The inferences from global circulation model simulations indicate that earth's mean surface air temperature warming for a doubling of atmospheric CO2 is expected to increase by 2°C - 4.5°C (IPCC, 2007). Additionally, it is projected that heat waves will be more intense, more frequent and longer lasting in future warmer climates (Meehl and Tebaldi, 2004). Daily minimum temperatures (nighttime temperature) are projected to increase faster than daily maximum temperature (daytime), leading to decreases in diurnal temperature (IPCC, 2007). For example, in summer 2003, Europe experienced an extreme climate anomaly that caused July temperatures to increase 6°C above the long-term mean and resulted in an approximate 30% reduction in terrestrial gross productivity across Europe (Ciais et al., 2005). Day/night temperatures greater than 36°/30°C commonly occur during a crop's life cycle in most of the world's tropical growing regions where daytime temperatures can occasionally reach up to 45 °C (Warrag and Hall, 1983, Ismail and Hall, 1998; Hall, 2004a; National Climate Data Center, 2008). The projected global temperature increase will subject these locations to an even higher temperature regime, particularly for night temperatures (IPCC, 2007).
Temperature is the most important abiotic factor that determines plant adaptation to different climatic zones and seasons of the year. Most annual crops can be described as being adapted to either the cool season or warm season (Hall, 2001; Cutforth et al., 2007) depending on their temperature range of survival (Tmax - Tmin; Reddy and Kakani, 2007). Temperature also plays an important role in determining the sowing date of a crop species based on seed germination and survival of the seedlings. The minimum threshold for seed germination differs among crop species (e.g., soybean 10°C, cowpea 18°C, upland cotton 16°C, and maize 14°C; Ismail and Hall, 1997; Hall, 2001; Cutforth et al., 2007). Similarly, optimum temperatures depend upon the developmental stage of the plant and species. The optimum temperature for peanut growth and development is between 25 °C and 30°C (Williams and Boote, 1995), whereas the optimum temperature for pollen germination and tube growth ranges between 30°C and 34°C (Kakani et al., 2002). The cardinal temperatures for growth and development of a crop species are also process dependant (Reddy et al., 1997a; Reddy and Kakani, 2007; Reddy et al., 2007a). A temperature stress could be anything below and/or above the optimum which influences the functionality and success of the biochemical pathway. This may reduce efficiency of the particular phase of development, resulting in a loss of economic yield (Singh et al., 2008b). Studies on cowpea and common bean have shown that heat stress during floral bud development can reduce fruit set because of damage to the pollen mother cells, resulting in poor anther dehiscence and reduced pollen number and viability (Warrag and Hall, 1983; Warrag and Hall, 1984; Gross and Kigel, 1994). Peanut (Prasad et al., 1999) and sorghum (Prasad et al., 2008) plants were more sensitive to high-temperature stress during microsporogenesis (just prior to flowering) and at flowering. High-temperature stress during pre-flowering stages mainly influences viability of male or female gametes, whereas at flowering, high-temperature stress decreases pollen dehiscence, germination, and tube growth, resulting in decreased fruit set and grain numbers. A negative association between increased daily mean temperature and reduction in yield has been reported in many crops (Ismail and Hall, 1998; Walton et al., 1999). Lobell and Asner (2003) projected an approximate 17% yield reduction in corn and soybean for each degree centigrade increase in average growing season temperature above the optimum in the U.S.. Most grain crops (i.e., peanuts, rice, wheat, soybean, and maize) are already being grown above optimum growth temperature; further increases in temperature due to climate change or increased frequencies of high-temperature stress during sensitive periods of reproduction will decrease crop yields (Reddy et al., 1997b; Lobell and Asner, 2003; Prasad et al., 2003a; Boote et al., 2005; Prasad et al., 2006a; Prasad et al., 2009).
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