Analyses To The Relations Between The Driving Force For Water Uptake And Ion Absorption In Maize Roots Under Salt Stress

The xylem pressure in response to a series of salt induced osmotic stress was monitored and calculated to yield the radial reflection coefficients of the roots in hydroponically cultured maize seedlings.The dynamic xylem pressure changes in relation to the water absorption was analysed accordingly.In contrast,the radial reflection coefficients of the roots were also investigated based on the ion concentrations of the root xylem sap and those of the bathing medium of the roots,measured with an atomic absorption spectrophotometer.Because the experiments with the conductivity method were carried out to evaluate the permeability of the cell membranes in both root and the leaves to get an overall estimation of the permeability features of the various types of cells in maize seedlings,in order to get an insight into the mechanisms of the selective absorption of salt and water in maize roots,and into theories of enhancing the resistance of the seedlings to salt stress.The results showed that the Na~+and K~+absorption both increased with the elevation of the salt concentration in the root ambient,and there was a tendency that the salt concentration difference between the xylem sap and the root bathing solution increased with the increased salt stress.At lower concentrations of salt stresses(25-75mmol/L),the K~+absorption into the xylem sap increased with the increase in the intensity of the salt stress,while at higher concentrations of salt stress(above 75mmol/L),K~+absorption into the xylem sap ceased to increase.The radial reflection coefficient of the root was between 0.4 and 0.7 according to the results based on the analysis of the ion concentrations in the xylem sap and in the root bathing solutions,no significant difference was seen for different salt stresses,showing that the permeability of the membrane was little affected by the salt concentrations.The absolute root xylem pressure in maize seedlings went down with the increase in the increase in salt stress.The fact that the salt stress or light was removed,the xylem pressure could recover or even went up to above the atmospheric pressure,showing the root pressure,and that the xylem pressure could go up to zero pressure even in high concentrations of salt solution indicated that the regulation of the xylem pressure in maize seedlings was extremely high,well above the salt induced high osmotic stresses.The radial reflection coefficient of the root based on the xylem pressure measurements and the osmotic potential of the salt stress was relatively smaller,being0.33-0.42,but this did not mean that the root had a high permeability to salt,but rather reflected the buffering effect of the cortical cells,because the parameters were obtained in a more normal status compared with the ion concentration of the xylem sap which was collected with a pressure chamber.This was also a reflection that the ultrafiltration ability of the root was highly effective,which could continuously prevent the massive salt from entering the xylem.On the other hand,this was an indication that the ultrafiltration in maize root was still follow the laws of thermodynamics.The increased salt concentration in xylem sap obtained with the pressure chamber was due to the ultrafiltration of the endodermis,allowing more water and less salt to enter the xylem,leaving more and more salt outside the endodermis,even when the reflection coefficient of the membrane was unchanged,because if the concentration of salt was increasing on one side of the membrane,there should be more salt penetrating the membrane,leading to a reflection coefficient significantly lower than 1.However,this did not mean an increase in the permeability of the membrane or the injury of the membrane.In like manner,the relative conductivity of the cell membrane in maize roots and leaves increased,and with the increase in the intensity of salt stress,resulting in the depolarization of the membrane potentials,this as well did not mean that the cell membrane was suffering from injury or damage,because the membrane potential was well within the normal physiological range.To sum up with all the results,it could be concluded that the maize roots could absorb water with rather small negative xylem pressures against the water potential gradient,and could highly regulate the xylem pressures when subjected to salt stress,thus being able to adapt to saline environment without the absorption of excessive amount of sodium ions.