Quantitative Interactions Between Phloem-Xylem Sap Flows, Root Pressure
and Predawn Water Potentials.
For many years, mechanistic studies of root pressure and xylem
exudation in decapitated plants have focused on the uptake of mineral
nutrients from the soil and the resulting osmotic potential and influx
of water into the xylem. Theoretical considerations and published
evidence of downward water flow through the phloem into the apoplast of
root tips in intact (non-decapitated) plants support an old concept that
phloem sap flow may contribute significantly to root pressure and upward
xylem flow (i.e. a “push-pull” effect). Phloem loading and osmotic
influx of water into leaf sieve tubes at night may contribute to
negative predawn water potentials.