Effects Of Manganese Treatment On Physiological And Biochemical Characteristics Of Brassica Rapa Ssp. Chinensis L. Under Cadmium Stress

The cadmium?Cd?pollution in vegetable fields in China has the characteristics of slight,multiple,and scattered plots.Through the application of trace element fertilizers and the use of antagonism between elements,the Cd content of vegetables can be effectively reduced,and it can be used to repair Cd pollution in vegetable fields with the advantages of flexibility,simplicity,rapidity,and low cost.Studies have shown that the application of manganese?Mn?fertilizer can significantly reduce the Cd content of plants,and its regulation mechanism is that the interaction of manganese and cadmium inhibits Cd uptake and mitigation of plants and their mechanisms of action are still lacking.In this paper,the common rapeseed variety“Hanlv”and low-Cd accumulation variety“Huajun”were selected as the test materials.The hydroponic experiment was adopted to set the Cd stress and different Mn treatment concentrations to study the effects of different Mn nutrition treatments on Brassica rapa ssp.Chinensis L.growth under Cd stress conditions.The effects of Cd subcellular distribution,root morphology,photosynthetic system,and nitrogen metabolism system were designed to reveal the mechanism of interaction between Mn and Cd on plant cadmium toxicity.The main conclusions are as follows:?1?Cd mainly distributed in vacuoles in the rapeseed,followed by the cell wall.Exogenous addition of Mn²⁺can change the distribution ratio of Cd in each subcellular fraction.Increasing Mn²⁺concentration can reduce the content of Cd in subcellular fraction.Cd mainly distributed in the vacuole in the shoots,followed by the cell wall.Exogenous addition of Mn²⁺could change the distribution ratio of Cd in various subcellular components.Increasing the concentration of Mn²⁺could reduce the content of Cd in the subcellular fraction.For“Hanlv”shoots,the proportion of Cd in the cell wall increased by 36.6%and 40.7%when treated with 0.1 and 0.5?mol·L^-1Mn increased Cd in the cell wall.Different concentrations of Mn treatment have no significant effect on the subcellular distribution of Cd in root.For the“Huajun”shoots,0.5,5 and 10?mol·L^-1Mn treatments all increased the proportion of Cd in the cytosolic fraction,which is increased by 51.1%under the treatment of 0.5?mol·L^-1Mn;for“Huajun”,when treated with 0.1?mol·L^-1Mn,the distribution ratio of Cd in the cytosolic fraction increased by 19.5%in the root,and the distribution ratio of cell wall increased by21.28%and 41.9%when treated with 0.5 and 5?mol·L^-1Mn,respectively.In conclusion,the addition of different concentrations of Mn treatment have different effects on the subcellular distribution of two genotypes of Brassica rapa ssp.Chinensis L..For“Hanlv”,the Cd toxicity is mainly reduced by increasing the proportion of Cd in the aerial shoot cell wall components;for“Huajun”,the toxicity of Cd is mainly reduced by increasing the proportion of Cd in the cytosol fraction of the aerial parts and the cell wall of the root.In addition,there is no significant changes in biomass of shoots of both genotypes of Brassica rapa ssp.Chinensis L.under different Mn treatment conditions,indicating that the retention of vacuolar compartment and cell wall play an important role in improving tolerance to Cd in Brassica rapa ssp.Chinensis L..?2?Compared with Mn1 treatment,the root length,surface area,volume,average diameter,and number of root tips of“Huajun”under Mn deficiency treatment are significantly reduced;the root length of“Hanlv”under Mn-excess treatment;the root surface area and the volume of“Huajun”significantly reduced under the Mn deficiency,the ratio of the surface area of both genotypes of Brassica rapa ssp.Chinensis L.fine roots increased,and the ratio of the surface area of the coarse root decreased;when the Mn was excessive,the proportion of the coarse root surface area