The Effects of Transverse Cuts Through the Stems of Transpiring Woody Plants on Water Transport and Stress in the Leaves

Both single and double-overlapping transverse cuts were madein the stems of transpiring plants (Acer pseudoplatanus andGossypium hirsutum) and their effects on transpiration and waterstress in the leaves studied. Water stress was measured as depressionof water potential and ß-gauge reading. Results weresimilar with both species. Single cuts severing as much as 90 per cent of the cross-sectionalarea of the stem had no detectable effect on the rate of transpirationor on stress in the leaves above. Double overlapping cross cutscaused irreversible increased stress and cessation of sap flowonly if their vertical separation was less than a critical distance,specific for each species and related to the respective vessellengths. Cuts separated by more than this distance had no detectableeffect on stress and transpiration rate was unchanged. Alsono effect was detected if a length of the stem between suchcuts was cooled to near-freezing temperature, or killed by steamtreatment. Patterns of heat pulse velocities showed how thesap was channelled around the cuts through remaining intactxylem.