Effects Of Different Nitrogen Form Ratios On Carbon And Nitrogen Metabolism And Photosynthate Transport And Distribution In Leaf-vegetable Sweet Potato

Sweet potato（Ipomoea batatas（L.）Lam.）is annual crop of the genus Ipomoea of Convolvulaceae,and is an important food crop,industrial raw materials and feed crop in the world.As a high-quality crop,leaf-vegetable sweet potato has attracted much attention,and its yield and quality have received increasing attention.Over the past few decades,under the pressure of huge food demand,excessive application of chemical fertilizers has led to low fertilizer utilization and serious fertilizer losses,resulting in serious environmental pollution and threatening human health and sustainable agricultural development.Nitrogen is one of the important factors affecting the yield and quality of leaf-vegetable sweet potato.Therefore,reasonable application of nitrogen fertilizer is the inevitable requirement of improving quality and efficiency and green production of leaf-vegetable sweet potato.In this study,a two-factor split-plot design was used.Fushu 18（F18）and Eshu 10（E10）were used as test materials.Five nitrogen ratios were set according to the three forms of nitrogen,namely,ammonium nitrogen（NH₄⁺-N）:nitrate nitrogen（NO₃^--N）:amide nitrogen（CONH₂-N）=1:1:1（N1）,1:0:2（N2）,2:0:1（N3）,1:2:0（N4）and 2:1:0（N5）,and no fertilizer was used as control CK.Through the study on yield,quality,carbon metabolism characteristics,nitrogen metabolism characteristics,and endogenous hormones of leaf-vegetable sweet potato,the mechanism of transportation and distribution of photosynthate of leaf-vegetable sweet potato and its effect on nitrogen metabolism characteristics and physiological mechanism were clarified,which provided theoretical support and technical guidance for reasonable fertilization to improve the quality and yield of leaf-vegetable sweet potato.The results are as follows:The shoot tip yields of F18 and E10 varieties in two years were measured,and it was found that the change of shoot tip yields of the two leaf-vegetable sweet potato varieties increased first and then decreased with time.The shoot tip yields under N4treatment were the highest,and those under N2 and N0 treatments were the lowest in the two years,respectively.It was speculated that the yield of leaf-vegetable sweet potato was related to the application amount of NO₃^--N.The contents of nitrate,soluble sugar,total phenols,and total flavonoids in shoot tips of leaf-vegetable sweet potato varied with different nitrogen forms and ratios.The nitrate content increased first and then decreased with time,and the nitrate content was the highest under N4 and N5 treatments in the two growth stages,respectively.The nitrate content was the highest under N4 and N5.The soluble sugar content increased first and then decreased with the growth period.The soluble sugar content under N4 treatment was the highest in the whole growth period,and the lowest under N0 and N3 treatments in2020 and 2021,respectively.The contents of total phenols and total flavonoids were the highest under N4 treatment,and the contents were significantly different in the middle growth stage.It can be seen from the quality-related indicators of leaf-vegetable sweet potato that N4 treatment has a significant indigenous promoting effect on the quality improvement.The effects of nitrogen forms on photosynthetic characteristics and dry matter distribution were different.The change of chlorophyll content showed a decreasing trend at the early growth stage,and then increased and then decreased.The chlorophyll content of the leaves of the two cultivars under N4 and N5 treatments was higher,and that under N0 and N2 treatments were the lowest.The changes in net photosynthetic rate and chlorophyll content in the whole growth period also showed a trend of first decreasing,then increasing and then decreasing,and first increasing and then decreasing.The maximum net photosynthetic rate appeared under N4 and N5 treatments,and the minimum values appeared under N0,N2,and N3 treatments.In other words,the combined application of ammonium and nitrate could improve the net photosynthetic rate of leaf-vegetable sweet potato.The highest sucrose content appeared in leaves and roots before and during the growth period,and the highest sucrose content appeared in stem apex before and during the growth period,while the sucrose content in upper stem and lower stem was at a high level in the middle period.The sucrose content in different parts was the highest under N4 and N5 treatments,and the sucrose content was at a low level under N0 and N2 treatments.The combined application of ammonium and nitrate could significantly improve the