Ameliorating Effects Of Exogenous Calcium On The Photosynthetic Physiology Of Honeysuckle Under Salt Stress

Objective:Honeysuckle,as one of the indispensable medicinal plants in China,is widely utilized as tea beverage traditional,Chinese medicine for thousands of years due to its unique aroma and pharmacological activity.Early research reported that the root system of honeysuckle is densely developed and can be grown in saline-alkali land.Therefore,honeysuckle seems to be a promising plant for cultivation in saline land.The purpose of this work was to uncover the ameliorating effects of exogenous calcium on chlorophyll fluorescence,ion distribution and physiological parameters of honeysuckle under salt stress.On the basis of ensuring the yield and quality of honeysuckle,it provides a theoretical basis for expanding the cultivation of honeysuckle in saline-alkali land.Methods:Chl a,Chl b contents were estimated by the ethanol-acetone extraction method.Determination of MDA content in leaves by thiobarbituric acid method（TBA method）.The gas exchange parameters were determined by a portable photosynthesis system which includes photosynthetic rate（Pn）,transpiration rate（Tr）,stomatal conductance（Gs）,intercellular CO₂ concentration（Ci）.Chlorogenic acid and Luteoloside in honeysuckle leaves were estimated by high-performance liquid chromatography（HPLC）.Chlorophyll a fluorescence transient was conducted by a multifunctional plant efficiency analyzer（Handy PEA）,the extracted parameters then led to the calculation and deduction of a variety of new parameters according to JIP-test.The expression of LHCB coding gene Lj Cab and Rubisco large subunit coding gene rbc L was conducted with Real-time PCR system.Na⁺,K⁺,Ca²⁺,Mg²⁺contents were determined by ICP-OES.Data was sorted using Excel 2010,statistical analysis was performed by SPSS software,and Graphpad Prism software was used for mapping.Results:（1）The contents of DW,chlorogenic acid and luteoloside in honeysuckle were significantly decreased under salt stress.10 mmol·L^-1,20 mmol·L^-1 Ca Cl₂ increased the DW of honeysuckle treated with 50 mmol·L^-1 and 100 mmol·L^-1 Na Cl,of which 20mmol·L^-1 Ca Cl₂ was stronger.10 mmol·L^-1,20 mmol·L^-1 Ca Cl₂ increased the accumulation of chlorogenic acid in all treatments,and the positive effect of 20mmol·L^-1 Ca Cl₂ was more significant.（2）Na Cl does not necessarily have a negative impact on DW,Chl a content and MDA content of honeysuckle.100 mmol·L^-1 and 200mmol·L^-1 Na Cl reduced the DW,chlorophyll content and gas exchange parameters of honeysuckle,increased the MDA content,and increased the relative expression of Lj Cab and rbc L.Ca Cl₂ combination treatments increased the DW and gas exchange parameters of honeysuckle under mild to moderate salt stress,increased the content of Chl a of honeysuckle under moderate and severe salt stress,and alleviated the increase of MDA content under moderate and severe salt stress.Besides,Ca Cl₂ promoted overexpression of cab under almost all treatments.（3）Ca Cl₂treatments exhibited better performance against salt stress while the salt-treated plants showed a great P-step depression.Ca Cl₂ effectively maintained efficiencies beyond Q_A^-(φP₀,ψE₀,φE_0,andσR₀),reduced the specific energy fluxes per active PSII reaction center（ABS/RC,TR₀/RC,ET₀/RC,RE₀/RC）and promoted the phenomenological energy fluxes per cross-section（ABS/CS,TR₀/CS,ET₀/CS,RE₀/CS）.Ca Cl₂ application induced a greater Area,Sm,and RC/ABS while the N showed lower values.（4）Na⁺concentration was significantly increased under salt stress,K⁺,Ca²⁺concentration decreased,but Mg²⁺concentrations were not significantly affected by Na Cl.The ratios of K⁺/Na⁺,Ca²⁺/Na⁺,Mg²⁺/Na⁺have the similar trend for all treatments,which is inversely proportional to the Na Cl concentration.The addition of Ca Cl₂ significantly increased the ratio of K⁺/Na⁺,Ca²⁺/Na⁺,Mg²⁺/Na⁺.（5）Na⁺,Ca²⁺concentration and physiological parameters revealed significant correlations,but the relationships between the K⁺,Mg²⁺and physiological parameters were not remarkably related.The Na⁺concentration was negatively correlated with Ca²⁺,DW,Pn,Area,ψE₀,PI_ABS,PI_total,and positively related to MDA,N,ABS/RC,DI₀/RC,while both reached the significant level（P<0.05）.However,the opposite relationships were uncovered between the Ca²⁺concentration and these physiological parameters.Conclusion:Exogenous Ca²⁺mitigates the damage caused by ionic toxicity and osmotic stress on salt-stressed honeysuckle by promoting the selective absorption and upward transport of Na⁺,K⁺,Ca²⁺and Mg²⁺.At the same time,the application of Ca²⁺facilitated the electron transport of PSII（especially after Q_A^-）and PSII photochemistry activity.This protective role of calcium is further confirmed by the overexpression of rbc L and Lj Cab,a high level of Ci,Gs.