Drought and high temperature often occurs simultaneously, causing significant yield losses in wheat (Triticum aestivum L.). The objectives of this study were to: (i) quantify independent and combined effects of drought and high temperature stress on synthetic hexaploid wheat genotypes at anthesis and at 21 days after anthesis; and (ii) determine whether responses to stress varied among genotypes. Four synthetic hexaploid and two spring wheat genotypes were grown from emergence to anthesis (Experiment I) and emergence to 21 days after anthesis (Experiment II), with full irrigation and 21/15°C day/night temperature. Thereafter, four treatments were imposed for 16 days as (a) optimum condition: irrigation + 21/15°C, (b) drought stress: withhold irrigation + 21/15°C, (c) high temperature stress: irrigation + 36/30°C and (d) combined stress: withhold irrigation + 36/30°C. Results indicated a decrease in leaf chlorophyll, individual grain weight and grain yield in an increasing magnitude of drought < high temperature < combined stress. There were 69, 81 and 92% grain yield decreases in Experiment I and 26, 37 and 50% in Experiment II under drought, high temperature and combined stress respectively. Synthetic hexaploid wheat genotypes varied in their response to stresses. Genotypes ALTAR 84/AO’S’ and ALTAR 84/Aegilops tauschii Coss. (WX 193) were least affected by combined stress in Experiments I and II respectively. Overall, combined effect of drought + high temperature stress was more detrimental than the individual stress and the interaction effect was hypo-additive in nature.

W heat (Triticum spp.) is one of the most important food crops in the world in terms of the area harvested, production, and nutrition as it supplies about 19% of the calories and 21% of the protein to the world's population (FAO, 2011). Over 90% of world wheat area is planted to common or bread wheat (Triticum aestivum L., 2n = 6x = 42, genomes AABBDD). The polyploidy has a highly buff ered genotype and has enormous genetic variability as each locus may harbor three divergent alleles. This genetic attribute enables bread wheat to exhibit an array of phenological responses to wide ranges of photoperiod and temperature regimes, including vernalization (Slafer and Rawson, 1994). Thus, wheat can be grown from tropical to temperate climates and from a few meters to more than 3800 m above sea level (Aase et al., 2010). Although wheat has a wide range of climatic adaptability, many biotic and abiotic factors limit its yield. High temperature stress is one of the most important environmental factors ABSTRACTHigh temperature (HT) stress is highly detrimental to crop productivity but there is limited variability for this trait among wheat (Triticum aestivum L.) cultivars. The objectives of this research were to screen wild wheats (Aegilops species) for tolerance to HT stress at the reproductive stage and to measure physiological and yield traits associated with the tolerance. Fiftytwo accessions belonging to fi ve Aegilops species were evaluated at optimum temperature (OT) (25/19°C day/night) and HT (36/30°C) with a photoperiod of 18 h. Stress was imposed at anthesis and continued for 16 d. Across species, HT decreased chlorophyll, grain number per spike, individual grain weight, and grain yield per plant by 38, 40, 56, and 70%, respectively. Based on a decrease in grain yield, A. speltoides Tausch and A. geniculata Roth were most tolerant and A. longissima Schweinf. & Muschl. was highly susceptible to HT stress. Tolerance was associated with higher grain number per spike and/or heavier grains. Within A. speltoides, accession TA 2348 was highly tolerant to HT with 13.5% decline in grain yield and a heat susceptibility index (HSI) of 0.23 whereas TA 1787 and TA 2097 were highly susceptible with >82% yield decline and HSIs > 1.4. Among A. geniculata, two moderately HT tolerant accessions, TA 2899 and TA 1819, were identifi ed, with an HSI of 0.80. Our results suggest that there is genetic variability among Aegilops species that can be utilized in breeding wheat for HT tolerance at reproductive stage.

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Nitrogen (N) is a major limiting nutrient to sustain crop yields and quality. As a result, N fertilizer is usually applied in large quantity to increase crop production throughout the world. Application of N fertilizers has increased crop yields and resulted in achievement of self-sufficiency in food production in many developing countries. Excessive application of N fertilizers beyond crops' demand, however, has resulted in undesirable consequences of degradation in soil, water, and air quality. These include soil acidification, N leaching in groundwater, and emissions of nitrous oxide (N 2 O), a potent greenhouse gas that contributes to global warming. Long-term application of ammonia-based N fertilizers, such as urea, has increased soil acidity which rendered to soil infertility where crops fail to respond with further application of N fertilizers. Another problem is the groundwater contamination of nitrate-N (NO 3-N) which can be a health hazard to human and livestock if its concentration goes above 10 mg L À1 in drinking water. The third problem is emissions of N 2 O gas which is 300 times more powerful than carbon dioxide in terms of global warming potential. This chapter examines the effect of N fertilization on soil and environmental quality and crop yields.

