Effects Of Manganese Regulation On Physiology And Biochemistry Of Tea Plant Under Acidic Condition

Tea plant[Camellia sinensis（L.）O.Kuntze]is an important economic crop in China.The plant grows well in wide range of soil conditions.In recent years,soil acidification has become of serious concern in tea cultivation as it is a bottleneck problem hindering growth of the tea plant,safe production and economic development of tea industry.Soil acidification results in the activation of manganese in the soil.Tea is an acidophilic and manganese-rich crop,thus,can tolerate higher concentration of manganese than other crop varieties,indicating that it has manganese tolerance ability.Due to its multi-functional utilization,tea products are eaten directly.The manganese accumulated in tea is in turn deposited and accumulated in human body,thus harmful to human health.Therefore,it is necessary to explore the effects of Mn²⁺concentration on tea plant under different acidic conditions（soil pH）.In this study,the degree of soil acidification and the content of metal elements in several tea gardens in Yixing City,Jiangsu Province were investigated,In addition,different pH gradients and manganese concentrations were set up to explore the effects of Mn²⁺on growth,antioxidant enzyme activities,manganese accumulation in tea plants under different acidic conditions,The effects of manganese on the transport and accumulation of metal elements in different tissues of tea plants,and the expression of Mn-CDF family genes related to manganese transport were also measured.The purpose of this study was to provide a research basis for efficient cultivation and management of tea,evaluation of tea quality and safety and mechanism of manganese tolerance of tea.The main results are as follows:1.The soil of tea garden in Yixing city was slightly acidic,and the average value of pH was 4.4,which was significantly lower than that of the control natural soil（pH 5.0）.The soil pH of most tea gardens was in the range 4.5-5.5,which is the required pH for efficient production of quality organic tea garden.However,some tea plantations have higher soil acidicity,with pH lower than 4,thus,obvious soil acidification.The soil manganese content ranges from 617.5-2326 mg·kg^-1,which is positively correlated with soil pH（P<0.01）.There was no strong correlation between other metal elements and pH,however,there was also strong correlation between the element,aluminum and iron,potassium and magnesium,iron and potassium,magnesium,potassium and magnesium,copper and manganese.2.Moderate increase in acidity or increase in Mn²⁺concentration could promote the increase in manganese content in tea,but the increase varied among different treatment groups.With the extension of the treatment time,compared with the pH5 treatment group,the enhanced acidity promoted the transfer and accumulation of manganese to the shoot,especially in the young leaves,which was proportional to the manganese concentration.The manganese content in young leaves is the highest at pH 4 and manganese concentration of 1000μM,which reached 3707 mg·kg^-1.However,the strong acidic condition inhibited the absorption of manganese by tea,thus,decreasing the manganese content in tea.The transport and accumulation of manganese is related to the tissue-specific expression of Mn-CDF family genes.Under different treatment conditions,increasing manganese concentration significantly promoted the expression of MTP11.1 and MTP11.2 in roots,and the promoting effect was enhanced with the increase in acidity,and the expression of MTP9also increased.MTP9 and MTP10 in old leaves responded significantly to pH4 and 250μM manganese concentration,but there was no significant difference in MTP8.1 expression among different treatment groups.The enhancement of acidity promoted the expression of MTP8.1 in stems and young leaves,and the promoting effect was enhanced with the increase in manganese concentration.3.Moderate increase in Mn²⁺concentration promoted the growth of tea roots,and the effect was the most significant when the concentration of Mn²⁺was 5μM.Moderately enhanced acidity improved the manganese tolerance of tea plants,mainly in the activation of antioxidant system.Strong acidic conditions reduced the tolerance of tea plants to manganese,and aggravated the toxicity of manganese in tea.When pH was 5 and Mn²⁺concentration was 250μM,mild manganese toxicity was induced,which showed that the old leaves were yellowing and easy to fall off.Correspondingly,the root SOD activity reached the strongest at this manganese concentration,and then decreased sharply,while at the same manganese concentration of pH2.5,the plant showed serious manganese toxicity,leaf margin scorched and root browning.The sensitivity of POD and SOD activities of roots and leaves to acid manganese treatment was different.Under strong acid conditions,the root POD activity was significantly affected by acid manganese treatment,the activity changed greatly,and showed a decreasing trend,resulting in a decrease in the tolerance threshold of tea to manganese,indicating that tea plants were more vulnerable to manganese toxicity under acidified soil conditions.As the main part of manganese accumulation in tea plants,manganese concentration enhanced the enzyme activity to a certain extent.4.Under different pH conditions,the absorption and transport of metal elements by tea plants were significantly different under different manganese concentrations.Increased acidity or increasing manganese concentration could promote the absorption of potassium and copper by tea roots,but inhibit the absorption of iron to a certain extent.With the increase of acidity,the presence of manganese promoted the long-distance transport of aluminum and magnesium from roots to leaves,and the promoting effect was the strongest at pH4 and pH2.5,respectively.Under strong acidity,high concentration of manganese could promote the accumulation of copper in roots and inhibit the translocation of copper to shoot.In the determination of metal elements,the content of potassium in tea plant was the highest and the copper was the lowest.