Metabolic Response Of Brassica Rapa L.ssp. Chinensis L. Under Chromium Pollution Stress

In recent years,the pollution of chromium has become serious problem.In China,chromium is one of the major heavy metals which will contaminate soil,seriously inhibit the growth of plants,damage plant cells,and even cause plant death.In this study,bok choy was used to investigate the metabolic response mechanism under chromium stress.Bok choy(Brassica rapa L.ssp.chinensis L.)belongs to Cruciferae and is a typical representative of leafy vegetables.The nutrient solution culture was conducted to investigate physiological indices(Photosynthetic rate,chlorophyll a,chlorophyll b,carotenoids,SOD,POD,and CAT)of the leaves of bok choy under different concentrations of Cr(VI)stress at different time(0,7,14,21,28 days).In addition,GC-MS was used to explore the change of metabolites in leaves and roots.In the physical level: The results showed that low concentration of chromium(0.5 mg/L)promoted the growth of bok choy.Compared to the control,the height of plants which were growed in low concentration of chromium solution was higher and the size of leaves were larger.However,under the high concentration of chromium stress,a large amount of ROS can be produced in bok choy.The growth rate,photosynthetic rate,the concentration of chlorophyll a and chlorophyll b decreased.The results also showed that the content of carotenoids,the activities of SOD,POD and CAT increased within 7 days,which indicated that the defense system of bok choy was activated at the initial stage of chromium stress,and it had certain resistance to chromium damage.In the metabolic level: 145 metabolites in leaves and 101 metabolites in roots were detected in different chromium stress treatments according to GC-MS analysis,and these metabolites were organic acids,amino acids,sugars and others.There was no significant change of metabolites in bok choy(leaf and root)under the low concentration of chromium stress.But some metabolites had significant changes under the high concentration of chromium stress.Compared with the control sample,in leaves of bok choy,10 and 24 metabolites respond in 2 mg/L and 8 mg/L chromium stress treatments for 7 days remarkably changed,also 26 and 24 metabolites were changed at 14 days of chromium stress,respectively.The response of 14,18,17 and 18 metabolites in 2 mg/L,4 mg/L,8 mg/L and 10 mg/L chromium stress treatments for 10 days remarkably changed,respectively,in roots of bok choy.The results revealed that chromium stress caused an significant increase in citric acid,succinic acid andglucose,an significant decrease in malic acid,aconitic acid,isocitrate and fructose 6-phosphate,which are involved in TCA cycle and glycolysis.It also caused amino acids(aspartic acid,cysteine,arginine)and sugars(xylose,lyxose,fructose)increased dramatically.At the same time,higher chromium stress induced more organic acids accumulation,which can alleviate the damage to plants under stress.Among them,the most remarkable metabolite was citric acid,which increased up to 13.68 times.Chromium stress caused systems alterations in metabolic networks including TCA cycle,GS/GOGAT cycle,glycolysis,alanine,aspartate and glutamate metabolism,starch and sucrose metabolism.It affects the anabolism of organic acid,amino acid and sugar,and thus affects the growth and development of bok choy.