Adaptive responses of eremophyte Pugionium cornutum seedlings to different concentrations of NaCl
To investigate the adaptive mechanisms of Pugionium cornutum seedlings at different NaCl concentrations,5-week-old seedlings were treated with a series of external NaCl concentrations (5,25,50,100 and 200 mmol/L).The seedlings exhibited different adaptive mechanisms at different NaCl concentrations.At low salt concentrations (5 mmol/L NaCl),leaf dry weight and water content significantly increased by 25％ and 35％ respectively compared to the control (P＜0.05).The adaptation of P.cornutum to low salt concentrations was mainly due to the rapid growth and increased water content of the plant,which could dilute the excessively absorbed Na+ to below toxic levels.At moderate salt concentrations (50 mmol/L NaCl),a strong controlled K+ and promoted Na+ transportation from root to leaf was observed,and leaf Na+ concentration was 8-fold higher than that in the control plants (P＜0.05).This adaptation might be explained by the plant's ability to compartmentalize excess Na+ into the vacuole in leaf cells,which could protect leaves from Na+ toxicity and produce a 75％ decrease in leaf osmotic potential (P＜0.05).Therefore,the ability of plants to resist the osmotic stress induced by NaCl was enhanced and normal plant growth could be maintained.The adaptation of P.cornutum to high concentrations of salt (200 mmol/L NaCl) was caused by a strong controlled Na+ and promoted K + transportation from the outside into the plant and then from root to leaf.This resulted in a 55 ％ increase in leaf K+ concentration under the high-concentration treatment compared to the 50 mmol/L treatment (P＜0.05),with a corresponding increase from 10.28 ％ to 12.51％ in the contribution of K+ to osmotic potential.This mechanism could help to maintain a low Na+/K+ within the cytoplasm,reduce Na+ toxicity to cells,balance the osmotic pressure from vacuole and apoplast to cytoplasm,and reduce osmotic stress on cells due to the primary location of K+ mainly in the cytoplasm.However,leaf dry weight significantly decreased by 51％at 200 mmol/L NaCl grown for 7 d compared to control (P＜0.05),indicating that P.cornutum seedling has limited salt tolerance.
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