| In this study,we conducted pot experiments using quartz sands to study the effects of salt stress on the growth,physiology and secondary metabolism of Schizonepeta tenuifolia Briq.Sodium chloride was used to simulate salt stress and there were total five groups of treatments in the study,including the control group(0),mild treatment group(25 mmol·L-1),moderate treatment group(50 mmol·L-1)and severe treatment groups(75 and 100 mmol·L-1).For S.tenuifolia seedlings,changes of the growth,membrane stability,osmotic adjustment,antioxidant enzyme system,ion absorption and distribution,antioxidant capacity under salt treatments were studied.For flowering S.tenuifolia plants,biomass of aerial parts,ion absorption and distribution,antioxidant capacity,essential oil yield of aerial parts and components under salt treatments were studied.The results obtained were as follows.1 Significant changes of growth and physiological characteristics of S.tenuifolia seedlings exposed to salt stress were observed.As salt concentration increased,salt injury index of S.tenuifolia seedlings rose gradually,increasing rate of plant height and specific leaf area decreased,but the dry weight and chlorophyll content increased initially and then reduced,in which both indexes were the highest at 25 mmol·L-1 NaCl.Malondialdehyde(MDA)content and electrolyte leakage percentage(ELP)was induced to increase significantly by salt treatment.Contents of soluble protein and proline increased and then decreased as concentration rose further,in which the both were the highest at 50 and 75 mmol·L-1,respectively,while a continuously rising trend in soluble sugar content was shown;the activities of superoxide dismutase(SOD)and catalase(CAT)were enhanced at 25 mmol·L-1 NaCl but then inhibited at higher concentrations,while that of peroxidase(POD)declined continuously.Under salt stress,accumulation of Na+in root,stem and leaf was enhanced,especially in the aerial parts,but K+and Ca2+content,values of K+/Na+,Ca2+/Na+decreased remarkably.In summary,S.tenuifolia seedlings were sensitive to salt stress,but could be tolerant of mild salt stress(25 mmol·L-1 NaCl).2 Significant changes of antioxidant capacity,glandular trichome density,and volatile exudates of S.tenuifolia seedlings exposed to salt stress were observed.Contents of antioxidants,including phenolics and flavonoids,increased at low(25 mmol·L-1)or moderate(50 mmol·L-1)levels,but declined at severe(75 and 100 mmol·L-1)levels.On leaf surfaces,peltate and capitate glandular trichomes(GTs)were found.Salt treatments,especially the moderate and severe treatments,enhanced the density of total GTs on both leaf sides.The most abundant compound in GT volatile exudates was pulegone.Under salinity,relative contents of this component and other monoterpenes decreased significantly.While the biosynthesis and accumulation of esters were enhanced,particularly sulfurous acid,2-ethylhexyl hexyl ester,which gradually became the second major compound as salinity increased.3 The aerial biomass yield,ion absorption and distribution of flowering S.tenuifolia plants was significantly influced by salt stress.The aerial biomass yield was not affected by mild(25 mmol·L-1 NaCl)and moderate(50 mmol·L-1 NaCl)salt stress but significantly reduced by severe(75 and 100 mmol·L-1 NaCl)treatments.As NaCl concentration increased,Na+content of root,stem,leaf and flower increased significantly,and that of the aerial parts was in a higher level than the root.Regarding the K+ content,it decreased in the root but increased in stem,leaf and flower.Some changes were detected in the Ca2+ content,but not significant on the whole.The value of K+/Na+and Ca2+/Na+reduced as a result of increasing NaCl concentrations.4 Salt stress had obvious effects on the antioxidant capacity,essential oil yield and components on flowering S.tenuifolia plants.The biosynthesis of antioxidants which were polyphenols and flavonoids was enhanced by mild and moderate stress,and contents of the two compounds in leaf and flower parts were higher than those in stem.Under severe stress,contents of polyphenols and flavonoids in leaf and flower were significantly lower than those in the control treatments and the decrease in the two parts was more obvious than that in stem.In the antioxidant capacity test,changes of DPPH scavenging capacity of stem were insignificant under salt stress.No significant changes were observed in the DPPH scavenging capacity of leaf under mild and moderate salt treatment,but that of flower reached the highest value under mild salt treatment.As salt concentration increased,the DPPH scavenging capacity of leaf and flower was obviously inhibited.The ABTS·+ radical scavenging capacity of aerial parts were enhanced initially and then inhibited,reaching the highest value at moderate salt stress.The correlation analysis result revealed a notably positive correlation between the contents of antioxidants and antioxidant capacity.The content of essential oil increased under medium salt treatment(50 mmol·L-1 NaCl).However,the synthesis and accumulation of essential oil was inhibited by the serious salt treatment(100 mmol·L-1 NaCl).Over 98%of the essential oil components were terpenes,in which pulegone and menthone were the most two abundant compounds.Varieties of essential oil components did not change significantly under salt stress but their relative proportions did.The transformation of pulegone to menthone was enhanced and the value of pulegone/menthone based on their relative contents decreased with NaCl concentration increasing.Consequently,menthone ranked the most abundant compound by replacing pulegone.Relative content of d-limonene increased under medium and serious salt stress,and that of β-caryophyllene only increased under mild treatments. |
