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 • 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 surface • Tried this with glass pressure chambers – Dramatic results: explosion – Ideas were right on • Scholander did it later with aluminum chambers P Simple modern evidence Z 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 gauge— relative to atmosphere 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).