| Vegetable, as large consumed food in daily life, its qulity and safty were paid attention by Chinese peple. Leaf vegetables are classified as high nitrate accumulation crops and this phenomenon is aggravated by high nitrogen fertilizer application in vegetable production. Nitrate itself is not harmful to human, while its reductant may increase risks of thyroid and gastric illnesses. Widly planted of Chinese cabbage [Brassica campestris L. ssp. Chinensis (L.)] in Wuhan and high consumption force us to mininise any adverse effects on human health. Based on various cultivars in Chinese cabbge, we have conducted high and low nitrate accumulate cultivars screening and studied nitrate uptake, translocation and assimilation differences between cultivars. Differences of nitrate uptake and assimilation between cultivars in response to nitrate and molybdenum were also examined. The main results are as follow:1. Nitrate accumulation in Chinese cabbage in Wuhan and nitrate accumulators screening experimentsThe nitrate content and biomass of168Chinese cabbage cultivars in Wuhan have been studied. The experimental results show that there were differences in nitrate concentration and biomass were observed in cultivars. The highest nitrate concentration and biomass were observed in the third and second screening experiments respectively. Nitrate concentration exceeding3100mg/kg were observed in24.4%,97.6%and100%of the cultivars in the three screening experiments respectively. On the basis of nitrate content and biomass, we choosed7cultivars defined as high nitrate high biomass cultivars,9cultivars defined as high nitrate low biomass cultivars,7cultivars defined as low nitrate high biomass cultivars and2cultivars defined as low nitrate low biomass cultivars. Nitrate content in different plant tissues were also determined. Petiole nitrate content in the third and fourth experiments were1.69and2times higher than in leaves. Due to high sensitivity, leaves were considered to be the best tissues for evaluating nitrate accumulation in plant. L18was defined as a low nitrate leaf accumulatior and H96was defined as a high leaf nitrate accumulator.2. Differences in the mechanism of nitrate accumulation between cultivarsThe high nitrate accumulator-H96and the low nitrate accumulator-L18, from the field screening experiments, were used in a hydroponic culture to investigate genotypic differences in nitrate uptake, translocation and assimilation between the two Chinese cabbage cultivars. H96could uptake more nitrate than L18in the root but had lower transport into leaf cells and assimilation in the leaf. It was show that root morphology parameters (length, surface area and volume) of H96were18.0%,31.6%and46.5%higher than for L18respectively. Nitrate transporters NRT1.l and NRT2.1transcription levels in roots were41.6%and269.6%higher than those of L18respectively. In process of nitrate translocation, NRT1.1and NRT2.1expressions in the leaf blades of the two cultivars were opposite to these in the roots, L18NRT1.1and NRT2.1expressions were279.2%and80.0%higher than H96. In addition, nitrate assimilation capacity of L18was significantly higher than H96in leaves. It was shown that nitrate assimilation enzymes-NR, GS and those gene-NIAã€Gln1ã€Gln2relative expressions of L18were234.0%,43.9%,105.4%,331.5%and124.8%higher than those of H96respectively. Both chlorophyll content and photosynthesis of L18were higher than those of H96. Nitrate assimilation products-Glu, total amino acid, soluble protein content in the leaf of L18were all significantly higher than those of H96. The results above suggested that nitrate accumulation differences were due to differential capacities for uptake, mechanisms for nitrate transport in leaves and assimilation of nitrate. Comparing the contribution of three aspects in nitrate accumulation, the latter two aspects contributed more of low nitrate concentration in the leaf blade.3. Genotypic differences of nitrate uptake, translocation and assimilation in response to nitrate provisionA hydroponic culture experiment was conducted to investigate genotypic difference of nitrate uptake, translocation and assimilation in response to nitrate provision. The results showed that NRT1.1and NRT2.1expressions in roots of the two cultivars were sharply increased in response to nitrate supply.15N accumulate rate and contents in roots was higher than in leaves. Meanwhile NRT1.1and NRT2.1expressions and5N contents in tissueses of H96were significantly higher than for L18. However, the results in leaves were reversed, NRT1.1and NRT2.1expressions in leaves of L18were peaked at12h and24h