RELATIONSHIP BETWEEN FREEZING TOLERANCE AND SHOOT WATER RELATIONS OF WESTERN RED CEDAR

Freezing tolerance and shoot water relations parameters of western red cedar (Thuja plicata Donn) seedlings were measured every 2 weeks from October 1989 to April 1990. Freezing tolerance, measured by freeze-induced electrolyte leakage, showed seasonal shifts in the temperature causing 50% foliage electrolyte leakage (LT50). The LT50 value was -4-degrees-C in October, it decreased to -20-degrees-C in February and then increased to -6-degrees-C in April. The foliage index of injury at -10-degrees-C (II-10) also showed seasonal shifts from a high of 98% in October to a low of 18% in February followed by an increase to 82% in April. Osmotic potentials at saturation (PSI(s(sat)) and turgor loss point (PSI(s(tlp)) were, respectively, -1.07 and -1.26 MPa in October, -1.57 and -2.43 MPa in January, and -1.04 and -1.86 MPa in April. Dry weight fraction (DWF) increased and symplastic volume at full turgor (V(o)) decreased during the fall-winter acclimation phase, whereas DWF decreased and V(o) increased during die late winter-spring deacclimation phase. Relationships between seasonal patterns of freezing tolerance and shoot water relations parameters showed that LT50 and II-10 decreased linearly as PSI(s(tlp)) and V(o) decreased and DWF increased. There was no discernible difference in the relationship during fall acclimation or spring deacclimation. The freezing dehydration index at -10-degrees-C (FDI-10) declined from 0.69 in November to 0.41 in February and increased to 0.56 in April. The value of II-10 decreased linearly as FDI-10 decreased, although a measurement made on actively growing spring foliage did not fit this relationship. The results indicate that seasonal changes in freezing tolerance of western red cedar are partially due to changes in tissue water content, symplastic volume, passive osmotic adjustment and FDI-10.