© 2003 Heron Publishing—Victoria, Canada
Effects of simulated thaw on xylem cavitation, residual embolism, spring dieback and shoot growth in yellow birch
R. M. Cox (1, 2) and X. B. Zhu (1)
1. Canadian Forest Service—Atlantic Forestry Centre, Fredericton, NB E3B 5P7, Canada / 2. Author to whom correspondence should be addressed (rcox@nrcan.gc.ca) / Received June 26, 2002; accepted December 14, 2002; published online May 15, 2003
Summary
Yellow birch seedlings (Betula alleghaniensis Britt.) that had lost more than 90% of their stem hydraulic conductivity during ambient winter temperatures were exposed
to 0 and 20 days of a simulated winter thaw followed by a 48-h freezing treatment at 0, –5, –10, –20 and –30 °C. After measuring
freezing injury to shoots and roots, the seedlings were placed in a greenhouse where recovery of xylem conductivity and new
growth were measured. Shoot xylem cavitation was measured as percent loss of hydraulic conductivity. Shoot freezing injury
was assessed by electrolyte leakage (EL) and root freezing injury was assessed by EL and triphenyl tetrazolium chloride reduction.
Seedlings pretreated with thaw had higher stem water contents and suffered more freezing damage to roots and shoots (at –20
and –30 °C, respectively) than unthawed seedlings. After 3 weeks in a greenhouse, seedlings from the 0, –5 and –10 °C freezing
treatments showed complete recovery of xylem conductivity, with substantially increased stem water contents. Poor recovery
of hydraulic conductivity was observed only in seedlings that were subjected to freezing treatments at –20 and –30 °C, regardless
of thaw treatment. Of these embolized seedlings, however, only those not previously thawed showed recovery of hydraulic conductivity
or regained stem water content after 9 weeks in the greenhouse. Shoot dieback, bud burst and length of new shoots were significantly
related to the extent of stem xylem cavitation and freezing injury. We conclude that (1) the simulated winter thaw predisposed
yellow birch seedlings to freezing damage in shoots and roots by dehardening tissues and increasing their water content; (2)
root freezing damage in turn affected the seedlings’ ability to refill embolized stem xylem, resulting in considerable residual
xylem embolism after spring refilling; (3) further recovery of stem xylem conductivity was attributable to growth of new vessels;
(4) and the permanent residual embolism, together with root and shoot freezing injury, caused increased dieback, bud mortality
and reduced growth of new shoots.
Keywords:
climate change, root pressure and freezing injury, thaw–freeze treatments, water content.
