Effect Of Low Nitrogen Nutrition On The Growth And Metabolism Of Isatis Indigotica Fort.

In this paper,we studied the effects of low nitrogen nutrition on the growth and metabolism of Isatis indigotica Fort.by both pot and field experiments.In the pot experiment,five different nitrogen levels were applied,which were 0（N0）,2.5 mmol·L^-1（N1）,5 mmol·L^-1（n2）,10 mmol·L^-1（n3）and 15 mmol·L^-1（CK）,respectively.The indicators of I.indigotica at the seedling stage were measured,including growth characteristics,photosynthetic paraMeters,photosynthetic pigments contents,chlorophyll fluorescence parameters,antioxidant enzyme activities,nitrogen assimilation contents,nitrogen metabolism enzyme activities,contents of indigo,indirubin and total fiavonoid in leaves and epigoitrin in roots.The biomass accumulation,the photosynthetic parameters,the contents of soluble sugar,fRee amino acid,nitrate nitrogen and main active components of I.indigotica at the later period of vegetative growth were explored.In the field experiment,four nitrogen levels were designed to evaluate the effects of low nitrogen nutrition on the growth and secondary metabolism,which were 0（N0）,168.75 kg·hm^-2（N1）,337.5 kg·hm^-2（N2）,675 kg·hm^-2（CK）,respectively.The biomass accumulation and main active components of I.indigotica were investigated.The results obtained were as follows.1.Low nitrogen nutrition had significant effects on growth and metabolism of I.indigotica at the seedling stage.The plant height,main root diameter and dry weights of I.indigotica decreased significantly under N0～N3 treatments(0～10 mmol·L^-1),but the root-shoot ratio increased.With the increase of nitrogen level,the activities of SOD and POD decreased initially and then increased but finally decreased,and CAT activity showed a trend of increasing initially and then decreasing.The content of MDA in leaves declined with the increase of nitrogen concentration.The contents of soluble protein,free amino acid,nitrate nitrogen in leaves reduced under low nitrogen nutrition,nitrate reductase activity and glutamine synthase activity were inhibited.With nitrogen concentration decreasing,the chlorophyll contents,carotenoid content,net photo synthetic rate,transpiration rate and stomatal conductance gradually reduced.On the contrary,the intercellular carbon concentration increased.Low nitrogen nutrition induced a significant decrease in ΦPSII,Fv/Fm,qP and an increase in qN.2.Low nitrogen nutrition caused remarkable effects on the contents of main active components of I.indigotica at the seedling stage.The contents of indirubin and total flavonoid in leaves under N0～N2 treatments(0～5 mmol·L^-1)were found at higher levels.In contrast,contents of indigo in leaves reduced significantly under low nitrogen nutrition and epigoitrin content in roots reached the minimum value at NO treatment.The contents of indigo in leaves and epigoitrin contents in roots declined under low nitrogen conditions,however the accumulations of indigo and total flavonoid were enhanced.3.The response of growth and active components of I.indigotica at the later period of vegetative growth were different under various low nitrogen levels.The fresh and dry weights of leaves and roots increased with the increase of nitrogen concentration,and the root-shoot ratios increased initially and then decreased.The net photosynthetic rate,transpiration rate and stomatal conductance increased initially and then decreased with the increasing nitrogen concentration,and the intercellular CO₂ concentration decreased.The content of soluble sugar in leaves showed a trend of decreasing initially and then increasing with the increase of nitrogen concentration.The soluble sugar content in roots showed the different trend as that in leaves.From the concentration of 2.5 to 15 mmol·L^-1,soluble sugar content in roots increased initially and then decreased.The contents of free amino acid in roots and leaves increased at first and then declined with the increasing nitrogen concentration.The content of free amino acid under N3 treatment(10 mmol·L^-1)reached the highest.The nitrate nitrogen contents in roots and leaves showed a trend of decreasing initially and then increasing.The nitrate nitrogen content under N3 treatment was significantly lower than that of the other treatments.Compared with the nitrate nitrogen content in roots,the nitrate content in leaves at the same nitrogen level was higher.The content of indigo in leaves gradually increased with the increase of nitrogen concentration,and the content of indirubin showed a trend of decreasing initially and then increasing.The content of epigoitrin in roots increased initially and then decreased with the increasing nitrogen concentration,and the content of total flavonoid in leaves showed a tendency of decreasing.It was demonstrated that appropriate nitrogen supply is conducive to the accumulations of indigo and epigoitrin,while low nitrogen condition is favorable for increasing the contents of indirubin and total flavonoid.The epigoitrin yield in per plant was the highest under N3 treatment,which indicated that suitably reducing nitrogen application could obtain Radix Isatidis with higher contents of active components.In cultivation production,nitrogen levels could be adjusted to regulate the levels of the secondary metabolites and obtain high-quality medicinal materials.4.In the field experiment,with the decrease of nitrogen supply level,the fresh and dry weights of leaves and roots were gradually decreasing.There was no significant difference in fresh and dry weights of leaves and roots between N2(337.5 kg·hm^-2)and CK(675 kg·hm^-2)treatments（P>0.05）.The root-shoot ratio decreased initially and then increased with the decrease of nitrogen application rate.With decreasing nitrogen supply,the contents of indigo in leaves increased initially and then decreased and the contents of indirubin and total flavonoid showed an upward trend.Furthermore,the content of epigoitrin in roots showed a decreasing trend with the decreasing nitrogen application rate.Compared with CK,the yields of indigo,total flavonoid,epigoitrin in per plant reduced by 0.5%,2.8%,and 13.5%under N2 treatment and the yield of indirubin in per plant increased by 23.1%.Appropriately reducing nitrogen application had insignificant effects on contents of indigo,total flavonoid,epigoitrin,while increased the content of indirubin significantly.