JIA-2018-09

1980 TAO Zhi-qiang et al. Journal of Integrative Agriculture 2018, 17(9): 1979–1990 for more high quality wheat. Wheat quality is determined by gluten strength (ranging from weak to strong gluten wheats) and is affected by the proteins in wheat flour. Total protein content of wheat and its protein components determine flour processing quality and the commercial value of flour products (Goesaert et al . 2005). Genes, environmental factors, and cultivation methods control the content of protein and its components in wheat grain. During the grain filling stage, high temperature stress (HTS) can not only reduce grain yield, but also affect flour protein components and its functional properties, and in turn, affect the rheological properties and baking quality of the flour (Blumenthal et al . 1993; Asseng et al . 2015). However, Nuttall et al . (2017) reported that, due to climate change, the daily frequency of extreme high temperatures has been increasing, hence the frequency of HTS occurring in the late growth stages of wheat (e.g., seed production) will likely increase and detrimentally affect the future yield and quality of wheat. The nutritional status of a plant is one of the most important factors that affect the protein content of wheat grain. At present, many studies have reported the effects of macronutrients and micronutrients, such as nitrogen and sulfur, on wheat grain protein content (Tea et al . 2007; Klikocka et al . 2016) and effect of copper deficiency on dough extensibility (Flynn et al . 1987). The micronutrient zinc is known to play an important role in grain protein formation and nitrogen assimilation in winter wheat (Li et al . 2011). Zinc affects monomeric protein structure, the composition of high molecular weight glutenin subunits, and the proportions of glutenin and gliadin in total proteins, all of which affect the quality of flour (Liu et al . 2015). However, the mechanisms driving the effects are not clear (Starks and Johnson 1985; Peck et al . 2008). In China, Turkey, India, Pakistan, Australia, and other countries, most of the available zinc in the soil is low (Ozturk et al . 2006). Increasing the content of available zinc in soil could improve the protein content of wheat flour (Hemantaranjan and Garg 1988). Zhao et al . (2013) reported that the activities of nitrate reductase (NR) and glutamine synthetase (GS) in wheat flag leaves could significantly influence the content of protein in wheat flour. Zinc fertilizer can increase the activity of NR and GS in flag leaves, and thus, affect the content of protein components (Crawford 1995; Liu et al . 2015). Furthermore, zinc can change the proportion of cysteine residues (Peck et al . 2008), and thus, affect the two disulfide bonds and the end-to-end formation of linear peptide molecules in the non-repetitive region of the peptide chain of high molecular weight glutenin subunits, and subsequently, affect the structure and rheological properties of gluten (Tamás et al . 2002). At present, we lack research on how zinc regulates the protein content of wheat grains that have experienced high temperature stress. Therefore, this study examined the response characteristics of wheat grain protein and its components of four different glutens in strong gluten and medium gluten wheat cultivars that were grown with increasing levels of zinc fertilizer and exposed to a short- termHTS after anthesis. The aim of this study was to provide empirical data to help elucidate the potential physiological mechanisms in response to additional zinc fertilizer and HTS effects, as well as a theoretical basis and technical support to food producers and researchers for improving the grain yield and flour from wheat that may be growing under more stressful conditions due to climate change. 2. Materials and methods 2.1. Cultivars Seeds of two varieties of winter wheat ( Triticum aestivum L.) were obtained from wheat fields in northern North China. The strong gluten wheat, Gaoyou 2018 (GY2018), is considered a high quality wheat and the medium gluten wheat, Zhongmai 8 (ZM8) is considered of average quality. The optimal temperature range to grow these varieties is 15–32°C. In northern North China, the recorded average, maximum, and minimum daytime temperatures were 11.2, 42, and –16°C, respectively, between 2000 and 2017. 2.2. Experimental design The experiment was conducted at test fields in the Institute of Crop Sciences, ChineseAcademy of Agricultural Sciences (39°57´40´´N, 116°19´23´´E). Loam soil for pot experiments was collected from the top 30 cm of the soil at the test field. The content of organic matter, total nitrogen, available nitrogen, available phosphorus, available potassium, and diethylenetriamene pentaacetate zinc (DTPA-Zn) in the soil was 16.5, 0.92, 55.2, 35.1, 189.6, and 0.85 mg kg –1 , respectively. A total of 10 kg of sieved (2-mm mesh), dry soil filled each of 256 pots that were 25 cm in height and 25 cm in diameter. Seeds were sown on 15 October 2015 and 12 October 2016 and harvested on 5 June 2016 and 3 June 2017. We fertilized the potting soil with urea, superphosphate, and potassium chloride at respective concentrations of 90 mg N, 90 mg P, and 80 mg K per kg soil before sowing. When plants reached a three-leaf stage, seedlings were thinned to 10 per pot. Topdressing each pot with 1 g of urea dissolved in deionized water (DI) water occurred at the jointing stage of wheat growth. Zinc fertilizer was made from a solution of analytically pure chemical reagents and deionized water (ZnSO 4 ·7H 2 O) and applied after fertilizing the potting soil with urea, superphosphate, and potassium chloride. Application of all fertilizers