Adaptation Of Dimorphic Seeds Of Suaeda Salse To Saline Environment

Responses of dimorphic seeds of two Suaeda salsa population to salineenvironment during seed development and germination from seeds collected in fieldor from plants grown with different concentrations of NaCl were investigated in thisstudy, in an attempt to understand the eco-physiological mechanism how halophytesadapt to saline environments.1. Ion accumulation and germination of dimorphic seeds fromSuaeda salsa (the inland population) grown with different concentra-tions of NaCl and nitrate nitrogenSeeds of the euhalophyte Suaeda salsa were harvested from S. salsa plants thathad been treated with1or500mM combined with0.5or5mM NO3--N for115daysin a glasshouse. Germination was evaluated under different concentrations of NaCland NO3--N. High salinity (500mM NaCl) and low NO3--N (0.5mM) tended toproduce heavy seeds. Either high salinity (500mM NaCl) or high nitrogen (5mMNO3--N) supply increased the brown seeds/black seeds ratio. The concentrations ofNa+, K+and Cl-in brown seeds were higher than those in black seeds regardless ofNaCl and NO3--N treatments during plant culture, but the opposite trend was shown inthe concentration of NO3-. In comparison with black seeds, lower NO3--N supply hadless positive effect on seed germination of brown seeds than that at higher NO3--Nsupply during growth of the plants. In comparison with black seeds, exogenousNO3--N during seed germination had less positive effect on seed germination ofbrown seeds. In conclusion, producing more brown seeds and heavy black or brownseeds at higher salinity should be an adaptation of S. salsa to saline environment,while S. salsa produces more black seeds which keep dormancy to regulate properabundance of S. salsa plants to allocate limited nitrogen resource in response tovariable environments. Nitrate provided exogenously or by mother plants to the produced black seeds rather than brown seeds may act as a signal molecule favouringthe germination of black seeds in S. salsa.2. The mechanism of ion accumulation in seeds of Suaeda salsa fromthe intertidal zoneThe results showed that Na+and K+content in embryos was less affected bysalinity compared to those in leaves and pericarps of S. salsa during plant growth.Salinity increased the relative quantity of SsSOS1and decreased the relative quantityof SsHKT1in leaves and pericarps, but the expression of the two genes was notaffected by salinity in embryo. The results showed that SsSOS1may play a role in Na+efflux in pericarps, while the gene SsHKT1may play a role in K+influx, thus tomaintain ion homeostasis in the embryos. Salinity increased the relative quantity ofSsSOS1and SsHKT1in the embryos of seeds harvested from plants grown with1or500mM NaCl, especially in1mM NaCl. The relation between the relative quantity ofSsSOS1and SsHKT1in the pericarps and embryos of maturing seeds and in theembryos of mature seeds treated with different concentrations of NaCl and the seedtolerance remains to be further evaluated.3. The protective role of seed coat in dimorphic seeds of Suaeda salsafrom saline inland soilThe component of organic substances from seed coats of black seeds was almostthe same as that in brown seeds. However, the content of organic substances fromseed coats of black seeds was higher than that in brown seeds.Waxiness was found inseed coats of S. salsa seeds. In comparison with brown seeds, the seed coats of blackseeds had higher ability in inhibiting Na+influx into and K+efflux from the embryoand water uptake. When the seed coat was destructed by CHCl3, these effects of seedcoats were reduced obviously. The germination rate of intact black seeds was slowerthan that of intact brown seeds. The germination rate in destructed black seedsincreased significantly compared to that of intact black seeds, but destroying seed coatwith CHCl3had no effect on the germination of brown seeds. This can be suggested that waxiness in seed coats, especially in black seeds may protect seeds from salineenvironment. The difference structure of seed coat in dimorphic seeds may be anadaptation to saline environment in S. salsa.4. Adaptations of seedlings from dimorphic seeds of Suaeda salsa tosaline environmentsWe investigated the salt tolerance of seedlings from dimorphic seeds, and theeffect of salinity on seedling growth, ion content and photosynthetic characteristics inleaves of plants from dimorphic seeds of Suaeda salsa L. in sand culture. Comparedto0mM NaCl,100and400mM NaCl did not inhibit the elongation of seedlingsfrom brown seeds, but the opposite trend was found for seedlings from black seeds.The elongation of seedlings from either brown seeds or black seeds markedlydecreased in800mM NaCl. Compared to0mM NaCl, shoot dry weight significantlyincreased in200mM NaCl, whereas600mM NaCl decreased shoot dry weight ofplants from dimorphic seeds. Lower salinity (200mM NaCl) had no effect on thenumber of the side branches along the main stem of plants from dimorphic seeds,while higher salinity (600mM NaCl) markedly decreased this number in plants fromdimorphic seeds, especially for plants from black seeds. Shoot dry weight and thenumber of side branches along the main stem of plants from brown seeds were muchhigher than those of plants from black seeds under salinity. High salinity (600mMNaCl) decreased the rate of photosynthetic oxygen evolution in leaves of plants fromdimorphic seeds. Salinity (200and600mM NaCl) markedly decreased concentrationsof chlorophyll a and chlorophyll b in leaves of plants from black seeds, while200mMNaCl had no adverse effect on these parameters in leaves of plants from brown seeds.Leaf Na+concentrations increased with increasing concentration of NaCl, whereasleaf K+concentrations decreased with increasing concentration of NaCl for dimorphicseeds. There was no significant difference in ion concentration, concentration ofchlorophyll and the rate of photosynthetic oxygen evolution in leaves of plants fromdimorphic seeds of S. salsa. The present results indicate that more branches may berelated to higher biomass of shoots for seedlings form brown seeds. Brown seeds may play a main role in seedling establishment in spring, while black seeds may play asustaining role in seedling establishment under unfavourable conditions.