| In recent years,heavy metal pollution in farmland soils is becoming more and more serious.Chromium is one of the important pollutants,which seriously affects the growth and development of crops,endangering the quality and safety of agricultural products as well.Studies have shown that the resistance obtained by primary crops under stress induction can be passed on to offspring,regulating the response of offspring to environmental stress.Moreover,the composition and content of organic acid metabolites can reflect the growth and metabolism of crops.Therefore,studying the effect of passaging memory of organic acid metabolism in crops is of great significance for studying the metabolic response mechanism of crops under heavy metal stress,and for improving and breeding new stress-resistant varieties.The study started from the metabolic point of view,using the Brassica rapa L.ssp.chinensis L.as experimental materials to study the metabolic response of organic acids and the intergenerational memory effect of the response under chromium stress.The main research contents,methods and results are as follows:1.Physiological response of the Brassica rapa L.ssp.chinensis L.under chromium stressA 7-day chromium stress and a two-week stress relief period were set up to propose a short-term chromium stress and short-term recovery culture environment.The physiological indexes such as total chromium,net photosynthetic rate,photosynthetic pigment content and resistance oxidase activity of the Brassica rapa L.ssp.chinensis L.were determined,in order to explain the corresponding changes of organic acids from the perspective of physiological indicators,and establish a link between physiological response and metabolic response.The results showed that0.5 mg/L chromium treatment had promoted the growth of the Brassicarapa L.ssp.chinensis L.,with the leaves growing well;while the high concentration of chromium treatment inhibited the growth of the Brassica rapa L.ssp.chinensis L.,with the leaves sallowing.When the stress was relieved,the Brassica rapa L.ssp.chinensis L.could slowly return to normal growth level.The activities of oxide enzyme(POD)and superoxide dismutase(SOD)changed significantly during chromium stress and stress release,which played important roles in resisting high concentration of chromium stress.The same as net photosynthetic rate,they can be used to characterize the chromium stress and to be important indicators of the extent of growth recovery of the Brassica rapa L.ssp.chinensis L..2.Response of organic acid metabolism of the Brassica rapa L.ssp.chinensis L.to chromium stressThe changes of organic acid content in leaves of Brassica rapa L.ssp.chinensis L.and the key pathway of organic acid metabolism was determined during the 7-day stress period and the relief period to transfer the study from contemporary plants exposed to chromium stress--chromium stress relief to primary plants exposed to chromium stress--progeny plants not subjected to the stress,being the basis for the research on passive memory effect of the Brassica rapa L.ssp.chinensis L.under the chrome pollution stress.The results showed that low concentration of chromium stress had no significant effect on organic acid metabolites.High concentration of chromium stress induced the increase of succinic acid and citric acid content,oxalic acid and malic acid content decreased,citric acid and oxalic acid changed more significantly;oxalic acid,succinic acid and citric acid is suitable to be an organic acid metabolite to characterize the degree of chromium stress.Oxalic acid and succinic acid can characterize the degree of stress-relieving the recovery of the Brassica rapa L.ssp.chinensis L..The key pathway for organic acid metabolism is the TCA cycle,in which citrate synthase,isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are the key metabolic enzymes.3.Passage memory effect of organic acid metabolism in the Brassica rapa L.ssp.chinensis L.under chromium stressIn the experiment,the differences in phenotypic and organic acid content between the offspring(referred to as F1)and the original(referred to as F0)Brassica rapa L.ssp.chinensis L.were investigated under both chromium-free environment and chromium stress.The organic acid changes were explained by measuring the activity of key enzymes in TCA cycle metabolism to reveal its relationship with the metabolic process of TCA cycle,which is beneficial to study the molecular mechanism of the passage memory effects of organic acids.The results showed that the Brassica rapa L.ssp.chinensis L.treated with high concentration of chromium stress could produce high resistance to chromium in the untreated progeny.The F1 generation of the Brassica rapa L.ssp.chinensis L.showed low resistance to low concentration chromium and high resistance to high concentration chromium.F0 generation organic acid had stress response to Chromium and could produce passage memory.With the increasing of chromium stress concentration,the content of oxalic acid and malic acid in leaves and the accumulation of citric acid in the F0 and F1 generations were stable.Under high concentration of chromium stress,the Brassica rapa L.ssp.chinensis L.accelerated the metabolic process of TCA cycle by increasing the activity of CS and IDH,and then regulated the metabolic activities of related substances in this pathway to promote the accumulation of citric acid and resist chromium toxicity.The conclusions of this study are as follows: oxalic acid,succinic acid and citric acid can be used to characterize the degree of chromium stress in the Brassica rapa L.ssp.chinensis L.,and oxalic acid and succinic acid can be used to characterize the recovery of the Brassica rapa L.ssp.chinensis L.during stress relief.The content changes are related to POD and SOD activities.High concentration of chromium stress induces a significant increase in citric acid in the leaves of the stems,and a significant decrease in oxalic acid,and this change remaines stable for at least two generations.The increase of CS and IDH activity accelerates the TCA cycle and promotes the accumulation of citric acid in leaves so that the TCA cycle plays an important role in the resistance of C.chinensis to chromium stress.The study provides a quantitative parameter standard for chromium pollution stress in farmland,and also provides a theoretical basis for the excavation of excellent genes and the breeding of new varieties of resistance to the Brassica rapa L.ssp.chinensis L.,which has a certainreference for the related research on the evolution of agricultural products. |
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