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Heat Tolerance in Cotton: Morphological, Physiological, and Genetic Perspectives

Muhammad Tehseen Azhar1, Shabir Hussain Wani2, Muhammad Tanees Chaudhary1, Tariq Jameel1, Parwinder Kaur3, and Xiongming Du4

1Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan

2Mountain Research Centre for Field Crops, Khudwani, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, Jammu and Kashmir, India

3UWA School of Agriculture and Environment, The University of Western Australia, Crawley, Western Australia, Australia

4Institute of Cotton Research of Chinese Academy of Agricultural Sciences, State Key Laboratory of Cotton Biology, Anyang, Henan, China

1.1 Introduction

Cotton belongings to the genus Gossypium and four species, namely G. hirsutum L., G. barbadense L., G. arboreum L., and G. herbaceum L., that were grown independently for the purpose of textile fiber (Fryxell 1992). Cotton is mainly grown in tropical and sub-tropical areas having temperatures varying from 40 to 45?°C (Ashraf et al. 1994). The phenological development and accumulation of plant biomass depend upon temperature during its growing season (Echer et al. 2014). Cotton sheds its flowers and squares when the temperature reaches 36?°C (Reddy et al. 1991). The plant reproduction, metabolism, and productivity of cotton are adversely affected by high-temperature stress (Demirel et al. 2014). The response of plants to heat stress depends upon the duration and degree of heat stress (Hasanuzzaman et al. 2013). Furthermore, high-temperature stress is closely related to water deficit and can be further exacerbated by limited or unreliable availability of water in cotton-growing areas (Rizhsky et al. 2004). The presence of genetic variation within a species is a prerequisite for a breeding program for the development of genotypes responsive to heat stress (Azhar et al. 2009). All metabolic and biochemical activities of plants require an optimum temperature range, which is termed the thermal kinetic window (TKW). Plant temperature should be within range of the TKW. The plant comes under heat stress if its temperature goes below or above TKW. A TKW of 23.5-32?°C is required for proper plant growth (Sawan et al. 2009). Plant breeders are continually screening cotton germplasm for heat stress due to climate change. Like other physiological traits, seed cotton yield (SCY) is considered a beneficial trait for the development of cotton germplasm against high temperature but it is complex, and affected by climatic conditions, and so requires very specialized breeding programs, but even then one cannot avoid environmental variations (DemIrel et al. 2016). High temperature exerts negative impact due to inhibition of photosynthesis process (DeRidder and Salvucci 2007). The modifications induced by high temperature may be direct, owing to changes in the physiological process, or can be indirect by altering the developmental patterns. For example, developing seeds may be affected by heat stress that may delay the germination or loss of vigor. Ultimately it will lead to reduced emergence and seedling establishment (Wahid et al. 2007). The exposure of cotton to high temperature causes shedding of 35% premature flower and 50-75% of bolls in variable climatic conditions.

1.1.1 Morphological and Physiological Traits

One of the most economical and suitable ways to mitigate the adverse effects of heat stress is to develop or identify heat-tolerant cultivars. Under various heat stress regimes, plants exhibit different kinds of survival mechanism, which include various heat stress avoidance mechanisms, phonological alterations like variable leaf angle, transpirational mechanism, or changes in lipid membrane composition, and various morphological changes (Niinemets 2010). Plants' stomata closing mechanism, reduced water loss, larger stomatal and trichomatous densities and widening xylem vessels are the most common approaches followed by the plant under different conditions of heat stress (Hasanuzzaman et al. 2013).

Research work about the use of various plant and physiological traits is reviewed in the proceeding paragraphs.

