
%0 Journal Article
%A Munns, R 
%A Passioura, JB 
%T Effect of Prolonged Exposure to Nacl on the Osmotic Pressure of Leaf Xylem Sap From Intact, Transpiring Barley Plants
%J Functional Plant Biology
%D 1984/12/01
%V 11
%N 6
%P 497-507
%K 
%R https://doi.org/10.1071/PP9840497
%U https://www.publish.csiro.au/paper/PP9840497 
%X Xylem sap was collected from the leaf tip of intact transpiring barley plants at low and high external [NaCI], by applying pressure to roots of plants growing in sand containing nutrient solution plus 0, 100, 150 or 200 mol m-3 NaCl. Transpiration rates during the collection period were manipulated by varying the light and humidity level around the shoot. The osmotic pressure of the xylem sap (&#960;x) from the oldest leaf of plants without added NaCl increased hyperbolically with decreasing transpiration rate, and rose to values 2-3 times that of the nutrient solution at very low rates of transpiration. A similar pattern applied to plants exposed to high external [NaCl] for several days, except that &#960;x was about 30% higher than that of plants without NaCl, for a given rate of transpiration. However, after a week or more at high external [NaCl], &#960;x of the oldest leaf increased dramatically, and the leaf died a day or two after &#960;x increased by 200-300 kPa. Growth of the youngest leaf was independent of these changes. Simultaneous collections of &#960;x from leaves of all ages showed that &#960;x was lowest in the youngest leaf. We suggest that ions in the transpiration stream build up in the cell walls when ion contents of the cells reach a maximum, and net uptake rates become zero. Such an accumulation would cause loss of turgor then dehydration of cells, and may be the primary cause of death of the old leaves. 
