**Abstract**

Increase in global warming poses a severe threat on agricultural production thereby affecting food security. A drastic reduction in yield at elevated temperature is a resultant of several agro-morphological, physiological and biochemical modifications in plants. Heat tolerance is a complex mechanism under polygenic inheritance. Development of tolerant genotypes suited to heat extremes will be more advantageous to tropical and sub tropical regimes. A clear understanding on heat tolerance mechanism is needed for bringing trait based improvement in a crop species. Heat tolerance is often correlated with undesirable traits which limits the economic yield. In addition, high environmental interactions coupled with poor phenotyping techniques limit the progress of breeding programme. Recent advances in molecular technique led to precise introgression of thermo-tolerant genes into elite genetic background which has been reviewed briefly in this chapter.

**Keywords:** global warming, high temperature, polygenic inheritance, breeding approaches, thermo-tolerant genes

### **1. Introduction**

Increase in global temperature had major impact on crop productivity especially in tropical and sub tropical regimes. Based on climate model predictions, around 1.8–4.0°C rise in air temperature was expected in 21st century [1]. The increase in temperature beyond a certain threshold level tends to induce detrimental effects in plant growth and development. In general, the elevation in temperature of 10–15°C above ambient triggers heat shock in crop plants. The extent of induced heat stress depends on the duration, intensity and rate of increase in global air temperature [2]. Indian lowlands share 15 per cent of global wheat production. The change in global climate would shift these fertile lowlands into heat stressed unproductive environment [3]. Similarly, the cultivation of cereals in Southern Africa and South East Asia was predicted to be heat stressed zone in near future [4]. Around 4–14% yield decline in rice was encountered due to elevated temperature of 1°C in South-East Asia [5]. The declined productivity due to elevated temperature imposes the urgent need for development of climate resilience genotypes. Evolving heat tolerant cultivars would highly benefit the livelihood of developing countries as around 70–80% of population relies on agriculture. Understanding the effect of heat stress on crop plants and its adaptation mechanisms would help in framing out the breeding strategies for high temperature tolerance.

Heat tolerance in crop plants is a complex mechanism involving adaptations through altered physiological process, morpho-anatomical features and induction of several biochemical pathways. On exposure to high temperature, several signal transduction pathways were triggered leading to changes in gene expression. As a result, varied stress related proteins were synthesized contributing heat tolerance in plants [6]. The tolerance mechanism to high temperature stress varies within genotypes of a plant species. The existing variation between and within species provide scope for evolving heat tolerant lines through conventional breeding approaches [7]. Dissecting out genetic information through molecular tools would hasten the development of climate resilient cultivars contributing to food security in near future. A brief review on plant response, adaptation mechanisms and genetic approaches to combat heat stress were presented in this chapter.

### **2. Effect of heat stress on crop plants**

Heat stress had varying impact on different phenological stages *viz*., germination, seedling, vegetative, flowering and reproductive of crop plants [8]. The plant response to heat stress depends on the duration, degree of rise in temperature and plant type. Under tropical regimes, high temperature with intense solar radiation poses a major limiting factor for yield by inducing leaf abscission, leaf senescence, scorching of leaves, branches and stems, growth inhibition, pollen infertility and poor seed formation [9, 10]. A significant decline in relative growth rate, shoot dry weight and net assimilation rate was recorded in sugarcane, maize and pearl millet on exposure to high temperature stress [11]. High reduction in grain quality was recorded in most of the cereal crops grown under heat stress environments [12]. Several physiological processes such as partitioning of assimilates, plant-water relations and shoot growth was affected due to heat stress in common bean [13]. In general, the susceptibility to heat stress was found higher at reproductive stage of plant development. An excessive yield loss is recorded in legumes on exposure to high temperature (30–35°C) during anthesis stage [14]. Drastic reduction in grain number and weight was observed in wheat at high temperature regimes [15]. Heat stress affects several metabolic pathways leading to accumulation of reactive oxygen species (ROS) which is a major component for oxidative stress in crop plants [16]. The photosystem centres (PS I and PS II) of chloroplast, mitochondria and peroxisomes are the major sites for generation of ROS in plants [17]. High temperature stress disrupts the stability of cell membrane through protein denaturation [18]. The induction of ROS due to high temperature stress was correlated with premature leaf senescence in *Gossypium* sp. [19]. Accumulation of ROS in root cells was evidenced in wheat on exposure to high temperature for two days [20].

### **3. Adaptation mechanisms**

Plants tend to adapt several complex mechanisms through phenological and morphological changes to combat high temperature stress (**Figure 1**). On heat stress regimes, plants exhibit varied short term escape/avoidance mechanisms *viz*., altered leaf orientation, transpirational cooling, altered membrane lipid properties, early maturation and so on for its survival. Plants show varied degree of leaf rolling

*Breeding Mechanisms for High Temperature Tolerance in Crop Plants DOI: http://dx.doi.org/10.5772/intechopen.94693*

upon intensity of solar radiation. A significant tolerance to high temperature was observed in wheat by maintenance of water potential in flag leaf through adoption of leaf rolling under heat shock conditions [21]. Increase in trichomatous and stomatal densities, waxy layer on leaves, and larger xylem vessels are the common features induced during heat stress [22]. On contrary, plants also evolve long term tolerance mechanisms for its effective survival and productivity under high temperature. Induction of osmoprotectants, antioxidants, late embryogenesis abundant proteins, dehydrins, and heat shock proteins are the major factors involved in counteracting the heat shocks. Accumulation of osmolytes such as proline, trehalose, and glycine betaine plays a vital role in imparting tolerance *via* cellular osmotic adjustment, detoxification of ROS, stabilization of enzymes and membrane proteins [23]. Several enzymatic and non-enzymatic antioxidant defense components are also involved in protection against oxidative stress induced by free radicals [24]. The activities of ROS scavenging enzymes are temperature specific. In general, most of the antioxidant enzymes show increased activity with elevation in temperatures. It is also influenced by genotype, growing season and phenological stages of plant [25]. Under high temperature conditions, several signaling molecules such as nitrous oxide, Ca-dependent protein kinases, Mitogen mediated protein kinase, sugars, and phytohormones play a role in stimulation of stress responsive genes *via* transduction pathways [26]. Evolving adaptation mechanisms (either tolerance or avoidance) to high temperature and drought would be more rewarding at arid conditions as it is often correlated.
