JIA-2018-09

1981 TAO Zhi-qiang et al. Journal of Integrative Agriculture 2018, 17(9): 1979–1990 occurred once. There were four different concentrations of zinc fertilizer treatments: 0, 15, 30, and 45 mg Zn kg –1 soil (named Zn0, Zn15, Zn30, and Zn45, respectively). To maintain soil field capacity at 75%, plants were watered accordingly with the aid of a TZS-1K soil moisture analyzer (Zhejiang Top CloudAgricultural Polytron Technologies Inc., Zhejiang Province, China) which was used to measure soil moisture content every 3 d. The daytime temperature range was 15–38°C, the relative humidity reached an average of (50±15)% by the time wheat plants began to flower. Over the last 10 years in the winter wheat region of northern North China, daily maximum temperatures had reached approximately 38°C for 1.8 consecutive days during the grain filling period in wheat growth. Thus, we exposed plants that reached 20 d after flowering (D20) to a 2-d HTS treatment at 38°C to simulate the potential growing trend of increased days of high temperatures in northern North China occurring during the wheat grain-filling period. We transferred plants to an artificial climate chamber where they were exposed to the heat stress for 5 h from 11:00 to 16:00 each day. The relative humidity of the indoor air was (45±5)%. After the 2-d heat treatment, all pots were returned to their previous locations in the field to continue growing until seed set. Control plants (NT) were grown at ambient temperatures in the field until harvest time. Pots were placed in a randomized block design. There were a total of 16 replicates per zinc treatment. Eight of these replicates were used to determine NR and GS activities in flag leaves that were sampled at D10 and D22. Leaves were sampled during mornings with full sun. The other eight replicates were used to determine grain yield, weight, protein content and protein components. The field portion of the experiment was first conducted on 15 October 2015 to 5 June 2016 and repeated 12 October 2016 to 3 June 2017. 2.3. Sampling methods Uniform-sized ears were selected at the same flowering stage, and plants were repeatedly sampled at D10 and D20. Grain yield and mean weight per grain were calculated after seeds harvested from pots and dried. The seeds were air-dried for 30 d, and then crushed and ground into flour with a grinding machine, ZH10852 (Zhonghui Tiancheng Technology Co., Ltd., Beijing, China). The flour was used to determine protein content, protein yield, and content of four protein components (albumin, gliadin, glutenin, and globulin). The content of protein was determined by semi- micro Kjeldahl nitrogen method, and then protein yield was determined by the protein content multiplied by the mean weight per grain (Zhao et al . 2013). NR and GS activities in flag leaves were determined by following the method described in Yu and Zhang (2012). The protein components of grains were determined by the sequential extraction method, i.e., albumin, globulin, gliadin, and glutenin were sequentially extracted with distilled water, 2% NaCl, 70% ethanol, and 0.5% KOH (Liu et al . 2015). 2.4. Data analysis Using SPSS 21.0 software (SPSS Inc., Chicago, IL, USA), a variance analysis and interaction effect and factor contribution analysis (Eta 2 ) were performed with a General Linear Model process, from “Analyze” to “General Linear Model” to “Univariate”. Eta 2 values range between 0 and 1 and represent the contributing proportion of effect from a factor in a model. The measured variables of grain weight, protein yield, total protein content, protein yield, and content of four protein components (albumin, gliadin, glutenin, and globulin), and flag leaf NR and GS activities were subjected to a post-hoc multiple comparisons test using Duncan’s method with a significance level at P =0.05. 3. Results 3.1. Effects of zinc fertilizer and high temperature stress on grain yield Exposure to HTS post-anthesis significantly reduced grain yield and weight ( P <0.05) below that of the NT group for plants of both cultivars. The influence of HTS on GY2018 was significantly higher than that on ZM8 (Figs. 1 and 2). Zinc fertilizer significantly increased grain yield and weight of the two cultivars ( P <0.05). A consistent pattern was observed in yield and weight in both cultivars and both experimental years. The measures of grain yield and weight were consistently the highest in the Zn15 treatment and the lowest in the Zn0 treatment. Furthermore, yield and weight consistently decreased from Zn15 with greater additions of zinc. On average, yield and weight in the three zinc treatments of GY2018 of both years increased by a respective 34.3 and 30.7% (Zn15), 28.4 and 23.0% (Zn30), and 17.1 and 10.3% (Zn45) compared to the control (Zn0). In Zm8, for both years, average increases in yield and weight were approximately, 12.2 and 14.9% (Zn15), 6.1 and 7.5% (Zn30), and 3.5 and 4.0% (Zn45) respectively greater than Zn0 (Figs. 1 and 2). The results indicated that HTS reduced grain yield and grain weight in both cultivars; whereas, zinc fertilizer increased grain yield and grain weight compared to the control (Zn0). Together, zinc fertilizer and HTS also caused increases in grain yield and grain weight in both cultivars. The interaction effects of cultivar×temperature, cultivar×zinc fertilizer, temperature×zinc fertilizer, and cultivar×temperature×zinc fertilizer on grain yield and weight