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

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Abstract
Both single and double-overlapping transverse cuts were made in the stems of transpiring plants (Acer pseudoplatanus and Gossypium hirsutum) and their effects on transpiration and water stress in the leaves studied. Water stress was measured as depression of water potential and β-gauge reading. Results were similar with both species. Single cuts severing as much as 90 per cent of the cross-sectional area of the stem had no detectable effect on the rate of transpiration or on stress in the leaves above. Double overlapping cross cuts caused irreversible increased stress and cessation of sap flow only if their vertical separation was less than a critical distance, specific for each species and related to the respective vessel lengths. Cuts separated by more than this distance had no detectable effect on stress and transpiration rate was unchanged. Also no effect was detected if a length of the stem between such cuts was cooled to near-freezing temperature, or killed by steam treatment. Patterns of heat pulse velocities showed how the sap was channelled around the cuts through remaining intact xylem.