| Nitrogen（N）is the most desirable nutrient during the growth and development of cotton and is one of the decisive factors for cotton yield and quality.With the development of the cotton industry,reducing the application of nitrogenous fertilizer while stabilizing the yield and quality of cotton is an important issue to be solved in cotton production.Improving the N use efficiency through the physiological mechanism of high N utilization efficiency in cotton,and tapping the genetic potential of high nitrogen utilization in cotton may be one of the economical and eco-friendly ways to reduce the amount of nitrogenous fertilizer.Earlier our group has selected cultivars with different nitrogen efficiency from 270 cultivars.Among the selected cultivars,CRI 69 and ZZM 1017 were the N efficient cultivars,while ZZM GD 89 and XLZ 30 were the N inefficient cultivars.A combination of pot and field experiments were carried out with four N treatments in the pot(N0,0 g N kg-1;N1,0.075 g N kg-1;N2,0.150 g N kg-1;N3,0.225 g N kg-1)and field experiments(N0,0 kg N ha-1;N1,112.5 kg N ha-1;N2,225 kg N ha-1;N3,337.5 kg N ha-1).The aim was to study the correlation between N metabolism,carbon metabolism,antioxidant system,and cotton N use efficiency to explore the key physiological process and mechanism of cotton N utilization.Finally,the variation between the two types of cotton cultivars for the economic and ecological effects on cotton production was discussed.The main results are summarized as follows:1.The application of N fertilizer increased the plant N accumulation,leaf N concentration,key enzyme activity of N metabolism,and amino acid content.Under the same level of N fertilizer,the leaf N concentration and the activities of key enzymes in N metabolism of the CRI 69 and ZZM 1017 were significantly higher than that of the ZZM GD89 and XLZ 30.Moreover,the difference between cultivars was greatest at the boll setting stage.Under both pot and field conditions,the nitrate reductase（NR）activity in the leaves increased at the N0-N3 level by 73.1%,87.6%,73.3%,41.1%and 45.0%,51.1%,41.9%,23.8%in N efficient cultivars than that of the N inefficient cultivars,and the free amino acid content increased by 20.7%,55.1%,56.4%,31.3%and 37.9%,56.8%,45.0%,40.5%,respectively.This result indicated that the N efficient cotton cultivars had strong N assimilation ability,especially under low N levels.2.Compared with the N inefficient cultivars,the N efficient cultivars exhibited a stronger net photosynthetic rate（Pn）and the key enzyme activities of carbon metabolism like sucrose synthase（SS）and sucrose phosphate synthase（SPS）under the same treatment.The difference in physiological traits between cultivars was greatest at the boll setting stage.Under both pot and field conditions,compared with the N inefficient cultivars,the Pn of the N efficient cultivars at the N0-N3 level increased by 88.2%,96.2%,83.1%,76.3%and 58.9%,75.5%,54.9%,26.8%,and the SPS activity increased by 19.2%,46.3%,38.0%,24.2%and 36.6%,43.5%,26.3%,9.2%,respectively,which indicated that the N efficient cultivars had strong carbon metabolism and photosynthetic capacity,especially under low N level.3.The application of N fertilizer could reduce the malondialdehyde（MDA）content and enhance the activity of antioxidant enzymes such as superoxide dismutase（SOD）,peroxidase（POD）,and catalase（CAT）.Under both pot and field conditions,the MDA content of the subtending leaf of cotton boll in the N efficient cultivars at the N0-N3 level decreased by 24.8%,18.3%,20.0%,24.2%and 22.3%,22.0%,18.8%,17.2%than that of the N inefficient cultivars,respectively.However,the SOD activity increased by 70.8%,68.4%,77.2%,41.2%and 37.7%,67.6%,26.6%,19.9%.The results showed that the N efficient cultivars had stronger ability to adapt to low nitrogen.4.The yield and value cost ratio of the N efficient cultivars was higher than the N inefficient cultivars.Under both pot and field conditions,the yield of N efficient cultivars increased at N0-N3 levels by 54.3%,68.0%,73.9%,70.7%and 29.0%,29.6%,19.7%,14.9%than that of the N inefficient cultivars,respectively.The value cost ratio increased by 42.3%,45.2%,62.7%,56.9%and 59.7%,29.7%,23.3%,7.6%.Furthermore,under the N1-N3 level,the N use efficiency increased by 74.8%,72.9%,and 64.0%in the pot and24.9%,19.5%,and 12.9%in the field conditions.In summary,the N efficient cultivars showed higher N concentration in leaves,and the enzymatic activities like NR,increasing the Pn and the enzyme of SPS.Meanwhile,the N efficient cultivars had the antioxidant enzyme activity and the lower MDA content,which delayed the leaf senescence,and increased the time for carbon and N metabolism,and provided more material for the bolls.Thus,the N efficient cotton cultivars improved N use efficiency,yield and net return. |
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