and significantly higher than those of H96. In process of nitrate reduction, NR activity and NIA expression of two cultivars were induced by nitrate supply, and NR activity and NIA expression of L18were significantly higher than those of H96, except for12h in leaves. It was suggested that nitrate assimilation capacity of L18was stronger than H96.4. Genotypic difference of two Chinese cabbage cultivars in response to N levelsDifferences of nitrate concentration between cultivars and in tissues were decreased in response to N deficiency. Nitrate content in tissues decreased under a-N treatment. Nitrate contents in petioles of L18and H96were5.3times and2.3times higher than in leaves. The leaf nitrate content of H96was higher than of L18significantly under the+N treatment, while no significant difference was observed in tissues or between cultivars under the-N treatment. Nitrate uptake was different between the two cultivars under N treatments. Nitrate uptake rate and amount of H96were higher than for L18under+N, while the results were reversed under-N. Similar results were observed in root nitrate transporters expressions. NRT1.1and NRT2.1expressions in roots of H96were41.7%and264.7%higher than those of L18under the+N treatment, while NRT2.1expression of L18was117.8%higher than H96, no significant difference was observed in NRT1.1. It was suggested that differences of nitrate uptake between cultivars was based on NRT2.1expression differences. In the process of nitrate translocation, NRT1.1and NRT2.1expression in leaves of L18were3.8times and1.8times higher than those of H96under the+N treatment, while NRT2.1expression in leaves of H96was3times higher than L18under the-N treatment. Thus, more nitrate were transported into leaves for reduction than for L18. NR and GS activities were decreased by the-N treatment, with no difference between cultivars. NR activity in leaves of L18was significantly higher, but its GS activity and nitrate assimilation production-soluble protein contents were lower than for H96under the-N treatment. These above results suggest that nitrate assimilation capacity of H96was higher than L18under the-N treatment.5. Genotypic difference of two Chinese cabbage cultivars in response to Mo levelsMo deficiency significantly increased nitrate uptake and nitrate transporters-NRT1.1and NRT2.1expressions in roots, decreased nitrate transport into leaves and enzyme (NR, GS) activities which were involved in nitrate assimilation, while genotypic difference of two cultivars were shown in response to Mo treatments. High nitrate accumulator-H96was more sensitive to Mo deficiency. It was shown that increase rate of nitrate uptake, NRT1.1and NRT2.1expressions in root of H96were (195.9%,44.2%and52.1%) higher than for L18(146.9%,5.0%and10.3%) response to Mo deficiency. In the process of nitrate translocation, decreased rates of NRT1.1and NRT2.1expressions in leaves of H96under Mo deficiency (1252.0%,4181.3%) were higher than those of L18(702.0%,4024.5%). NR and GS activities of H96were increased47.1times and64.6%which higher than L18(11.0times and36.3%). Nitrate assimilation, especially NR activity was most impacted by Mo deficiency. In addition, the nitrate content in leaves of L18was lower than for H96. Related to the nitrate uptake result, total N of H96was significantly higher than for L18. Comparing contributions of uptake, translocation and assimilation to nitrate accumulation under conditions of Mo deficiency, uptake and assimilation capacities were determined in nitrate accumulation in leaf. Based on high Mo utilization, the nitrate assimilation capacity of L18, especially NRA, contributed to the low nitrate content in response to Mo deficiency.In conclusion, inconsistency between root uptake and shoot assimilation capacity is main reason cause nitrate accumulation in leaf of Chinese cabbage. H96had a great capacity of nitrate uptake in roots, which was a basic reason for nitrate high accumulation in leaves. Nitrate high translocation and assimilation capacities in leaves were a key reason for nitrate low accumulation in leaves. Differences in root nitrate uptake, shoot accumulation and assimilation between the two cultivars were minimized under-N treatment, while the results of root nitrate uptake were reversed. This was caused by nitrate transporter expression changes in the roots. Changes of nitrate uptake, translocation and assimilation in response to Mo deficiency were based on Mo efficiency of two cultivars. Due to high NR activity and Mo efficiency, nitrate concentration in leaves of L18was less impacted by Mo deficiency. |
PDF Full Text Request