Cadmium Uptake And Distribution In Suaeda Salsa And Atriplex Triangularis And Their Modeling

The coastal and estuarine ecosystems in China are now facing increasing heavy metal pollution because metals discharge from various sources and ultimately accumulate in the coastal areas.In these areas,most plants can not survive due to high salinity.However,halophytes are most prominent in the coastal and estuarine ecosystems,and species like Suaeda salsa and Atriplex triangularis have their dominance in the particular areas.In recent years,halophytes are of special interest because these plants can be chosen for phytoremediation of heavy metal or be developed as seawater vegetables.Nevertheless,there exists a lack of knowledge about cadmium uptake and distribution in halophytes.The aim of this study was to evaluate the tolerance of S.salsa and A.triangularis to cadmium in relation of Cd subcellular distribution characteristics,and to distinguish and quantify the uptake of free and complexed cadmium with biotic ligand model at constant Cd²⁺ activity.1.Comparative Cd tolerance and subcellular distribution characteristics betweenSuaeda salsa and Atriplex triangularis and their modelingTolerance and accumulation of cadmium by halophytic species are important for phytoremediation and food safety of heavy metals contaminated saline soils.A hydroponic experiment was conducted to compare the growth response and the Cd and Na uptake,translocation and subcellular distribution of two halophytic species,Suaeda salsa and Atriplex triangularis,in order to improve the understanding of Cd accumulation and detoxification mechanisms at the subcellular level of these two types of halophytes.Both species showed decrease in root length,plant height and biomass under Cd stress.However,the growth response and Cd toxicity symptoms were quite different between both species,with S.salsa less severely affected by Cd stress than A.triangularis,implying that S.salsa had higher Cd tolerance than A.triangularis did.The capacity of Cd adsorption on root surface of two halophytes was also obviously different.Additionally,S.salsa had higher root Cd accumulation but a lower Cd translocation rate than A.triangularis.Cadmium tolerance was associated with the Cd accumulation in the roots and thus low transport to the shoots.The subcellular distribution of Cd was in the order of cell wall fraction>soluble fraction>organelle fraction.The cell wall and soluble fractions were the dominant storage compartments for Cd in the roots,stems and leaves.S.salsa could bind more Cd in the cell wall fractions than A.triangularis.Thus,immobilization of Cd by the cell walls of different organs appeared to be the main Cd detoxification mechanism at the subcellular level.Cadmium stress also influenced the content of Na in shoots and roots of two plants,interfering the normal Na homeostasis at the levels of organs and subcellular fractions.The use of a sigmoidal model in this study provided a promising method that could improve the understanding of Cd subcellular distribution and Cd toxicity in halophytes.2.Uptake and distribution of Cd by Suaeda salsa and Atriplex triangularis at constantCd²⁺ activity and their modelingNaCl salinity-enhanced Cd uptake by halophyte Suaeda salsa in soil experiment has been reported.However,it is difficult in soil-based experiments to determine whether this finding is due to increased rates of Cd²⁺ diffusion to plant roots or the direct uptake of complexes such as CdCl+.A controlled hydroponic experiment was designed to investigate the effects of free and complexed cadmium on Cd uptake,accumulation and translocation at constant Cd²⁺ activity and to model the uptake of Cd by S.salsa and A.triangularis with a 'biotic ligand model'.As expected,the activities of CdCl+,CdCl20 and CdSO4O increased with increasing Cl-and SO₄^2-concentrations respectively without significantly affecting the activity of the free ion Cd²⁺ and the ionic strength.The results showed that no significant differences were observed for root biomass and root physiological parameters in both halophytes.Higher Cl-concentrations in solution generally resulted in greater root Cd content but lower Cd translocation factor.The activity of Cd²⁺ alone could not fully explain the differences in Cd content between the Cl-treatments.Activities of Cd-Cl complexes showed significant correlations with the root Cd content.This hydroponic trial provided good evidence of uptake of CdCl+ complexes by halophytes.In addition,the use of BLM in this study also provided a good framework for explaining the competition by Cd²⁺ and CdCl+ for root sorption sites and the direct uptake of CdCl+.Nevertheless,the modelled reaction constants suggested that the efficiency of uptake of CdCl+ was lower than Cd²⁺.And it was also possible that more than one mechanism might influence Cd uptake.