The Adaptation Of Suaeda Physophora Seedlings To Salinity And Drought Environments

For the purpose of understanding the adaptation of Suaeda physophora to salinityand drought environments during germination and seedling stages, the effect ofcombined salinity and drought on seed germination and seedling growth wasinvestigated. In addition, the role of cotyledons in the establishment of S. physophoraseedlings was investigated. The results were as follows:1. The effect of combined salinity and drought on seed germinationSeeds without any treatment (as control), and seeds pretreated with700mmol·L-1NaCl and then desiccated at room temperature germinated in0mmol·L-1NaCl for8h.The germination of seeds pretreated with700mmol·L-1NaCl was65%, while thevalue was only10%in control. The water absortion rate of seeds pretreated with700mmol·L-1NaCl was higher than that in control. This indicated that seeds pretreatedwith700mmol·L-1NaCl and then desiccated can germinate as rapid as possible whenthe environment was suitable to ensure seedling establishment. There was nodifference in the relative conductivity, contents of MDA, chlorophyll, free radical O2·-and H2O2, activities of SOD and CAT between seeds pretreated with700mmol·L-1NaCl and the control. This indicated that seeds of S. physophora can maintain seedvigor when they are subjected to combined salinity and drought stress.2. The effect of combined salinity and drought on seedling survivalduring early seedling stageSeedling survival of seedlings pretreated with300mmol·L-1NaCl and thendesiccated at room temperature was lower than those pretreated with0mmol·L-1NaCl. MDA content in young seedlings pretreated with either0or300mmol·L-1NaCl and then desiccated at room temperature was higher than seedlings from germinated seeds in0or300mmol·L-1NaCl without drought treatment, respectively.However, there was no difference in relative conductivity and chlorophyll contentbetween seedlings pretreated with0or300mmol·L-1NaCl and then desiccated andseedlings from germinated seeds in0or300mmol·L-1NaCl without droughttreatment. It can be suggested that young seedlings of S. physophora have a highability to tolerate combined salinity and drought stress.3. The tolerance of seedlings to combined salinity and drought stressDrought stress increased leaf Na+and Cl－content, especially at400mmol·L-1NaCl pretreatment, while drought treatment had no effect on leaf K+content afterseedlings were pretreated with either1or400mmol·L-1NaCl. Shoot dry weight ofseedlings decreased during drought treatment compared to control (without droughttreatment) regardless the concentrations of salinity after seedlings were rewatered.Drought treatment had less adverse effect on the seedling and leaf survival inseedlings pretreated at400mmol·L-1NaCl than at1mmol·L-1NaCl after seedlingswere rewatered. The present results indicate that seedlings of S. physophora pretreatedwith salinity can increase seedling survival, and this can help them to adapt tocombined salinity and drought stress in arid areas.4. The role of cotyledons in the establishment of seedlingsThe role of cotyledons in seedling establishment of the euhalophyte S. physophoraunder non-saline and saline conditions (addition of1mmol·L-1or400mmol·L-1NaCl)was investigated. Survival and fresh and dry weights were greater for seedlings grownin the light (12-h light/12-h dark) than in the dark (24-h dark). The shading ofcotyledons tended to decrease shoot height, shoot organic dry weight, number ofleaves, and survival of seedlings regardless of NaCl concentration, but the effect ofcotyledon shading was greater with400mmol·L-1NaCl. Concentrations of Na+werehigher in cotyledons than in leaves, regardless of NaCl treatment. The K+/Na+ratiowas lower in cotyledons than in leaves for seedlings treated with1mmol·L-1NaCl butnot for seedlings treated with400mmol·L-1NaCl. Addition of400mmol·L-1NaCl decreased oxygen production in cotyledons but especially in leaves. These results areconsistent with the hypothesis that, by generating oxygen via photosynthesis and bycompartmentalizing Na+, cotyledons are crucial for the establishment of S.physophora seedlings in saline environments.