| Nitrogen (N) plays an important role throughout plant growth and development as it is a major constituent of DNA, RNA, protein and hence enzymes, chlorophyll, ATP, auxin and cytokinins. Barley (Hordeum vulgare L), the fourth largest cereal crop grown worldwide, is widely and efficiently used for malting, feedstock and human consumption. Genetic variation for nitrogen uptake, transportation and accumulation, as well as nitrogen use utilization (NUE) is a well-known phenomenon in crops like barley. However the mechanism underlying barley genotypes is still not much understood. Therefore, there is an imperative to reveal the mechanisms of low nitrogen tolerance in plants and to develop the crops with high NUE. The current study was conducted to evaluate the genetic differences for physiological, agronomic and biochemical mechanisms in wild and cultivated barley under low N condition. Meanwhile we also studied the expression level of some important N uptake and metabolism related genes under N limitation. The main results attained are summarized as follows:Improvement and enhancement of sustainable agriculture under low N application can be achieved by developing crop cultivars with high yield under low N supply, which is dependent on the exploration and utilization of the elite germplasm. In this study,4 barley genotypes (two Tibetan wild and two cultivated), differing in N use efficiency (NUE), were characterized for their growth and physiological responses to low N stress. The genotypes ZD9 (cultivated) and XZ149 (wild) with high NUE performed better in terms of shoot dry weight and photosynthetic parameters under both low and normal N levels, and also had higher antioxidative enzyme activities, N concentration and accumulation in both shoots and roots. The current results showed the substantial difference among barley genotypes in low N tolerance and verified the significance of Tibetan wild barley in genetic improvement of cultivated barley in NUE.A greenhouse hydroponic experiment was carried out to study the effects of different N level (0, 0.2 mM and 2mM N) on ultrastructure, mineral concentration and N metabolism related enzymes in the four contrasting barley genotypes (two Tibetan wild and two cultivated), differing in N use efficiency (NUE). The genotypes ZD9 (cultivated) and XZ149 (wild) with high NUE performed better in terms of leaf area, chlorophyll contents and maximum photochemical efficiency of photo system II at low N levels. Higher N levels significantly increased the contents of other essential nutrients (P, K, Ca, Fe, Cu and Mn), and the increase was predominant for the N-efficient genotypes (ZD9 and XZ149). The observation of ultrastructure by a transmission electron microscope showed that chloroplast structure was severely damaged under low nitrogen, and the two high N efficient genotypes were relatively less affected. The activities of the five N metabolism related enzymes, i.e. nitrate reductase (NR), glutamine synthetase (GS), nitrite reductase (NiR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) all showed the substantial increase with the increased N level in the culture medium. However the increased extent differed among the four genotypes, with the two N efficient genotypes showing more increase in comparison with the other two genotypes (HXRL and XZ56). The current findings would be of great interest that a huge difference exists in low N tolerance among barley genotypes, and improvement of some physiological traits (such as enzymes) could be helpful for increasing N utilization efficiency.In the current study, four barley genotypes, ZD9 & XZ149 (higher NUE) and HXRL & XZ56 (lower NUE) were used to investigate the expression pattern for N uptake (NRT2.1) and N assimilatory (GS1 & GS2) genes in barley leaves and roots at seedling stage in response to low nitrogen (0.1 mM) and normal nitrogen (2mM) levels. Compared to the normal N supply, under low N, all the genotypes expressed fewer tillers per plant, lower soluble protein, chlorophyll and N concentration in both roots and shoots. However, significant difference was found among the genotypes, with N efficient genotypes (ZD9 & XZ149) showing higher N concentration, more tillers per plant, higher chlorophyll and soluble proteins contents in both roots and shoots as compared to the inefficient genotypes (HXRL & XZ56). Furthermore, nitrate transporter gene NRT2.1 expressed more under low N in both roots and leaves of N efficient genotypes at different times while glutamine synthetase GS1 and GS2 expressed more under normal N in leaves. Compared to the lower NUE genotype (HXRL & XZ56), the higher NUE genotype (ZD9 & XZ149) under low N re-supply performed better in response to N stress, and may require relatively less N fertilizer application for normal growth and development. |
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