Study On The Mechanism Of Pb Uptake And Accumalation,Physiological Response And Tolerance Of Sugarcane

Sugarcane industry is a pillar industry in Guangxi.The planting area and yield of sugarcane rank first in China.However,heavy metal pollution is one of the prominent environmental problems in Guangxi.The absorption and accumulation of heavy metals in sugarcane directly affect the quality of sugarcane,so the safety of sugarcane food has aroused great concern.In this research,the sugarcane "taiyou" was used as the experimental material,and the pot experiment was adopted to study the Pb absorption and accumulation characteristics of sugarcane and the influence of Pb on the growth of sugarcane under the different concentrations of Pb.The physiological response mechanism of sugarcane on Pb was explored from photosynthetic fluorescence characteristics and antioxidant system,and the tolerance and detoxification mechanism of sugarcane on Pb was revealed at the cellular and molecular level.The research provides a theoretical basis for the cultivation of sugarcane varieties with high tolerance and low accumulation,and it is of great significance to guide the safe production of sugarcane in heavy metal contaminated agricultural land.The main research results are as follows:(1)As high Pb tolerance plant,sugarcane is observed grown normally without obvious toxic symptoms under all Pb stress condition.Within the body of sugarcane,the accumulation of Pb is shown as:root>leaves>stems.The accumulation in sugarcane root system is apparently higher than its above-ground parts.The mobilities of Pb from root to stem and leaves are identified relatively weak,and Pb concentration in edible sugarcane stems is the lowest.The phenomenon of "low to promote,high to inhibit" of Pb on sugarcane is identified:below the stress level of 250.0 mg/kg,Pb promotes the growth of sugarcane;on the level of 1250 mg/kg and above,the grow-th of sugarcane is inhibited.Pb is fixed and remained in sugarcane roots and the transferring to above-ground organs is restricted,so the toxic effect of Pb to sugarcane is reduced and it is believed one of its tolerance mechanisms.(2)Low concentrations of Pb enhance the effects of photosynthesis and thus strengthen the growth of sugarcane.High levels of Pb inhibit the effects of photosynthesis.Rates of photosynthesis decline as the opening of stoma is reduced.With high level Pb stressed,the closing of stoma could be a protective mechanism of sugarcane to reduce Pb entering the body.When sugarcane is under Pb stress,its non-photochemical quenching is increased.Sugarcane increases the thermal dissipation to weaken Pb stress,which reduces the toxic effects of Pb on photosynthesis in cells;When the stress level of Pb is over 1250.0 mg/kg,the potential activities of PS ? reaction centre and the transport of electrons between PS ? and PSI are inhibited.As a result,the electron transport process is hindered,which affects photosynthesis and inhibits the growth of sugarcane.(3)Sugarcane can improve its defensive performance against Pb by regulating the activities of superoxide dismutase(SOD)?catalase(CAT)and peroxidase(POD).The activities of SOD?CAT and POD in root and leaves of sugarcane increase first and then decrease as Pb stress level increase.When the stress level of Pb is lower than 1250.0 mg/kg,SOD?CAT and POD activities in sugarcane increase obviously,which can effectively eliminated large amount of active oxygen and H2O2 caused by Pb stress,and reduce Pb damage to antioxidant system of sugarcane;However,when Pb stress concentration is 2500.0 mg/kg,SOD and CAT activities in sugarcane decrease significantly,which can not eliminate a large amount of reactive oxygen and H2O2,and sugarcane is obviously poisoned by oxidative stress.(4)Low levels of Pb enhance the assimilations of macro-elements such as K,Ca and Mg and also micro-elements such as Cu,Fe,Zn and Mn by sugarcane,while high concentrations of Pb inhibit the assimilation of these minerals by sugarcane.When sugarcane is stressed by low Pb concentration,the toxicity of Pb to sugarcane may be alleviated by increasing the concentration of cationic ions such as K and Ca in cells.However,under high Pb stress,Pb destroys the dynamic balance of mineral nutrients in sugarcane,causing insufficient or unbalanced absorption of mineral nutrients in sugarcane and inhibiting the growth of sugarcane.(5)The subcellular distribution of Pb in the root and leaves of sugarcane is as follows:cell wall>organelle>soluble.Most of Pb is deposited in the cell walls of sugarcane roots and leaves,and the chemical extraction forms of Pb in sugarcane roots and leaves are dominated by acetic acid extraction and hydrochloric acid extraction.The sugarcane reduces Pb damage to itself by binding Pb to cell wall and converting partially soluble Pb into low activity phosphate or oxalate,which is one of the main detoxification and tolerance mechanisms of sugarcane.(6)There are 32238 differential genes in sugarcane root,of which 10544 are up-regulated and 21694 are down-regulated.In mesophyll cells,there are 6263 differential genes,of which 2774 are up-regulated,3498 are down-regulated.Through the analysis of differential genes in sugarcane under Pb stress,it is found that 11 transport proteins,17 genes related to sulfur metabolism and 15 antioxidant enzyme genes are excessive expressions in sugarcane.The results show that sugarcane mainly detoxified and defended against Pb stress by activating transporter gene,antioxidant enzyme gene and sulfur metabolism related gene.