Journal of Crop Science and Biotechnology 57 Physiological and Genetic Mechanisms for Nitrogen-Use Efficiency in Maize Guohua Mi, Fanjun Chen, Fusuo Zhang * The Key Laboratory of Plant Nutrition and Nutrient Cycling, MOA, Key Laboratory of Plant-Soil Interactions, MOE, Department of Plant Nutrition, China Agricultural University, Beijing 100094, China Abstract Due to the strong influence of nitrogen (N) on plant productivity, a vast amount of N fertilizers is used to maximize crop yield. Over-use of N fertilizers leads to severe pollution of the environment, especially the aquatic ecosystem, as well as reducing farmer's income. Growing of N-efficient cultivars is an important prerequisite for integrated nutrient management strategies in both low- and high-input agriculture. Taking maize as a sample crop, this paper reviews the response of plants to low N stress, the physiological processes which may control N-use efficiency in low-N input conditions, and the genetic and molecular biological aspects of N-use efficiency. Since the harvest index (HI) of modern cultivars is quite high, further improvement of these cultivars to adapt to low N soils should aim to increase their capacity to accumulate N at low N levels. To achieve this goal, establishment and maintenance of a large root system during the growth period may be essential. To reduce the cost of N and carbon for root growth, a strong response of lateral root growth to nitrate-rich patches may be desired. Furthermore, a large proportion of N accumulated in roots at early growth stages should be remobilized for grain growth in the late filling stage to increase N-utilization efficiency. Some QTLs and genes related to maize yield as well as root traits have been identified. However, their significance in improving maize NUE at low N inputs in the field need to be elucidated. Key words: nitrogen use efficiency, root response, maize (Zea mays L.) J. Crop Sci. Biotech. 10 (2) : 57 ~ 63 REVIEW ARTICLES Maize is an important multi-purpose crop used for food, fodder, chemicals and biofuels. According to the FAO, total world maize production reached 6.37×10 12 tons (http://nue.okstate.edu/ Crop_Information/) in 2003, higher than wheat or rice. In devel- oping countries such as China, a huge demand for maize is expected due to the increase in animal production and biofuel requirements. Yield increase in maize is largely due to larger nitrogen (N) fertilizer inputs (Dai, 1998). While N fertilizer is a limiting factor for maize production in marginal areas and in developing countries, excessive input of N fertilizers in inten- sive agricultural areas is causing serous environmental problems such as nitrate leaching and nitrous oxide emissions, especially where there is heavy rainfall during the maize growing season. In the latter situation, the N fertilizer recovery rate is typically around 20-30% (Ju et al. 2004). Improving N fertilizer applica- tion techniques can greatly increase N-use efficiency. Developing N-efficient cultivars which take up N and/or utilize plant N efficiently provides an alternative strategy. Knowledge on the physiology and genetics of N uptake and utilization is crucial to the development of an N-efficient cultivar. Taking maize as a model crop in this review, we discuss the response of maize plants to low N stress and the possible physiological and genetic mechanisms determining N-use efficiency. Response of maize plants to N supply Spatial nutrient availability is largely determined by root dis- tribution in the soil. A typical response of maize plants to low N supply is an increase in root-to-shoot ratio resulting from rela- tively more assimilate allocated from shoot to root. The change in root system architecture (RSA) on response to N supply is much more complicated. In general, elongation of the axial and the lateral roots is enhanced at relatively low N supply (Chun et al. 2005a; Tian et al. 2005; Wang et al. 2003) (Fig. 1). Nevertheless, lateral root elongation is inhibited if the N supply is extremely low (Chun et al. 2005b; Guo et al. 2005a). Under this situation, a local supply of nitrate can increase lateral root elongation significantly (Guo et al. 2005b). In arable soils in which N distribution is extremely heterogeneous, changes in maize root morphology may represent a combination of the above-mentioned responses, and it is difficult to distinguish one * To whom correspondence should be addressed Fusuo Zhang E-mail: zhangfs@cau.edu.cn Tel: +86-10-62732499 / Fax: +86-10-62731016