INFLUENCE OF THAWING RATE AND FUNGAL INFECTION BY RHIZOSPHAERA-KALKHOFFII ON FREEZING-INJURY IN RED-SPRUCE (PICEA-RUBENS) NEEDLES

Red spruce (Picea rubens Sarg.) decline has been observed in northeast ern North America fur the last 30 years. A major inciting stress invol ved in this decline is freezing injury of foliage. The objectives of t his study were the following: (i) to examine how photosynthesis, needl e electrolyte leakage, chlorophyll loss, needle reddening, needle loss and bud break respond to single freezing events down to -45 degrees C on 3-year-old seedlings; (ii)to test if faster thawing rates increase the amount of freezing injury; and (iii) to measure how Rhizosphaera kalkhoffii Bubak inoculations interact with freeze-injured needles. Tw o trials, one of 60 seedlings and one of SO seedlings, were conducted. The second trial had half the seedlings covered with plastic bags for doubling the thawing time. Photosynthesis, as measured by gas exchang e, was consistently the most sensitive measure, detecting nonvisible i njury on uncovered seedlings (p < 0.05) at -25 degrees C. Measurements detecting freezing damage on covered, slower thawing seedlings were p hotosynthesis, chlorophyll loss, and percent budbreak. Faster thawing rates increased the amount of injury ca. 2: to 3-fold after freezing t o -35 of -45 degrees C for all measures. Infection by R. kalkhoffii in creased 40-83% after freezing needles to -40 or -45 degrees C. Fungal inoculations caused ca. 40-60% reduction in photosynthesis on needles frozen to -40 or -45 degrees C. This study suggests that two new facto rs can increase freezing injury on red spruce needles: a faster thawin g rate and fungal (R. kalkhoffii) infection. These results are consist ent with the growing knowledge that freezing injury is a complex pheno menon in red spruce.