**Table 1.** Critical growth stages and threshold temperatures of important 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 toxic compounds and reactive oxygen species (ROS) [10, 11].

development has been the prime factor for reduced yield under HS [13, 24]. Continuing HS beyond a successful fertilization can also halt further development of the embryo [12]. HS during seed development may result in reduced germination and loss of vigor, leading to reduced emergence and seedling establishment as noted among several crop plants [25, 26]. Both grain weight and grain number appears to be affected by HS in many temperate cereal crops, wherein decline in grain number was found directly proportional to increasing temperatures during flowering and grain filling [27, 28]. HS during seed development in several crop species has been found to cause reductions in quality parameters such as starch, protein, and total oil yield [29]. High temperature affects different stage and part of the crop growth in terms of morphology. The anatomical and phenological changes also affect plant growth

Breeding Cultivars for Heat Stress Tolerance in Staple Food Crops

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

49

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].

and development.

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

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 been discussed in detail elsewhere [17].

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 development has been the prime factor for reduced yield under HS [13, 24]. Continuing HS beyond a successful fertilization can also halt further development of the embryo [12]. HS during seed development may result in reduced germination and loss of vigor, leading to reduced emergence and seedling establishment as noted among several crop plants [25, 26]. Both grain weight and grain number appears to be affected by HS in many temperate cereal crops, wherein decline in grain number was found directly proportional to increasing temperatures during flowering and grain filling [27, 28]. HS during seed development in several crop species has been found to cause reductions in quality parameters such as starch, protein, and total oil yield [29]. High temperature affects different stage and part of the crop growth in terms of morphology. The anatomical and phenological changes also affect plant growth and development.
