Comparison of structural changes in red spruce ( Picea rubens Sarg.) during cold hardening in mature trees and in seedlings used in pollutant exposure studies

Abstract

One of the concerns in understanding the causes of tree decline has been extrapolating results obtained in controlled experiments on seedlings to mature trees in the field. The goal of this project was to compare results from trees growing under natural conditions at several sites, with varying levels of red spruce ( Picea rubens Sarg.) decline, with results from experiments conducted at the Boyce Thompson Institute on ozone and acid precipitation exposures of seedlings. One of the major hypotheses for decline of red spruce is winter injury. There is accumulating evidence that pollutants such as ozone or acidic deposition may increase susceptibility to freezing injury in early winter. The transition from a summer active state, to a state which can tolerate the conditions in winter, involves concerted changes in biochemistry, physiology, membrane composition and cellular structure. Mature tree foliage differs from seedlings in several respects, but alterations in cellular structure between mid-summer and mid-winter are similar. Changes in chloroplast numbers and orientation occur in red spruce foliage from trees of all ages, and under approximately the same conditions. Differences in cellular health are obvious from the different field sites and treatments, yet a similar pattern of response to low temperatures in the fall was seen. Individual mesophyll cells undergo alterations in the amount of cytoplasm and in chloroplast arrangement. Results indicate that foliage from young seedlings is a valid model for the response of foliage on mature trees to cold hardening in the fall.