PHYSIOLOGICAL-RESPONSE TO CONTROLLED FREEZING OF ATTACHED RED SPRUCE BRANCHES

Previous studies of foliar cold tolerance in red spruce (Picea rubens Sarg.) have been conducted on isolated shoots or needles. These studie s therefore could not show either the type or amount of visible injury to attached foliage in a natural environment after controlled freezin g stress, or the physiological response, if any, of visibly undamaged foliage. It was hypothesized that even when visible injury did not dev elop, foliage on frozen, attached branches would show long-term altera tions in electrolyte leakage, water balance, and gas exchange capacity as a result of severe freezing stress. In mid-December 1990, intact, attached branches of red spruce saplings which had been exposed to con trolled ozone levels (1 x or 2 x ambient) and precipitation acidity (p H 3.1 or 5.1) were frozen to -48 or -54-degrees-C. Neither ozone nor p recipitation acidity affected any measure of freezing injury at P less -than-or-equal-to 0. 1. During the 3 months following freezing, curren t-year shoots developed either a brown needle discoloration, needle ab scission without visible discoloration, or no obvious needle discolora tion or abscission. This is the first experiment to show that severe f reezing stress can lead to abscission of visibly undamaged needles fro m an attached branch. All frozen current-year needles had elevated rel ative electrolyte loss (REL) with 24 hr after freezing, but REL subseq uently declined for needles without visible discoloration. One day aft er freezing, average relative water con tent (RWC) of needles on froze n and unfrozen shoots was about 85%, and RWC remained above 80% throug hout the winter for unfrozen shoots. By mid-january, needle RWC on vis ibly injured shoots averaged about 50% RWC of experimentally frozen ne edles without visible injury fell more slowly to between 65 and 76% by late February. Across all shoot types, REL just after freezing was ne gatively correlated with needle RWC 1 month later, and also with maxim um dark respiration (R), conductance to water vapor (g), and light-sat urated net photosynthesis (A) measured on detached shoots at 12-20-deg rees-C in late january to February. In unfrozen shoots, net photosynth esis reached a maximum of about 5 mumol m-2 s-1. Shoots with visible n eedle injury and shoots from branches which later showed needle abscis sion had very large reductions in mean maximum gas exchange (>50% for R, > 60% for g, and > 90% for A) compared to unfrozen shoots. On froze n branches which retained needles, non-visibly injured shoots had decl ining dark respiration rates and reductions of about 40% in maximum co nductance and net photosynthesis, suggesting physiological damage from the freezing stress despite a lack of visible needle injury. Net phot osynthesis and conductance remained lower in non-visibly injured froze n shoots compared to unfrozen shoots in late March and early April, mo re than 3 months after freezing. Initial net photosynthesis of frozen shoots placed in the growth chamber in late january was strongly corre lated with maximum net photosynthesis. Initial net photosynthesis divi ded by initial conductance (A(i)/g(i)) was even more strongly correlat ed with maximum net photosynthesis (r = 0.95). A(i)/g(i) was also nega tively correlated with visible needle injury estimated in late March. A(i)/g(i) measured on detached shoots, under favorable conditions for photosynthesis, may therefore be an early indicator of either latent v isible needle injury or reduced photosynthetic capacity for freeze-dam aged trees.