Polyaspartic Acid Improves Maize (Zea mays L.) Seedling Nitrogen Assimilation Mainly by Enhancing Nitrate Reductase Activity

Xuerui Dong,Yanli Xu,Hui Dong,Qingyan Wang,Guangyan Li,Zhiqiang Dong,Huihui Tang

doi:10.3390/agronomy8090188

Abstract

Improvement of nitrogen use efficiency is of great importance in maize (Zea mays L.) production. In the present study, an eco-friendly growth substance, polyaspartic acid (PASP), was applied to maize seedlings grown with different nitrate (NO3−) doses by foliar spraying, aimed at evaluating its effects on maize nitrogen assimilation at both the physiological and molecular level. The results showed that PASP promoted biomass and nitrogen accumulation in maize seedlings, especially under low NO3− doses. Among different NO3− conditions, the most noticeable increase in plant biomass by PASP addition was observed in seedlings grown with 1 mmol L−1 NO3−, which was a little less than the optimum concentration (2 mmol L−1) for plant growth. Furthermore, the total nitrogen accumulation increased greatly with additions of PASP to plants grown under suboptimal NO3− conditions. The promotion of nitrogen assimilation was mostly due to the increase of nitrate reductase (NR) activities. The NR activities in seedlings grown under low NO3− doses (0.5 and 1.0 mmol L−1) were extremely increased by PASP, while the activities of glutamine synthetase (GS), aspartate aminotransferase (AspAT), and alanine aminotransferase (AlaAT) were slightly changed. Moreover, the regulation of PASP on NR activity was most probably due to the promotion of the protein accumulation rather than gene expression. Accumulation of NR protein was similarly affected as NR activity, which was markedly increased by PASP treatment. In conclusion, the present study provides insights into the promotion by PASP of nitrogen assimilation and identifies candidate regulatory enzymatic mechanisms, which warrant further investigation with the use of PASP in promoting nitrogen utilization in crops.

Highlights

Nitrogen is one of the most important nutrients that strikingly affects plant growth, development, and production
Nitrogen content per gram of dry matter did not significantly (p < 0.05) differ as a result of these treatments, as calculated in Figures 1 and 2. These results suggest that the promotion of nitrogen assimilation by polyaspartic acid (PASP) is probably simultaneous with other types of metabolism, such as carbon assimilation [19], which results in the extreme expansion in total nitrogen accumulation per plant, with a small influence in nitrogen concentration per gram of dry matter
The promotion by PASP of maize nitrogen accumulation was primarily due to the increase in nitrate reductase (NR) activity

Summary

Introduction

Nitrogen is one of the most important nutrients that strikingly affects plant growth, development, and production. Despite the importance of improving nitrogen use efficiency in maize, a number of previous studies had focused on various agronomic strategies to optimize nitrogen application and its biological mechanisms, such as the tillage type, rate and timing of nitrogen fertilizer application, and better sources of nitrogen fertilizer [7,8]. On the one hand, cultivars with high nitrogen use efficiency have been proven to be a great option for increasing grain yield under low nitrogen conditions while maintaining the health of the environment [9]; on the other hand, the development of highly efficient nitrogen fertilizers is another effective way to resolve these problems [10]. Controlled-release urea has been demonstrated to significantly improve grain yields and the nitrogen use efficiency of maize [11]. Polyaspartic acid is a hydrophilic, nontoxic, and biodegradable polymer of aspartic acid, with good dispersibility, chelating ability, and adsorption capacity [16]

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Conclusion