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Fluid Journal : Summer 2016
16 The Fluid Journal Summer 2016 food security. Gous et al. (2013) found that in barley (Hordeum vulgare L.) a genotype with “stay green” characteristics had greater potential for maintaining starch biosynthesis and grain quality under severe water stress compared with the genotype without these characteristics. Saint Pierre et al. (2008) observed protein composition in winter wheat was affected by water and N management with no variation among wheat cultivars in their flour protein and protein composition to water or N stress. However, de Mezer et al. (2014) observed differences among barley cultivars in their physiological and molecular responses to water deficits and recovery. A comparison of maize hybrids by Aydinsakir et al. (2013) found that different irrigation levels significantly affected all yield components (e.g. anthesis-silking interval, plant height, ear diameter, ear length, kernel number, and 1,000 grain weight) except for ear number. For both genotypes, water deficit stress significantly increased glucose, fructose, and sucrose contents while decreasing protein content. The effect of water deficits on grain quality and the physiological reactions will provide insights into potential methods to alleviate the effects of water stress and should be a focus of future studies to be able to provide high quality grain for future generations. Excess water also limits crop production, and often occurs in the mid- western United States, Southeast Asia and Europe when more intense rainfalls lead to excess soil moisture early during the growing season. Zaidi et al. (2004) screened maize genotypes and found the V2 (two leaves visible) and V7 (seven leaves visible) growth stages were the most susceptible to excess soil water. Excess moisture affected growth and biochemical processes, disrupted anthesis and silking, and resulted in poor kernel development and yield. The attributes of tolerant genotypes included good carbohydrate accumulation in stem tissues, moderate stomatal conductance, high root porosity, early brace root development, and less than 5d anthesis to silking interval (Zaidi et al, 2004). Earlier, Zaidi et al. (2003) concluded that tolerance to excess moisture conditions was primarily due to stress avoidance mechanisms due to anaerobic metabolic adjustments and morphological changes, e.g., brace root development. If we accept the premise of Sakschewski et al. (2014) that land productivity is a limiting factor, then yield improvements from occurrence of water shortage and excess water could be partially achieved by modifying soil water characteristics. Cairns et al. (2011) suggested that plant performance under drought is not simply defined as the ability to extract water, but attention must be given to soil physical environments to quantify the plant’s ability to distribute roots throughout the soil profile. Passioura (2006) suggested increased crop productivity in water limited conditions could be achieved through crop breeding by capturing more water for transpiration, increasing WUE by exchanging water for CO2 more effectively, and converting more biomass into harvestable products. Management practices that reduce soil water evaporation losses and better couple crop development with water supply would prove to be beneficial to ensure adequate water supply throughout the growing season (Passioura, 2006). The findings from Egli and Hatfield (2014a, 2014b), demonstrating average county maize and soybean yields were directly related to a soil productivity index suggest agronomists pay attention to building soil quality based on water availability, which would include both soil water holding capacity and rooting depth, and improved soil structure to relieve anoxic condition by facilitating oxygen exchange under excess soil water and improved infiltration to increase effective precipitation, defined as the amount of rainfall entering the soil and available for crop water use. Improvements in WUE through soil management have been summarized by Hatfield et al. (2001) and suggest that soil management plays a key role in being able to supply adequate water to achieve maximum productivity. Temperature High temperature exposure effects on crop productivity and yield were summarized for agronomic crops by Hatfield et al. (2011), and extensive literature documents specific effects on growth, pollination, and yield (Prasad et al., 2002). Rattalino Edreira et al. (2011) evaluated heat stress effects on temperate and tropical maize hybrids and found three sources of reduced productivity--decreased floret differentiation, pollination failure, and kernel abortion. They used normal temperatures and a daytime Figure 3. Frequency distribution of yield gaps for Story County, Iowa maize production since 1950.