Pathological anatomy of conifer needle necrosis

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Abstract
The pathological anatomy of pine needle necrosis caused by major environmental stresses was investigated to determine if histological responses could provide a diagnostic tool. All stresses studied caused a general hypertrophy of the epithelial tissue of the resin canals, occluding these ducts. Granulation of transfusion and mesophyll parenchyma cells was also a general response to stress. Hypertrophy of the phloem cells and transfusion parenchyma was caused by almost all stresses in some instances but was most consistent for necrosis caused by natural senescence, drought, or fluoride. Collapse of the mesophyll cells best characterized necrosis caused by ozone, sulfur dioxide, salt, or boron toxicity. The greatest value of microscopic examination of the tissues adjacent to the necrotic areas was found to lie in the elimination of certain pathogens, narrowing the possible causal agents. Histological examination could ascertain when certain air pollutants were not responsible for a particular injury; for instance, the absence of mesophyll collapse can definitely eliminate sulfur dioxide as a cause of necrosis. Conversely, though, presence of mesophyll cell collapse, because of its ubiquitous nature, could not establish etiology.

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