accumulation and transportation of sucrose in plants.N4 and N5 treatments mainly increased the accumulation of aboveground dry matter,and N1,N2,and N3 treatments promoted the accumulation of underground dry matter.Both leaf-vegetable sweet potatoes had higher aboveground biomass accumulation under the combined application of NO₃^--N and NH₄⁺-N,and had obvious underground dry matter accumulation under no or less NO₃^--N treatment.Different nitrogen forms had significant effects on the regulation of the main pathways of nitrogen metabolism.Nitrate reductase activity was at a low level in the early and late stages of growth,while it was high in the middle stage.The enzyme activity was the highest under N4 and N5 treatments,and the lowest under N0 and N2 treatments.The expression of Ib NR was also significantly up-regulated under N4 and N5.The change in GS activity mainly showed a trend of first decreasing,then increasing and then decreasing during the growth period.The GS activity of the two varieties was relatively low in the early stage.With the extension of the growth period,the enzyme activity increased.The GS enzyme activity was the highest under N4 and N5 treatments.The enzyme activity was low under N0,N1 and N2 treatments.The expression of the enzyme regulatory gene Ib GS was also significantly up-regulated in the late growth period,and the expression of the enzyme regulatory gene was up-regulated under the corresponding treatment as well as the enzyme activity.The GOGAT activity varied with the growth period.The enzyme activity was generally low in the early growth period and generally increased in the middle and late growth periods.The GOGAT activity mainly played an important role in improving the enzyme activity in the early growth period by N3 treatment,while it was mainly N4 treatment in the middle and late growth period.The enzyme activity under N2treatment was generally low in the growth period,while CONH₂-N was negatively regulated.According to the expression level of Ib GOGAT,the enzyme-regulated genes were also significantly up-regulated under N4 treatment,and the gene expression level was low under N2 treatment,which was consistent with the change of enzyme activity under this treatment.The GDH activity peaked mainly at the late growth stage and had higher enzyme activity under N4 and N5 treatments,and the lowest was under N0 and N2 treatments.In terms of enzyme regulation gene expression,the expression of the Ib GDH gene was significantly up-regulated at the late growth stage.Although the gene expression was also significantly up-regulated under N4 and N5 treatments,the expression of the Ib GDH gene was down-regulated under N3 treatment.The responses of different endogenous hormones to nitrogen forms were significantly different.The IAA content of E10 was mainly at a high level at the late growth stage,while that of F18 was mainly at the early growth stage.The IAA content was the highest under N3 treatment at the early stage,and the IAA content was the highest under N4 and N5 treatments at the middle and late stages.The hormone content was relatively low under N0 and N2.With the change of time,the variation trend of ABA content fluctuated less than that of IAA,but the ABA content of E10 was the highest at the late growth stage,while that of F18 was the highest at the middle and late growth stages.The two varieties reached the extreme values under different treatments,and the ABA content was at a high level under N3,N4,and N5 treatments,that is,NH₄⁺-N had a major impact on the regulation of endogenous ABA content in the two leaf-vegetable sweet potatoes.The CTK content of the two leaf-vegetable sweet potato varieties was the highest under the N4 treatment with the majority of NO₃^--N at the early and middle growth stages,while the CTK content of E10 was the lowest under the N1 and N5treatments at the later stage,and the CTK content of F18 was the lowest under the N0 treatment.Although both N1 and N5 treatments had the combined application of NO₃^--N,the CTK content of E10 was at a low level under the regulation of this treatment at the later stage,while the CTK content of F18 was the lowest under the treatment without NO₃^--N at the later stage.The GA content was relatively high under N2 and N3 treatments,and the hormone content was low under N4 and N5.The results showed that at the early stage of sweet potato growth and development,the combined application of NO₃^--N and NH₄⁺-N could significantly increase N accumulation,metabolic enzyme activity,and regulate gene expression.The N4treatment,the combined application scheme of ammonium nitrogen:nitrate nitrogen:amide nitrogen=1:2:0,was the optimal combination under this experimental condition.