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Fluid Journal : Summer 2016
17 The Fluid Journal Summer 2016 temperature between 33 and 40oC at ear level before and post-silking, and observed the effects of heating were greater for the anthesis stage with kernel abortion as the parameter most affected. Tropical hybrids were more resistant than temperate hybrids with respect to heat response. Li et al. (2013) conducted a similar study on durum wheat (Triticum turgidum L. ssp. durum) on nine different genotypes under drought and heat stress. They manipulated the planting date in northwestern Mexico to achieve the heat stress and managed the irrigation supply to induce drought stress. Heat and drought stress reduced grain yields across all genotypes and the G x E analysis showed E explained 90% of the variation in yield, 73% in the thousand kernel weight, and 60% in grain protein (Li et al. 2013). The G component was dominant for flour yellowness at 87% and they concluded that screening of durum wheat genotypes should account for both yield and quality parameters under abiotic stresses. In a screening study on grain sorghum [Sorghum biocolor (L.) Moench.], Djanaguiraman et al. (2014) found that exposure to 38/28oC for 10 d, compared with the normal temperature of 30/20oC at the boot stage, decreased quantum yield of PSII, electron transport rate and transcript levels of rubisco activase, glutathione peroxidase enzymes induced cell membrane damage, and decreased pollen viability, pollen germination and seed set. There was variation among genotypes in pollen response to high temperatures, with the tolerant genotypes exhibiting less oxidative damage in leaves and pollen grains than sensitive genotypes. These observations across multiple species suggest that exposure to high temperatures of short duration would affect the photosynthetic efficiency; however, the temperature effects may still not be as impactful as the drought effects on productivity. High temperature effects on plants are particularly evident during the pollination stage of development. This sensitivity is increased when plants are water stressed. There have been some general observations of the impact of heat-induced spikelet sterility on another dehiscence, reduced pollen shedding, poor germination of pollen grains and decreased elongation of pollen tubes (Prasad et al., 2001, 2003, 2006a, 2006b, 2008; Das et al, 2014. Observations in crops (e.g., rice) have found that air temperatures greater than 35oC for more than 1 hour caused sterility (Jagadish et al. 2010) and exposure to temperatures greater than 35oC for more than 5 days caused spikelets to be completely sterile (Rang et al. 2011). Temperature effects on pollination have been observed, but the question often arises about the impact of day vs. night temperatures. Shah et al. (2011) found high night temperatures to be more damaging than high day temperatures. From an earlier study, Ziska and Bunce (1998) observed the ratio of respiration to photosynthesis increased with increasing temperatures. For maize, temperatures above 35oC are lethal to pollen viability (Dupuis and Dumas, 1990) and pollen viability (before silk reception) is a function of pollen moisture content, which is strongly dependent on vapor pressure deficit (Fonseca and Westgate 2005). Quantifying the impact of episodes of temperature extremes on pollen viability and the disruption of reproductive processes may become more important with the projection that extreme temperature events will increase under climate change (Tebaldi et al. 2006). Butler and Huybers (2013) suggested that maize in the United States may be more adapted to hot temperatures and yield declines from a 2oC warming would only be 6%; however, the variation in precipitation and extreme events were not considered during this analysis. Hatfield (2016) observed that exposure of maize hybrids to temperatures 4oC above normal temperatures reduced grain yield by 75% in the absence of water stress. There was no difference in vegetative growth when exposed to high temperatures except the phonological advancement was faster in the warmer temperatures. The effect on grain yield was due to a combination of disruption of pollination and exposure to high nighttime temperatures during grain-filling. Any of the disruptions due to temperature or water stress will limit the photosynthetic efficiency, ep, of plants throughout the growth cycle. Rezaei et al. (2015) demonstrated that shifting phenology of wheat offers a potential solution to exposure to high temperature and would be a viable avoidance mechanism. An opportunity exists to evaluate genetic response to temperature stress and how changing phenology can potentially affect photosynthetic efficiency. An overlooked aspect of changing climate is the effect on soil temperature and exposure of roots to a warmer temperature regime. There have been no studies to document this impact; however, a study was conducted on the effect of warm water temperatures on rice by (Horai et al., 2014). During this study they increased water temperature by an average of 1.5 o C for two different rice cultivars, an adapted cultivar, and a late-maturing cultivar, and found the warmer temperatures increased the flowering date by 2 to 5 days, with both cultivars showing the same response. The adapted cultivar showed a significant increase in dry matter production before heading, compared with the adapted cultivar; however, both showed a positive response of dry matter to warming. Leaf senescence increased in response to the warmer temperature for both cultivars and RUE decreased. Thus, they concluded that analysis of leaf senescence rates would be necessary to evaluate crop response to a warming climate (Horai et al., 2014; Hatfield, 2016). Water and temperature stress will not occur in isolation from each other and will tend to exacerbate the effects of either stress. These effects will impact both the vegetative and reproductive growth of plants. One example of the complexity of this response was reported by Sadras et al. (2013) in which they compared 29 pea (Pisum sativum L.) accessions using a combination of field experiments and simulated results. The differences among accessions are larger for the favorable environments compared "Feeding the projected 9 billion global inhabitants of 2050 is a topic of concern."