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Recent studies have shown that, in some species, xylem hydraulic conductivity ( K  h ) increases with increasing cation concentration of xylem sap. Evidence indicates that  K  h  increases as a result of the de-swelling of pit membrane pectins caused by cation neutralization of polygalacturonanes. We tested whether this ionic effect partly compensates for the embolism-induced loss of stem hydraulic conductivity (PLC) by increasing  K  h  of functioning conduits. We report changes in PLC, leaf water status and potassium concentration ([K + ]) of xylem sap measured in April and July in two evergreens ( Ceratonia siliqua  L. and  Phytolacca dioica  L.) and one deciduous tree ( Platanus orientalis  L.) growing in the field in Sicily. In summer,  Ceratonia siliqua  and  Phytolacca  dioica  showed similar native embolism (PLC = 30–40%) and [K + ] of xylem sap (14 to 17 mM), and  K  h  of stems perfused with 10 to 25 mM KCl increased by 15 to 18% compared with  K  h  of stems perfused with a low concentration of a multi-ionic solution. In contrast, native [K + ] of sap of  Platanus  orientalis  was 50% of that in the two evergreens in summer, with a parallel lack of detectable changes in PLC that was below 10% in both spring and summer. The ionic effect was PLC-dependent: the enhancement of  K  h  induced by 10 to 25 mM KCl changed from 15% for fully hydrated stems to 50–75% for stems with PLC = 50%. In  Ceratonia siliqua , PLC was less than 10% in spring and about 40% in summer; concurrently, xylem sap [K + ] increased from 3 to about 15 mM. This [K + ] at the recorded PLC would cause an increase in residual  K  h  of about 30%. Hence, the actual reduction in water transport capacity of  Ceratonia  siliqua  stems in summer is about 20%. Similar calculations for  Phytolacca  dioica  suggest that the actual loss of hydraulic conductivity in stems of this species in summer would be only about 10%, and not 30% as suggested by hydraulic measurements performed in the laboratory. We conclude that an increase in [K + ] of xylem sap might be involved in the up-regulation of residual  K  h , thus substantially alleviating the embolism-induced reduction in leaf water supply.

Xylem embolism alleviated by ion-mediated increase in hydraulic conductivity of functional xylem: insights from field measurements