High Temperature Stress Responses

Plants subjected to heat stress show a range of responses and manifest mechanisms to cope with its adversaries. These changes can be discerned at whole plant to subcellular and molecular levels. An individualistic account of all these responses is briefly described below.

2.1 Growth and Phenology

High temperature is a major determinant of agricultural production throughout the world and its effects are evident at all critical growth stages starting from seed germination to final yield harvested. An account of changes in the phenology of plants has been described in the following lines.

Seeds put to germinate at supraoptimal temperatures show reduced or even inhibited germination. In soybean, heat stress changed the protein expression profiles and reduced the seed germination and seedling vigor, which appeared to determine the seed quality attributes (Egli et al. 2005; Ren et al. 2009). Seed germination, seedling emergence and its establishment is prone to increased temperature in most of the plant species (Grass and Burris 1995; Burke 2001; Ashraf and Hafeez 2004; Wahid et al. 2007). Columbo and Timmer (1992) demonstrated that black spruce plant seedlings are more susceptible to high temperature stress than adult plants. Maize shows optimal germination and growth at 20-30°C and 28-31°C, respectively (Hughes 1979; Medany et al. 2007; Farooq et al. 2008a, b, 2009) .

There are conflicting reports about the post-emergence seedling growth in maize under heat stress. For instance, some studies show that maize coleoptile was more heat tolerant at all stages of seedling development (Venter et al. 1997; Momcilovic and Ristic 2007) , while in some other studies on maize, upon exposure to 40°C, there was a substantial reduction in coleoptile growth and at 45°C growth was completely stopped (Weaich et al. 1996, Akman 2009). Heat stress lowered the activity of specific enzymes and thus reduced the protein synthesis in germinating maize embryos (Riley 1981). Likewise, seedling growth and development of cotton (Gossypium hirsutum L.) was also reduced under heat stress (Mahan and Mauget 2005) .

High temperature is a major environmental factor that determines the sustainability of crop growth and yield in some regions (Blum 1988; Al-Khatib and Paulsen 1999). Plants grown in warmer environments have much lower biomass than those grown at optimum or lower temperature (Kim et al. 2007). High temperature reduced the plant growth by affecting different mechanisms (Sibley et al. 1999; Wollenweber et al. 2003). For example, it decreased the dry weight,

Table 6.1 Effect ofheat stress on yield and yield components of the bread and durum wheat genotypes

Genotype

Control

Hat stress

Genotype x temperature

Grains per spike

Bread wheat mean

70 ± 0.85

70 ± 0.88

ns

Golia 69

69 ± 1.31 a

70 ± 1.43 a

ns

Sever

71 ± 1.10 a

71 ± 1.05 a

ns

Durum wheat mean

63 ± 0.75

64 ± 0.74

ns

Acalou

66 ± 0.98 a

65 ± 0.95 a

ns

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