JIA-2018-09

1987 TAO Zhi-qiang et al. Journal of Integrative Agriculture 2018, 17(9): 1979–1990 treatments than in the heat-stressed, Zn0 treatment in both replicates of both wheat varieties. Conversely, of the same treatment comparison, grain protein yield was significantly lower in both replicates of both wheat varieties. Compared with ZM8 under HTS, grain protein content was significantly greater in GY2018 under HTS. In both years, the reduction in grain protein content was significantly less in the Zn treatments in GY2018 than that in ZM8. Moreover, there was significantly more proteins in GY2018 than those in ZM8 (Figs. 5 and 6). Fig. 7 illustrates the proportions of protein components in the two cultivars across all treatment combinations. A ranking of the content of each component of grain protein in both cultivars is glutenin>gliadin>albumin>globulin. The high temperature treatment reduced the globulin content in both cultivars whereas, HTS increased the contents of the other components. The ratio of glutenin to gliadin increased in GY2018, but no change was observed in ZM8. The glutenin content and the ratio of glutenin to gliadin of strong gluten GY2018 was significantly higher than that of medium gluten ZM8. Lower concentrations of gliadin and glutenin were observed in both the Zn0 and Zn45 treatments than in the mid-level Zn treatments. Of all zinc fertilizer treatments, the highest gliadin content was measured in Zn15 in GY2018 and in Zn30 in ZM8. The highest glutenin content was measured in Zn15 in both GY2018 and ZM8. A similar trend was also found in albumin content and the opposite trend in globulin content. There was an increase of albumin in GY2018 and ZM8 with the addition of zinc fertilizer from 0–15 mg Zn kg –1 soil, followed by a decrease of albumin when zinc fertilizer additions increased from 15– 45 mg Zn kg –1 soil. The albumin content was higher in ZM8 than that in GY2018. A different trend was observed in globulin content where there was a decrease of globulin content in GY2018 and ZM8 with the increase of zinc fertilizer. The contents of glutenin, albumin, and globulin were significantly affected by main and interaction effects of cultivar, temperature, zinc fertilizer, cultivar×temperature, cultivar×zinc fertilizer, temperature×zinc fertilizer, and cultivar×temperature×zinc fertilizer ( P <0.01). The content of gliadin content was significantly affected by main and interaction effects of cultivar, temperature, zinc fertilizer, and cultivar×zinc fertilizer ( P <0.01 or P <0.05) (Table 1). The effect of cultivar had the greatest contribution to the total variation in glutenin, albumin, and globulin contents. This was followed by the contributions of zinc fertilizer, temperature, cultivar×zinc fertilizer, cultivar×temperature×zinc fertilizer, zinc fertilizer×temperature, and cultivar×temperature. The effect of cultivar had the greatest contribution to the total variation of gliadin content, followed by zinc fertilizer, cultivar×zinc fertilizer, and temperature (Table 2). In both years, comparison of zinc-added treatments in combination with HTS between the two cultivars indicated that glutenin, gliadin, albumin, and globulin contents were more strongly affected in GY2018 than those in ZM8. In contrast, in Zn15, Zn30, and Zn45 treatments under HTS compared to their respective zinc treatment levels that were not under HTS, glutenin, gliadin, albumin, and globulin contents were greater in both GY2018 and ZM8 in both years (Fig. 7). 4. Discussion 4.1. Response of NR and GS to zinc fertilizer and high temperature stress Nitrate reductase is important to the uptake and utilization of nitrogen because it affects crop yield and quality (Raun and Johnson 1999). GS is a multifunctional enzyme involved in nitrogen metabolism and the regulation of many other metabolic processes. In higher plants, more than 95% of NH 4 + is assimilated by glutamine synthetase/glutamate synthase (GS), GS is central to nitrogen metabolism where it participates in the regulation of key enzymes. A lack of GS activity can negatively impact a variety of nitrogen metabolizing enzymes as well as some glucose metabolizing enzymes (Foyer et al . 2003). Zinc fertilizer has been shown to enhance nitrogen assimilation (Shi et al . 2011). Additionally, studies showed that the activities of NR and GS increased with Zn fertilization of up to 10 mg Zn kg –1 soil (Ghosh and Srivastava 1994; Liu et al . 2015). In our study, 15–45 mg Zn kg –1 soil treatments increased the activities of NR and GS, but the magnitude of the increases in the two cultivars varied. Thus, the amount of zinc fertilizer available in soil can affect the NR and GS activities of flag leaves, but the mechanism of these changes in activity need to be further studied. Similar to our observations, other researchers have also observed that high temperature stress causes NR and GS activities in flag leaves to decrease (Liu et al . 2007). The observed reduction in both enzyme’s activity caused by HTS above 35°C may have accelerated senescence of the plant while in its later life stage of grain filling. The stress from the high temperature may have been so severe that plants were unable to recover from or compensate for the damage. Thus, the NR activity in the flag leaves decreased or possibly ceased completely. The likely consequence is that NO 3 – is deoxidized to NH 4 + and the NH 4 + is subsequently reduced. As a substrate of the GS reaction pathway, the loss of available NH 4 + can cause a decrease in GS activity (Liu et al . 2007). The results of this study also showed that the interaction effect of zinc fertilizer×temperature×cultivar had a significant effect on NR activities, and temperature×cultivar had a significant effect on GS activities. Overall, these