WHOLE-PLANT HYDRAULIC RESISTANCE AND VULNERABILITY SEGMENTATION IN ACER SACCHARINUM

Hydraulic properties were studied in Acer saccharinum L., a riparian s pecies that also grows well on a dry soil when transplanted. Hydraulic resistances were measured by two independent techniques: a new high-p ressure flowmeter (HPFM) method and a conventional evaporative flux (E F) method. Vulnerability to cavitation was also investigated on petiol es, stems and roots using a hydraulic conductivity technique. Vulnerab ility segmentation was found, i.e., roots, stems and petioles had diff erent vulnerabilities to xylem dysfunction. Petioles were most vulnera ble with 50% loss of hydraulic conductivity at -0.5 MPa, roots were le ast vulnerable (50% loss at -2.2 MPa) and stems were intermediate in v ulnerability. The HPFM and the EF methods gave comparable results, exc ept that the EF method gave a significantly higher value for resistanc e across petioles plus leaves. Native embolism was high enough to expl ain the discrepancy in resistance across petioles plus leaves between the HPFM and the EF methods, indicating that the HPFM estimates the mi nimum (potential) hydraulic resistance of plants. Whole-plant hydrauli c resistance of A. saccharinum was low compared to resistances of othe r temperate species. The hydraulic characteristics of A. saccharinum w ere consistent with adaptation to its typical environment: low whole-p lant resistance assures high transpiration rates in the presence of su fficient water, and vulnerability segmentation provides the ability to survive during droughts through shedding of expendable organs.