of“Hanlv”increased,and the ratio of fine roots decreased,while“Huajun”is the opposite.In conclusion,the morphological parameters of the roots of both genotypes of Brassica rapa ssp.Chinensis L.were significantly inhibited under Mn deficiency and excessive-treatment,but have different response patterns in the root surface area distribution.?3?Under normal Mn nutrition condition,10?mol·L^-1Cd treatment have no significant effect on the photosynthetic pigment content of both genotypes of Brassica rapa ssp.Chinensis L.,and had a stimulatory effect on“Hanlv”photosynthetic parameters,but“Huajun”is an inhibitory effect.The change of photosynthetic pigment content show that under the condition of Cd stress,0¹0?mol·L^-1Mn have no effect on the photosynthetic pigment content of both genotypes of Brassica rapa ssp.Chinensis L.compared with Mn1treatment,but under the treatment of 10?mol·L^-1Mn,chlorophyll-a,chlorophyll-b,carotenoids,and chlorophyll?a+b?in leaves of“Huajun”were reduced by 15.0%²5.0%under the same conditions;chlorophyll-a,carotenoids,and chlorophyll?a+b?in the leaves of both genotypes of Brassica rapa ssp.Chinensis L.treated with 100?mol·L^-11 Mn,the drop ranged from 15.0%to 31.4%,respectively.The changes of photosynthetic parameters are as follows:under the condition of Cd stress,the net photosynthetic?Pn?,stomatal conductance?Gs?and transpiration rate?Tr?of both genotypes of Brassica rapa ssp.Chinensis L.leaves are treated with 0?mol·L^-1Mn compared with Mn1 treatment.The decrease of the photosynthetic rate is 12.0%²9.4%;the photosynthetic rate parameters Gs,Tr and Ci of both genotypes of Brassica rapa ssp.Chinensis L.leaves decreased by 18.5%⁵5.6%under100?mol·L^-1Mn treatment,and the change trends of Pn,Gs,and Ci are basically the same.In conclusion,under Cd stress,Mn-lackness and Mn-excessive treatments all have an inhibitory effect on photosynthesis of leaves.Among them,Mn-excess treatment had a more significant effect;under Mn-excess,photosynthetic pigment content in both genotypes of Brassica rapa ssp.Chinensis L.leaves decreased significantly,and gas exchange parameters decreased.Both also decreased significantly,indicating that the decrease in photosynthetic rate caused by Mn deficiency and excess is mainly caused by the pore limitation factor.?4?Under normal Mn nutrition condition(1?mol·L^-1Mn),10?mol·L^-1Cd treatment resulted in the decrease of Pro,NO₂^--N and NH₄⁺-N contents,NR and GOGAT activities in leaves of“Hanlv”,Pr content and GS and GDH activity increased;for“Huajun”,the content of NH₄⁺-N and Pr and the activity of NR decreased,and the activity of GS,GOGAT,and GDH increased.When treated with 0?mol·L^-1Mn,the contents of NH₄⁺-N,NO₂^--N and NO₃^--N in leaves of both genotypes of Brassica rapa ssp.Chinensis L.decreased by 18.8%to66.7%,and the Pro content in“Hanlv”leaves increased by 20.3%.The activity of NR and GDH were reduced by 59.8%and 25.8%,respectively.Under Mn0.1 treatment,the activity of GS and GDH in“Hanlv”increased by 47.3%and 41.9%,respectively,and the activity of GOGAT decreased by 26.7%.When treated with 10?mol·L^-1Mn,the activities of NR and GOGAT in leaves of“Hanlv”decreased by 23.4%and 19.8%;under the treatment of 100?mol·L^-1Mn,the NR activity in leaves of both genotypes of Brassica rapa ssp.Chinensis L.decreased significantly,and decreased significantly in leaves of“Hanlv”.The activity of GOGAT and GDH are significantly reduced,and GDH activity is increased in leaves of“Huajun”.In general,the effects of different concentrations of Mn on the main pathways of nitrogen metabolism in both genotype Brassica rapa ssp.Chinensis L.are different under Cd stress.Treatment with 10?mol·L^-1Mn could promote the conversion of NH₄⁺-N through GDH pathway in“Hanlv”,and 0.1?mol·L^-1Mn treatment could inhibit the“Huajun”transformation of NH₄⁺-N via the GDH pathway.The absence of Mn and Mn could inhibit the NO₃^--N conversion.The degree of inhibitant is related to the degree of Mn stress.