Influence of thawing rate and fungal infection by Rhizosphaera kalkhoffii on freezing injury in red spruce (Picea rubens) needles

Abstract

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. The objectives of this study were the following: (i) to examine how photosynthesis, needle electrolyte leakage, chlorophyll loss, needle reddening, needle loss and bud break respond to single freezing events down to −45 °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 Bubák inoculations interact with freeze-injured needles. Two trials, one of 60 seedlings and one of 80 seedlings, were conducted. The second trial had half the seedlings covered with plastic bags for doubling the thawing time. Photosynthesis, as measured by gas exchange, was consistently the most sensitive measure, detecting nonvisible injury on uncovered seedlings (p < 0.05) at −25 °C. Measurements detecting freezing damage on covered, slower thawing seedlings were photosynthesis, chlorophyll loss, and percent budbreak. Faster thawing rates increased the amount of injury ca. 2- to 3-fold after freezing to −35 or −45 °C for all measures. Infection by R. kalkhoffii increased 40–83% after freezing needles to −40 or −45 °C. Fungal inoculations caused ca. 40–60% reduction in photosynthesis on needles frozen to −40 or −45 °C. This study suggests that two new factors can increase freezing injury on red spruce needles: a faster thawing rate and fungal (R. kalkhoffii) infection. These results are consistent with the growing knowledge that freezing injury is a complex phenomenon in red spruce.