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Water under tension in xylem

Water under tension in xylem. V. Berg UNI Plant physiology 2011. Water pulled or pushed?. Picric acid. Cut gap in trunk. Interpretation. 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

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Water under tension in xylem

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  1. Water under tension in xylem V. Berg UNI Plant physiology 2011

  2. Water pulled or pushed? Picric acid Cut gap in trunk

  3. Interpretation • 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

  4. 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

  5. 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 surface • Tried this with glass pressure chambers • Dramatic results: explosion • Ideas were right on • Scholander did it later with aluminum chambers

  6. P

  7. Simple modern evidence Z

  8. 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?

  9. 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

  10. Two kinds of scales 2 bars 1 bar Pressure (+ P) 0 bars 1 bar -1 bar 0 bars -2 bars -1 bar Tension (- P) Tire pressure gauge—relative to atmosphere Relative scale Absolute scale

  11. 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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