Water under tension

Water under tension in xylem
V. Berg
UNI Plant physiology
Water pulled or pushed?
Picric acid
Cut gap
in trunk
• Picric acid poisons all living cells
• Live pumps in trunk would stop working
– No acid (or water) would get to leaves
– Leaves would dry out
• If leaves pull up acid
– Leaves would die of acid
– Would still be wet
What happened?
• Leaves did not dry out
• They died of acid poisoning
• So the acid was pulled up by leaves
– Acid below trunk at ambient pressure
– Acid in leaves lower pressure
• Not pushed up by pumps in trunk
• Proof of principle
Dixon’s short career
• If water in trunks under tension…
– Would snap back if you cut transpiring trunk
– Actually used twigs with leaves
– Idea: could push on leaves to get water back to cut
• Tried this with glass pressure chambers
– Dramatic results: explosion
– Ideas were right on
• Scholander did it later with aluminum chambers
Simple modern evidence
Semisynthetic tree
• From RB Walker
• Fiddly to set up
• Get transpiring with water in capillary
– Then pour Hg in test tube
– Hg gets pulled up capillary
– How high? > 1 m
• Can substitute clay cup for plant
– How high?
Water pressure scale
• Tire pressure gauge
– Sitting there: 0 pressure
– Really 1 atmosphere (= 1 bar, = 0.1 Mpa)
– Relative measure
• Absolute measure
– Ambient: 1 bar pressure, not 0
– Between 1 bar (ambient) and 0 is still + press
– Below 0 (-1 on relative measure) is negative press
Two kinds of scales
0 bars
Pressure (+ P)
1 bar
-1 bar
Tire pressure
relative to
Relative scale
1 bar
0 bars
Tension (- P)
-2 bars
2 bars
-1 bar
Absolute scale
Holbrook, N. M. & Zwieniecki, M. A.
Embolism repair and xylem tension:
Do we need a miracle? Plant Physiol.
120, 7–10 (1999).

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