Adaptation Strategies In Dormancy And Germination Of Suaeda Salsa Seeds To Saline Habitat

In the process of long-term evolution,plants have developed a series of life strategies to cope with the unpredictable environments.In the study,an annual plant with dimorphic seeds,Suaeda salsa inhabiting in Yuncheng Saline Lake was taken as a case,and the combination methods of field investigations and laboratory experiments were used to examine that,(1)the differences in fruit-set patterns,morphology and biochemistry and physiology of dimorphic seeds;(2)responses of dimorphic seed germination to temperature,light and salinity and dormant characteristics;(3)the different influences of mixed salt((extract of salt soil)and NaCl on seed germination;(4)the seasonal dynamics of seed bank.The questions were solved to reveal the adaptive strategies of Suaeda salsa seeds in saline habitats.The results showed as follows:Firstly,Suaeda salsa produced two types of seeds.For the whole plant,the number of brown seed was larger than black seed,and most of brown seeds were yielded at the upper part of the plant,and black seeds mainly occurred at the lower part of the plant.Black seeds were small and testa was keratose and glossy,in contrast,brown seeds were big and testa was membranous and lusterless;POD activity in brown seeds was higher than black seeds,but the difference was not significant(P>0.05),activity of SOD and soluble content were slightly higher than those of black seeds,but the differences were not significant(P>0.05),respectively.However,the content of MDA and soluble protein of black seeds were significantly higher than those of brown seeds(P<0.05),respectively.It is that the differences in fruit-set patterns,morphological and physiological and biochemical characteristics of dimorphic seeds to enhance the adaptability to saline habitation.Secondly,the effects of different factors on seed germination of dimorphic seeds showed that:(1)Germinations of freshly matured brown seeds were more than 90% across temperature regimes,but germination of black seeds was only the highest at 15/25 °C(less than 85%),and germination rate of brown seeds was higher than that of black seeds.Alternating of light and darkness or full darkness had no significant impacts on germination of brown seeds under low salt conditions(<0.39 mol/L).However,under high salt conditions(> 0.39 mol/L),germination of black seeds under light was significantly higher than under darkness(P<0.05).Responses of dimorphic seeds to saline levels were different,and germination of brown seeds was more than 30% in 0.78 mol/L NaCl solution,but black seeds did not germinate beyond 0.59 mol/L.(3)Seed coat scarification,dry storage,cold stratification and GA3 significantly promoted germination of black seeds.And thus,black seeds had non-deep physiological dormancy,but brown seeds were non-dormant.Our study showed that differences in dormancy and germination behavior of dimorphic seeds in S.salsa,suggesting that bet-hedging strategy was adopted to increase the adaptability to the heterogeneous habitats.Thirdly,the experimental results of the effects of mixed salt and NaCl on germination of black seeds showed that:(1)Seeds of S.salsa germinated at the wide range of 5-30 oC,and the optimal temperature was 15/25 oC.(2)With the increase of salt concentration,germination percentage(GP)decreased and rate(GR)increased and then decreased,regardless of salt types.For GP and GR,there was no difference between mixed salt and single salt(NaCl)when salt concentration was less than 20 mS/cm;Beyond 20 mS/cm,GP and GR in NaCl solution was lower than that in mixed salt solution at the same of salt concentration,respectively.Recovery germination transferred from NaCl to distilled water was markedly higher than that transferred from mixed salt.(3)The Na+ content of the imbibed seeds decreased with increasing salt concentration regardless of salt types.However,when salt concentration was higher than 5 mS/cm,Na+ content of seeds was higher in NaCl solution than that in mixed salt solution at the same of salt concentration;K+ content of seeds did not show significant change with the increase of salt concentration,and there was no difference between the types of salt at the same of salt concentrations(P>0.05);At a given salt concentration,Ca2+ and Mg2+ contents in the seeds treated with mixed salt were higher than with NaCl,respectively.(4)With the increasing salt concentration,imbibition rate of seeds decreased regardless of salt types,and imbibition rate of seeds treated with NaCl was lower than in mixed salt at the same concentration.Our results suggested that,both NaCl and mixed salt do not cause specific toxicity on seed germination and seeds of Suaeda salsa inhibit Na+ absorption but increase Ca2+ and Mg2+ concentrations in seeds to alleviate specific toxicity and adjust the osmotic potential to adapt to the saline environments.Finally,investigation of the seasonal dynamics of seed banks showed that: during the observation period from October 2015 to August 2016,the seed bank density was biggest in March of 2016(9166.7 ± 227.3 seeds /m2),but the density of October 2015 and August 2016 were low(1166.7±112.8 and 1853.3±135.2 seeds / m2);the density of seedling bank in May 2016 was largest(900.0±220.6 plants/ m2),while the density was lesser on the April and August 2016(213.3±156.7and 273.3±220.6 plants/ m2).Seed dormancy showed the seasonal patterns: conditional dormancy in seed freshly mature-non dormancy in late winter and spring-secondary dormancy in summer and autumn-non dormancy in late winter and spring.This pattern might be regulated by seasonal changes of temperature.And thus,seed dormancy cycle of S.salsa ensure germination or seedling emergence to spread risk in time and increased the adaptability to the unpredictable environment.Case study of adaptive strategies of S.salsa with dimorphic seeds for saline habitation showed,there were great differences in morphology,biochemistry and physiology,and responses of germination to light,temperature and salinity,and the species might cope with the unpredictably saline environments and maintain the persistence and regeneration of the populations by the combined strategies of differentiation of dormancy and salt-tolerant levels in dimorphic seeds,selective absorption of ions in the process of germination,and seed dormancy cycle in soil seed bank.