- Manter, Daniel K.
Red spruce (Picea rubens Sarg.) decline has been observed in northeastern North America for the last 30 years. A major inciting stress involved in this decline is freezing injury of foliage (Dehayes 1992). The objectives of this study were to: (i) relate gas exchange and chlorophyll loss to visual symptoms and electrolyte leakage, (ii) determine if thawing rate can influence the amount of freezing injury, and (iii) measure how Rhizosphaera kalkhoffii Bubák interacts with freeze-injured needles. Two trials, 60 and 80 seedlings respectively, were conducted in which measurements of photosynthesis (4 branches per seedling), needle electrolyte leakage (1 sample per seedling), chlorophyll loss (1 sample per seedling), needle reddening and loss (4 branches per seedling) and bud break (1 sample per seedling) were performed to measure injury after a single freezing event. In addition, for each seedling, two of the branches used for photosynthesis measurements were inoculated with R. kalkhoffii after freezing. Seedlings were frozen to test temperatures of -5, -20, -25, -30, -35, and -45ºC in trial 1 and -5, -25, -35, and -45ºC in trial 2. In the second trial, half the seedlings were covered with plastic bags, which doubled the thawing time.
Measurements of freezing injury showed that photosynthesis was consistently the most sensitive measure, detecting non-visible injury on uncovered seedlings (p<0.05 at -25 to -30ºC for trial 1 and -25 to -35ºC for trial 2). All other measures were less sensitive to freezing injury and did not detect injury (p<0.05) until -30ºC in trial 1 and -35ºC in trial 2. For the seedlings, chlorophyll loss and % bud break were the only measures to detect damage at -35 and -45ºC, respectively. Faster thawing rates increased the amount of damage ca. 2-3 fold after a single freezing event (p<0.05) at -35 and -45•C for all measures. Infection by R. kalkhoffii increased significantly after colder freezing treatments, with an 83% increase in infection rate at -40ºC in trial 1, and 81% and 40% increase at -45ºC in trial 2 for uncovered and covered seedlings, respectively (p<0.05). Fungal inoculations caused a ca. 60% reduction in photosynthesis after treatment at -40ºC in trial 1 and ca. 40% and 60% reductions after treatment at -45ºC in trial 2 for covered and uncovered seedlings, respectively (p<0.05). This study suggests two new factors which influence freezing injury; thawing rate and fungal (R. kalkhoffii) infections. The results of this study are consistent with the growing knowledge that freezing injury is a complex phenomenon in red spruce.