XVI International Botanical Congess

Diurnal variation in petiole specific conductivity was measured in red maple, tulip tree, and fox grape. The involvement of living cells in maintaining hydraulic conductivity was examined using two treatments: phloem girdling and addition of HgCl2. There was a decrease in specific conductivity associated with lower leaf water potentials in maple and tulip tree. Specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem was still under tension. In maple and tulip tree, both girdling and HgCl2 treatments resulted in decrease in recovery of specific conductivity as xylem tension relaxed during the afternoon. Thus parenchyma cells play an important role in the maintenance of xylem transport.