The Effects Of Nitrate Stress On Nitrogen Assimilation In Maize(Zea Mays L.)Seedlings

In this study, two maize （Zea mays L.） inbred lines. Zheng58and Chang7-2and their hybrid line Zheng Dan958were used to investigate the change in biomass and root morphology of seedlings grown in liquid cultivated with such five different nitrate concentration conditions as0.02mM,0.2mM,2mM,10mM,20mM （2mM as control in this study）. The activities of glutamine synthetase （GS）, nitrate reductase （NR）. which are related to N assimilation, were also measured, revealing the physiological characteristics of seedlings at different nitrate supplements. At the same time, based on the maize database in NCBI, the EST sequences of genes NRT1.1（AY187878）, NRT2.2（AY659965）, NR（M27821） and GS（NM₀01112255） that are involved in nitrate transport and assimilation, were blast so as to design the specific primers for PCR amplification. After the cloning and sequencing, the Real-Time Quantitative PCR technology was employed to monitor the expression pattern of the genes preliminarily clarifing their expression mode under the same nitrate stress conditions. The results showed that:（1） Under the nitrate starvation stress, not only elongation of lateral root was inhibited,but the amount of root and shoot biomass were obviously restricted.and more affected in shoot biomass; the activity of NR and GS was enhanced with the increase of nitrate supplement, while the activity of SOD gradually decreased.（2） The exceeding nitrate supply inhibited the root length, density, and plant biomass accumulation, and also decreased the NR and GS activity, while continued to rise the nitrate content and SOD activity in maize seedlings. It showed that there was little relation to this luxury absortion phenomenon with the seedlings growth.（3） The expression of high-affinity nitrate transpoters NRT2.2gene was up-regulated at low nitrate stress. With the increasing of nitrate concentration, the gene expression start to decrease, while low-affinity nitrate transpoters NRT1.1gene was gradually induced.It indicated that NRT1and NRT2synergy complete the process of nitrate transport. Compared with the GS gene, NR and NRT genes are more sensitive to nitrate induced, which suggests that nitrate as a molecules signal regulate to the process in nitrate absorption and reduction. （4） The expressions of NRT, NR, GS genes were inhibited by high concentration of nitrate, it show that high concentration nitrate is not conductive to the nitrate uptake, transport and assimilation so as to influence plant growth and development.