1.1.1.1 Seedling and Root Growth

The germination and development of seedlings of crop plants depend upon remaining with the optimum temperature range of 28 to 30?°C. The base temperature is about 12?°C for seed germination and 15.5?°C for seedling growth. Cool temperature between 2 and 4?°C is a major problem in various places in the United States, mainly across the Delta region of Mississippi during germination and initial growth of the seedlings. Genotypic differences have been observed for germination and root development under cool soil temperatures (Mills et al. 2005). While at the time of sowing in northern India, the wind velocity and soil temperature are very high, resulting in a rapid loss of soil moisture (Lather et al. 2001). McMichael and Burke (1994) reveal that soil with a temperature range of between 20 and 32?°C is suitable for proper root growth and development. Root temperature stress ranges between 35 and 40?°C and badly affects the hydraulic conductivity and nutrient uptake ability of the plant and causes low hormone synthesis, and badly affects hormone transportation (Clark and Reinhard 1991; Burke and Upchurch 1995). Synthesis of cytokinins which originate predominantly in roots is among the most sensitive processes (Paulsen 1994). Many functions of roots - comprising the uptake of nutrients and water, assimilation and synthesis of metabolites, and translocation - are very sensitive to temperature. Root temperature may be more critical than shoot temperatures for plant growth because roots tolerate a shorter range of temperature and are less adaptable to extreme variations (Nielsen 1974).

1.1.1.2 Stomatal Conductance

Stomatal closures to reduce transpiration rates also reduce photosynthesis rates. Increased stomatal conductance helps with the cooling of leaves by evaporation and thereby reduces thermal stress. Optimum temperatures of <?30?°C during daytime in upland and Pima cotton are well below commonly occurring air temperatures in most cotton-growing areas (Radin et al. 1994). Stomatal conductance and net photosynthesis are inhibited by moderate heat stress in many plant species, owing to decreases in the activation state of rubisco (Crafts-Brander and Salvucci 2002; Morales et al. 2003). Stomata could be explored for the development of heat tolerance in crop plants because they regulate the transpiration rate and determine the degree to which water evaporation can cool down the leaves. If stomata open wider in higher-yielding lines, transpiration may be increased and photosynthetic rates improved because CO2 as well as water vapors must diffuse through the stomata. A broader examination of 16 species performed by Monteith (1995) supported the theory that stomata respond not to humidity levels but to respiration rates. Moneith (1995) also identified that stomata from all investigated species had patchy stomata closure in response to increased respiration rates to limit water loss. Many studies were intended to assess the germplasm of cotton and wheat by using stomatal conductance and photosynthesis as criteria for the identification of heat-tolerant genotypes (Cornish et al. 1991; Lu et al. 1998; Ulloa et al. 2000; Rahman 2005).

1.1.1.3 Cell Membrane Thermostability

The conventional screening methods of large germplasm are time consuming and laborious but the success of any breeding program depends on effective evaluation techniques (Asha and Lal Ahamed 2013). Cell membrane thermostability (CMT) protocol has been suggested by Sullivan (1972) as a dependable method for measuring heat tolerance by quantifying the amount of electrolyte leakage from leaf disks after exposure to heat stress, and this technique has been used efficiently in potato (Coria et al. 1998), rice (Maavimani et al. 2014), tomato (Golam et al. 2012), soybean (Martineau et al. 1979), wheat (Blum et al. 2001), sorghum (Marcum 1998), and barley (Wahid and Shabbir 2005). Membrane thermostability has been reported to have a strong genetic correlation with grain yield in wheat. Heritability of membrane thermostability in maize was estimated to be 73%. CMT has been used in cotton as a suitable screening and selection criterion for heat tolerance, owing to its ability to distinguish heat-sensitive and -tolerant genotypes within the species (Singh et al. 2007). Marsh et al. (2000) found a large portion of the variability for membrane stability to be controlled by a small number of genes. Rahman et al. (2004) indicate that exposure to high temperature preceding the CMT test produces better distinction between heat-tolerant and heat-susceptible cultivars; however, they caution regarding its indirect selection on the basis of SCY under non-heat-stressed environments, so Rahman et al. (2004) conclude...