Effects Of NaCl Stress On Physiological And Biochemical Characteristics Of Two Hibiscus Cultivars

Salt stress is one of the important factors affecting plant growth,and it is important to clarify the salt-tolerant mechanism of plants for the breeding of salt-tolerant varieties.Hibiscus Linn.plants are excellent garden tree species,which are highly adaptable,large in flower volume,and rich in flower colors and types.Among them,hibiscus（Hibiscus syriacus Linn.）and other excellent varieties have strong salt-tolerant ability and can be used as saline-alkali land.In this study,in order to explore the response characteristics of the photosynthetic characteristics of hibiscus to salt stress,two cultivars of hibiscus,H.syriacus’Duede Brabaul’and H.syriacus’Blueberry Smoothie’were used and treated with a setting of different concentrations of NaCl(0,50,100,150,200 mmol·L^-1),and the photosynthesis,osmotic adjustment substances,antioxidant enzyme activity,ion content and other parameters of hibiscus leaf were analized and discussed.The salt tolerance mechanism of hibiscus will provide a theoretical basis for improving the salt tolerance of hibiscus.The main research results are as follows:（1）Under 200 mmol·L^-1NaCl stress,both hibiscus plants produced stress phenotypes,with the leaves of losing green and falling off.The etiolation symptom of’Duede Brabaul’spread from petiole to vein and gradually expanded to leaf margin.While’Blueberry Smoothie’showed shriveled leaves with greenish patches and scorched leaf margins.The old leaves of both of them had fallen off obviously.In terms of phenotype,the growth of’Blueberry Smoothie’was severely inhibited under 150 mmol·L^-1NaCl stress,while the growth of’Duede Brabaul’was better and had stronger salt tolerance.（2）Under different concentration of NaCl stress,both of the hibiscus cultivar plants showed varied degrees of photosynthesis inhibition.The photosynthetic carbon assimilation process was restrained,and the content of chlorophyll（Chl a+b）,the maximum net photosynthetic rate(P_nmax),the largest PSII photochemical efficiency（Fv/Fm）,the actual photochemical efficiency(Φ_PSII)and electron transport rate（ETR）were significantly reduced.The inhibitory effect of NaCl treatment on P_nmax,Fv/Fm andΦ_PSIIin’Blueberry Smoothie’was significantly greater than that in’Duede Brabaul’.These results indicated that’Duede Brabaul’has higher salt tolerance than’Blueberry Smoothie’,and’Duede Brabaul’can maintain high photochemical efficiency,effectively utilize the captured light energy and reduce the generation of excess excitation energy under NaCl treatment.（3）Under different concentrations of NaCl stress,the content of MDA and relative conductivity of membrane lipid peroxidation products of two hibiscus cultivars’leaves were significantly increased,and the content of MDA and relative conductivity of’Blueberry Smoothie’were significantly higher than that of’Duede Brabaul’,indicating that’Duede Brabaul’had stronger salt tolerance.In order to grow normally under adverse conditions,the contents of organic osmotic substances such as proline,soluble sugar and soluble protein in both of the hibiscus cultivar plants increased significantly,indicating that both hibiscus plants had certain adaptability to NaCl stress.With the increase of NaCl concentration,the contents of Na⁺and Cl^-and Na⁺/K⁺of the two hibiscus cultivars’leaves showed a significant upward trend,while the content of K⁺showed a significant downward trend.However,the content of Na⁺and the value of Na⁺/K⁺in the leaves of’Duede Brabaul’were significantly lower than that of’Blueberry Smoothie’.These results showed that,compared with’Blueberry Smoothie’,’Duede Brabaul’is more conducive to maintaining the osmotic potential of cells,maintaining the absorption of water,alleviating the impact of stress on normal growth of plants,and having better salt tolerance.（4）With the increase of NaCl concentration,the activities of superoxide dismutase（SOD）,peroxidase（POD）,ascorbate peroxidase（APX）and catalase（CAT）in the leaves of both hibiscus cultivars showed a trend of first increasing and then decreasing.The activity of SOD,CAT and APX in the leaves of’Duede Brabaul’were significantly higher than those of’Blueberry Smoothie’,while the activity of POD was lower than that of’Blueberry Smoothie’under 50-150 mmol·L^-1NaCl concentration.Under the concentration of 200 mmol·L^-1NaCl,the antioxidant enzyme activities of’Duede Brabaul’and’Blueberry Smoothie’had little difference.These results showed that both of the hibiscus can improve the activity of antioxidant enzymes to effectively eliminate excessive reactive oxygen species（ROS）,and avoid the damage of ROS to light system and a variety of enzymes in the process of photosynthetic carbon assimilation,and therefore protected the photosynthetic mechanism and provided conditions to maintain higher photosynthetic capacity.Nevertheless,’Duede Brabaul’has stronger antioxidant stress ability and better resistance to salt stress compared with’Blueberry Smoothie’.