Adaptive Response Of Soybean Plants And The Regulating Effect Of Na~+ Under Drought And Salt Stresses

Drought is the most important environmental factor that restricts plant growth,resulting in the reduction of crop yields.In arid and semi-arid regions,drought and salt stresses often occurs at the same time,exerting a serious impact on plants.However,some studies have found that in arid environments,mild soil salinity could have a positive effect on plant growth,but the relevant mechanism is not clear.In this study,pot grown soybean plants were used as experimental materials,and a series of soil salt(NaCl)stresses in combination with a series of drought stresses were applied to the plants.The growth condition,photosynthesis,water status,osmotic adjustment,and material and energy utilization in different plant groups were measured to analyze the effect and possible mechanism of soybean's absorption and accumulation of Na~+and other salt ions in response to drought under drought conditions.The results showed:(1)Compared with the control,the drought or salt stress alone,and the combined treatment of drought and salt stresses all resulted in the inhibition of the leaf photosynthesis and growth of the soybean seedlings and the decrease in the biomass accumulation.However,the plant height,biomass,leaf net photosynthetic rate(Pn)and PS II photochemical efficiency(Fv/Fm)of the plants treated with combined drought and salt stresses were significantly higher than those in plants with single drought treatments,whereby the above indices was higher in the plants treated with the combined treatment of drought and 100mmol·L~-11 and 150 mmol·L~-11 NaCl than in plants treated with a single drought treatment(p<0.05).This shows that in arid environments,a certain amount of NaCl in the soil can alleviate the adverse and damaging effects to their photosynthetic system in the stressed plants.(2)The leaf relative water content,water potential and osmotic potential of soybean seedlings treated only with drought stress,or salt stress alone,and the combined drought and salt stresses were all lower than those of the control.However,compared with the drought treatment,the osmotic potential of soybean leaves treated with combined drought and salt decreased more significantly,and the cell turgor pressure and relative water content of leaves were both higher,with a significant difference(P<0.05);The Na~+content in soybeans treated with the combined drought and salt was significantly higher than in those of soybeans subjected to drought treatment alone(P<0.05),and there was a significant correlation between the osmotic potential and Na~+content of soybeans(p<0.01).These results show that,under drought stress,the salinity in the soil can promote the accumulation of Na~+in soybean plants,thus effectively lowered the osmotic potential of the tissue and enhanced the driving force of water absorption,which is conducive to maintaining a certain cell turgor pressure and high relative leaf water content.(3)The contents of soluble sugar,proline and betaine in leaves of soybean seedlings subjected to different salt stresses were significantly higher than those in the control group(P<0.05).However,the contents of soluble salt,proline and betaine in leaves of soybean seedlings subjected to combined drought and salt stresses were lower than those in the drought stressed group.Further analysis showed that that the osmotic concentration and content of organic osmoticum in the leaves of soybeans with drought treatment were significantly higher than in plants treated with combined drought and salt(P<0.05),and the osmotic concentration and content of inorganic osmoticum were significantly lower than those of soybeans treated with combined drought and salt(P<0.05).This indicated that the salinity in soil can promote the accumulation of the osmoticum for the osmotic adjustment when they are suffered from drought stress,thus reducing the consumption of organic matter,which maybe beneficial to the accumulation of biomass in soybean seedlings.(4)The caloric value of soybean seedlings treated with drought stress alone,salt stress alone,and the combination of drought and salt stresses was significantly lower than that of the control(P<0.05),which indicates that the stresses supressed the dry matter accumulation in those plants.However,under drought conditions,the ash content and specific leaf area of soybean leaves treated with combined drought and50-150 mmol·L~-11 NaCl were significantly higher than those of soybean leaves subjected only to drought stress(P<0.05);thus,the construction costs of unit mass and unit area were both significantly lower than in soybeans subjected only to drought stress(P<0.05).It shows that the addition of appropriate amount of salinity in soil under drought conditions could enhance the accumulation of minerals in the soybeans and reducing the energy consumption of biosome construction,which is thereby beneficial to the accumulation organic matter and hence the growth of the plants.To sum up,the presence of a certain amout of NaCl in soil could promote the absorption and accumulation of inorganic ions under drought stress in plants.On the one hand,the inorganic ions such as Na~+can lower the osmotic potential of the tissue through participating in the osmotic adjustment of cells and enhance the driving force for water absorption in soybean plants so that the plants can maintain higher cell turgor and relative water content,thereby reducing the inhibition effect of drought on the photosynthesis and growth of the plants;On the other hand,the accumulation of inorganic ions such as Na~+could reduce the synthesis of organic substances for osmoregulation,provide more substances for the biosome construction,reflected ultimately in the increase in the biomass accumulation.