摘要：

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 (πx) 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 πx 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], πx of the oldest leaf increased dramatically, and the leaf died a day or two after πx increased by 200-300 kPa. Growth of the youngest leaf was independent of these changes. Simultaneous collections of πx from leaves of all ages showed that πx 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.

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