Variation Of Nitrogen Uptake And Utilization Efficiency In Spinach Genotypes And Their Mechanisms

The excess application of nitrogen (N) fertilizers has led to a low efficiency of N sources in agricultural production. Besides, the high cost and accordingly N pollution of ecosystems and the atmosphere has also increased public concern. Breeding N-efficient genotypes is considered to be an effective way to improve nitrogen use efficiency, and thus overcome the related problems. A better understanding toward the underlying genetic variation and physiological mechanism has therefore become necessary and urgent. Although a lot of researches on genetic variation, evaluation and physiological mechanism had been done in the past decades, they were limited to crops. Because significant differences in growth cycle and fertilization pattern exist between crops and vegetables, it is doubtful that whether the same indexes for nitrogen use efficiency (NUE) evaluation in crops were feasible in vegetables. Thus, the variation of nitrogen uptake and utilization efficiency in spinach genotypes was studied in the present work, and parameters were evaluated to propose a proper evaluation index to aid the breeding process. Physiological mechanisms regarding N efficiency and the effects of nitrogen on plant quality and antioxidative responses were also investigated with two selected NUE contrasting genotypes. The main results obtained were summarized as follows:(1) A hydroponic experiment was carried out to evaluate the variation of NUE, N uptake efficiency, N utilization efficiency among 30 spinach genotypes grown under either N insufficient (2 mmol·L-1) or sufficient (10 mmol·L-1N) conditions. The results showed significant difference in all parameters of NUE among spinach genotypes tested. At high N level, plants revealed higher shoot dry weight, N concentration, N accumulation and lower N uptake efficiency, N utilization efficiency and N utilization ratio. Among all the parameters, shoot dry weight could be used as the crucial index for NUE evaluation in spinach genotypes due to its high coefficiency at both low and high N levels. Besides, shoot N accumulation, N uptake efficiency and N utilization ratio also showed high coefficiency of variation, and exhibited a significant positive correlation with shoot dry weight. Thus, theses parameters could also be adopted as assistant indexes aiding NUE evaluation in spinach genotypes. Based on shoot dry weight, efficient and inefficient genotypes were selected as those yielding 10% more or less, respectively, at low and high N levels. The typical efficient spinach genotypes showed higher shoot dry weight, N uptake efficiency, N utilization efficiency and N utilization ratio than those inefficient genotypes under low and high N conditions. The path coefficient analysis revealed that NUE was dominated by N uptake efficiency at low and high N level, while N utilization efficiency also contributed to NUE. These results suggest that improvements in both N uptake efficiency and N utilization efficiency are required in breeding N-efficient spinach genotypes under low and high availability of N conditions.(2) With selected N-efficient (YN398) and N-inefficient (DMWL) spinach genotypes, another hydroponic experiment was conducted to evaluate the variation of N uptake efficiency, root morphological and physiological characteristics under N-insufficient (2 mmol·L-1) and N-sufficiency (10 mmol·L-1N) conditions. The results showed significant difference in all parameters studied. Compared with DMWL, YN398 showed higher root fresh weight, root dry weight, total length, maximum root length, root surface area, root volume, root activity, Nitrate reductase (NR) and Glutamine synthetase (GS) activities under low and high N conditions. Besides, it was noticed that the root morphological and physiological characteristics exhibited a significant positive correlationship with N uptake efficiency. These results indicated that variation of root morphological and physiological characteristics might lead to the variation of N uptake efficiency. The better root growth condition, higher root activity and nitrogen metabolism enzyme activity of N-efficient spinach genotype significantly increased N uptake efficiency.(3) The typical N-efficient (YN398) and N-inefficient (DMWL) spinach genotypes grown under N-insufficient (2 mmol·L-1) and N-sufficient (10 mmol·L-1 N) conditions were further investigated into the variation of N utilization efficiency and related physiological characteristics. The results showed significant difference in all parameters studied. Compared with DMWL, YN398 showed higher nitrogen concentration, nitrogen redistribution efficiency, content of Chlorophyll, Carotenoids, soluble proteins and NR, GS activities under low and high N conditions. Besides, these physiological characteristics exhibited a significant positive correlation with N utilization efficiency. These results suggest that variation of N utilization efficiency is closely related to the nitrogen translocation and metabolism. The higher nitrogen translocation and NR, GS activities contributed to the higher N utilization efficiency of the typical efficient spinach genotype.(4) In order to investigate the effects of nitrogen on plant quality and antioxidative responses in selected spinach genotypes with contrasting N use efficiency, a hydroponic experiment was conducted in the selected typical N-efficient (YN398) and N-inefficient (DMWL) spinach genotypes grown under both N-insufficient (2 mmol·L-1) and sufficient (10 mmol·L-1 N) conditions. The results showed that, at high N level, plants revealed higher biomass, activity of superoxide (SOD), peroxidase (POD), nitrate content, total oxalate content and lower catalase (CAT) activity, As A content, GSH content, DPPH (1,1-diphenyl-2-picrydrazyl) radical scavenging rate and FRAP (ferric reducing antioxidant power) value. YN398 produced higher biomass and showed higher DPPH radical scavenging rate, FRAP value, AsA content, GSH content, and activity of SOD, POD, CAT, while with lower nitrate and total oxalate content than DMWL under both N conditions. These results suggest that breeding N-efficient spinach genotypes would promote N use efficiency, increase yield and improve plant antioxidant responses.