High temperature (HT, heat) stress is detrimental to wheat (Triticum aestivum L.) production. Wild relatives of bread wheat may offer sources of HT stress tolerance genes because they grow in stressed habitats. Wheat chromosome translocation lines, produced by introgressing small segments of chromosome from wild relatives to bread wheat, were evaluated for tolerance to HT stress during the grain filling stage. Sixteen translocation lines and four wheat cultivars were grown at optimum temperature (OT) of 22/14°C (day/night). Ten days after anthesis, half of the plants were exposed to HT stress of 34/26°C for 16 d, and other half remained at OT. Results showed that HT stress decreased grain yield by 43% compared with OT. Decrease in individual grain weight (by 44%) was the main reason for yield decline at HT. High temperature stress had adverse effects on leaf chlorophyll content and Fv/Fm; and a significant decrease in Fv/Fm was associated with a decline in individual grain weight. Based on the heat response (heat susceptibility indices, HSIs) of physiological and yield traits to each other and to yield HSI, TA5594, TA5617, and TA5088 were highly tolerant and TA5637 and TA5640 were highly susceptible to HT stress. Our results suggest that change in Fv/Fm is a highly useful trait in screening genotypes for HT stress tolerance. This study showed that there is genetic variability among wheat chromosome translocation lines for HT stress tolerance at the grain filling stage and we suggest further screening of a larger set of translocation lines.

Drought stress is an important abiotic factor limiting wheat yield. Thirty-one accessions of Aegilops species belonging to five species were screened to identify species/accessions tolerant to drought stress and to measure traits associated with the tolerance. Plants were grown at full irrigation, 25/19°C day/night temperature and an 18 h photoperiod. At anthesis (Feekes 10.5.1), drought stress was imposed by withholding water for 16 days. Controls were continuously irrigated. Drought stress decreased chlorophyll content, grain number, individual grain weight and grain yield by 31, 25, 68 and 76% compared with the control. Aegilops geniculata Roth had greater tolerance to drought stress for yield (48% decline from control) compared with other species (>73% decline from control). The tolerance was associated with higher grain number spike–1 and heavier grains. A. geniculata, GenBank accession number TA 10437, was highly tolerant to drought stress with <20% yield decline and a drought stress susceptibility index (DSI) <0.5, whereas TA 1802, TA 2061, TA 1814, TA 1819 were identified as moderately tolerant to drought stress (20–40% yield decline and DSI < 1.0). Our results suggest a presence of genetic variability among Aegilops species that can be utilised in breeding wheat for tolerance to drought stress at reproductive stages.

Drought is an important abiotic stress limiting wheat (Triticum aestivum L.) yield in the U.S. Southern High Plains. Although wheat breeding has improved drought tolerance in the area, the physiological traits conferring drought tolerance have not been well understood. Canopy temperature depression (CTD), the difference between air and canopy temperatures, has been suggested as a trait for identifying drought‐tolerant genotypes. The objective of this study was to investigate whether a higher CTD is one of the reasons for higher yield in new drought‐tolerant cultivars. Field experiments were conducted in five genotypes (TAM 111, TAM 112, TX86A5606, TX86A8072, and Dumas) under dryland conditions in 2009/2010, 2010/2011 and 2011/2012 seasons. The canopy temperature was measured continuously from late jointing to the middle of grain filling, using wireless infrared thermometers. Although CTD varied with sky conditions, growth stage, and time of day, the genotypic variation in CTD was consistent. In general, yield was positively correlated to daytime CTD whether the CTD was used from a single clear day or a season‐long mean. However, including Dumas weakened the correlation under severe drought in 2012. The nighttime CTD was not correlated to yield. Two new cultivars (TAM 111 and TAM 112) had up to 2.7°C higher CTD and 31% more yield than other genotypes. Therefore, cooler daytime canopy might be the reason for higher yield in the two new and drought‐tolerant cultivars under drought conditions. Further studies are needed to understand the physiological bases of differences in CTD among genotypes.

Wheat streak mosaic virus (WSMV) causes significant yield loss in hard red winter wheat in the U.S. Southern High Plains. Despite the prevalence of this pathogen, little is known about the physiological response of wheat to WSMV infection. A 2-year study was initiated to (i) investigate the effect of WSMV, inoculated at different development stages, on shoot and root growth, water use, water use efficiency (WUE), and photosynthesis and (ii) understand the relationships between yield and photosynthetic parameters during WSMV infection. Two greenhouse experiments were conducted with two wheat cultivars mechanically inoculated with WSMV at different developmental stages, from three-leaf to booting. WSMV inoculated early, at three- to five-leaf stage, resulted in a significant reduction in shoot biomass, root dry weight, and yield compared with wheat infected at the jointing and booting stages. However, even when inoculated as late as jointing, WSMV still reduced grain yield by at least 53%. Reduced tillers, shoot biomass, root dry weight, water use, and WUE contributed to yield loss under WSMV infection. However, infection by WSMV did not affect rooting depth and the number of seminal roots but reduced the number of nodal roots. Leaf photosynthetic parameters (chlorophyll [SPAD], net photosynthetic rate [Pn], stomatal conductance [Gs], intercellular CO2 concentration [Ci], and transpiration rate [Tr]) were reduced when infected by WSMV, and early infection reduced parameters more than late infection. Photosynthetic parameters had a linear relationship with grain yield and shoot biomass. The reduced Pn under WSMV infection was mainly in response to decreased Gs, Ci, and SPAD. The results of this study indicated that leaf chlorophyll and gas exchange parameters can be used to quantify WSMV effects on biomass and grain yield in wheat.