CHLOROPHYLL FLUORESCENCE AS A MEASURE OF COLD-HARDINESS AND FREEZING STRESS IN 1+1 DOUGLAS-FIR SEEDLINGS

Two-year-old seedlings of Douglas-fir (Pseudotsuga menziesii [Mirb.] France) from two seed lots with different levels of predicted cold hardiness were grown in separate nurseries. Seedlings were lifted every 4 wk from October 1991 through March 1992 and exposed to controlled freezing temperatures. Fluorescence emissions of the seedlings were measured before and 1 and 3 days after each freezing test. Destructive morphological assessments were made 7 days after freezing. Unstressed seedlings were measured during 6 additional test periods through September 1992. Chlorophyll fluorescence had a significant linear relationship to needle freezing damage and seedling survival, but there was no significant linear relationship between fluorescence of control seedlings and frost hardiness. The LT(50) (lethal temperature for 50% of the seedlings) fluorescence measurements, however, tended to be lower before and higher after the seedlings were hardened off. These results indicate that chlorophyll fluorescence is capable of detecting nonvisible damage to seedlings. Continued research may show this to be a valuable tool for rapidly identifying damaged seedlings in nurseries and hence for aiding vital management decisions.