Effect Of Exogenous Ca²⁺ On Salt Tolerance Of Malus Zumi Seedlings Under Salt Stress

Malus zumi is an important salt-tolerant wild fruit tree resource with a high survival rate in medium and heavy saline-alkali land.It is an excellent apple rootstock,and is used for the development and utilization of saline-alkali land,the greening of saline-alkali land,and the expansion of salt-tolerant alkalis.It is play important roles in orchards coastal defense afforestation and improving the ecological environment.In this study,the seedlings of M.zumi was used as potted materials,and 20 treatments were set up.The effects of different concentrations of Ca²⁺on the physiological and photosynthetic parameters were studied in M.zumi.under salt stress.This will provide the reference for the selection of the optimal Ca²⁺concentration for application in the saline environment in the future.The expression of Mz₂NHX₁ gene was explored in M.zumi in different treatments.It is will provides the refferences for exploring the function of Mz₂NHX₁ gene.As the aggravating of salt stress and the increasing of Ca²⁺concentration,the activities of superoxide dismutase（SOD）and peroxidase（POD）presented the trend of increasing firstly and decreasing secondly;and the activity of two kinds of enzymes reached the maximum when the concentration of salt was 100 mmol·L^-1and Ca²⁺was 10 mmol·L^-1.Soluble protein presented the trend of increasing firstly and decreasing secondly,as the different concentration of salt and Ca²⁺,the maximum value was reached when the Ca²⁺was 10 mmol·L^-1.Soluble sugar reached the maximum value when the Ca²⁺concentration was 5 mmol·L^-1 under high salt stress(300 mmol·L^-1).The application of Ca²⁺is beneficial to the accumulation of proline in M.zumi.The salt tolerance of M.zumi could be improved through the accumulation of a lot of osmoregulation substances.The content of malondialdehyde（MDA）with 20mmol·L^-1Ca²⁺treatment is only 17%of 5 mmol·L^-1 Ca²⁺treatment,under 300mmol·L^-1 salt.As the aggravating of salt stress and the increasing of Ca²⁺concentration,the activities of chlorophyll a and chlorophyll b presented the trend of increasing firstly and decreasing secondly,both of them presented the highest content with Ca²⁺15mmol·L^-1.Carotenoid content was significant in the high salt stress treatment group with a salt concentration of 300 mmol·L^-1.The net photosynthetic rate（Pn）value decreased significantly under high salt stress.The stomatal conductance（Gs）of M.zumi have a promoting effect when Ca²⁺was 10 mmol·L^-1.Salt stress could reduce transpiration rate（Tr）of M.zumi leaves.Excessive Ca²⁺uptake inhibits the use of CO₂,leading to a large accumulation of intercellular CO₂（Ci）.The expression characteristics of Mz₂NHX₁ gene in M.zumi under different concentrations of Na Cl stress were analyzed by real-time fluorescence quantitative PCR.The gene expression was increased by 11%compared with the control when the concentration of salt was 200 mmol·L^-1.In this study,Ca（NO₃）₂was watered to achieve the aim of applying different concentrations of Ca²⁺,at this time the change tendency of Mz₂NHX₁ gene expression increased rapidly and then decreased obviously.The gene expression reached a peak when Ca²⁺reached 15 mmol·L^-1.This indicates that the expression of Mz₂NHX₁ gene can be increasing when salt stress treatment and application of Ca²⁺,the activity of Mz₂NHX₁ protein was be increased,and reduces the harm caused by the stress.It is inferred that the salt tolerance of the seedlings of M.zumi can be increased when external application Ca²⁺and the expression of Mz₂NHX₁gene.