RESULTS AND DISCUSSION 

 Laboratory Study 



Pinus ponderosa and Pseudotsuga menziesii responded nearly 

 identically within treatments and the symptoms (responses to 

 treatment) described below apply to both species. Unless other- 

 wise stated, all observations in the laboratory study refer to 

 the chlorotic region within the transition zone on current-year 

 foliage. Figures 2 through 5 show macroscopic responses of both 

 species to control and sulfur dioxide fumigations. 



CONTROL 



Needles remained green and healthy throughout the experi- 

 ment (figs. 2, 4). Cells of all internal tissues appeared turgid and 

 normal, with little or no plasmolysis caused by the histological 

 procedures (fig. 6). Plastids were dispersed within the chloren- 

 chyma and were not granulated (table 2 and fig. 7). Xylem cells 

 were stained light pink and living cells light greenish-blue. Stain- 

 ing was normally differentiated with no obvious accumulation or 

 intensification within the vascular tissues (fig. 8). 



SALT 



Needles showed symptoms of salt toxicity within 15 to 20 days 

 following the initial treatment. Chlorosis appeared at the tip, 

 middle, or base and was independent of species or individual. 

 Usually the transition zone was diffuse, not distinct. In all cases, 

 symptoms intensified until the trees were dead, even though 

 fresh water replaced the salt treatments soon after the onset of 

 symptoms. Mesophyll chlorenchyma plasmolyzed extensively 

 and most plastids were destroyed (figs. 9, 10). Endodermal cells 

 in the region of plasmolyzed mesophyll remained turgid and in- 

 tact but xylem, phloem, and transfusion parenchyma collapsed 

 (table 2). Phloem elements virtually disintegrated, and there was 

 no obvious accumulation of stain within the transition zone 

 (fig. 10). 



DROUGHT 



Chlorosis most often appeared first at the needle base and pro- 

 gressed acropetally. No distinct transition zone was formed. 

 Mesophyll cells collapsed but did not plasmolyze as in the salt 

 treatment (table 2). All other living cell types collapsed except 

 the endodermis (fig. 1 1). No obvious accumulation of stain oc- 

 curred within the transition zone. 



WINTER DRYING 



The syndrome for winter drying (table 2) was virtually the 

 same as for drought except that injury appeared much sooner, 

 within 3 to 5 days (figs. 12, 13). 



SULFUR DIOXIDE 



Chlorosis appeared near or at the tips within 3 to 8 hours 

 following fumigation and an abrupt, distinct transition zone 

 developed (figs. 3, 5). Sporadic collapse of the mesophyll oc- 

 curred, and plastids clumped near the plasmalemma. Endo- 

 dermis cells in contact with a necrotic mesophyll chlorenchyma 

 collapsed (figs. 14, 15). Xylem, phloem, and transfusion paren- 

 chyma and phloem elements showed extensive hypertrophy and 

 hyperplasia. Epithelial cells hypertrophied, occluding the resin 

 canals (fig. 16). An intense, purple-red stain developed in the 

 vascular tissues and extended basipetally into the region of non- 

 damaged mesophyll (table 2). 



HYDROGEN FLUORIDE, HYDROGEN SULFIDE, AND 

 ETHYL MERCAPTAN 



The internal syndrome induced by these pollutants was in- 

 distinguishable from that caused by S0 2 (table 2 and figs. 

 17-19). However, H 2 S caused necrosis to only the needle tips, 

 whereas F~ and C 2 H 6 S injury initially appeared slightly below 

 the tips and progressed acropetally and basipetally. Also, 

 tracheid cell walls were stained yellowish in needles injured by 

 H 2 S, unlike the light red in needles injured by the other gases. 



Field Study 



Control specimens were histologically similar to those of the 

 laboratory study (fig. 20). Similarly, symptoms in the transition 

 zones of field-collected needles within polluted areas were iden- 

 tical with symptoms induced by gaseous pollutants in the green- 

 house study (figs. 23-28). Plastids clumped in the mesophyll 

 parenchyma; endodermis collapsed when in contact with 

 damaged mesophyll; and phloem, xylem, and transfusion paren- 

 chyma and epithelial cells divided excessively and became abnor- 

 mally large. Resin ducts were occluded by the hypertrophy of 

 epithelial cells. An intense purple-red stain developed in the 

 vascular tissue extending well into the area of nondamaged meso- 

 phyll. The total syndrome was similar regardless of pollutant or 

 species; for example, it was not possible to differentiate fluoride 

 injury from S0 2 , and all species and needle ages responded 

 similarly. 



Winter drying, however, was dissimilar to pollutant injury; 

 this difference also was observed in the laboratory study. 

 Mesophyll collapsed, endodermis remained turgid even when 

 in contact with collapsed, necrotic mesophyll, and no hyper- 

 trophy or hyperplasia occurred in the parenchyma (figs. 21, 

 22). Epithelial cells in winter-dried specimens were hyper- 

 trophied, often occluding the resin canals. No intense stain 

 developed in the vascular cylinder such as occurred with gas- 

 injured specimens; the symptoms were similar among the dif- 

 ferent species and needles of various ages. 



Histological differentiation of needle necrosis has not been 

 clearly defined. Solberg and others (1955) and Solberg and 

 Adams (1956) showed that HF and S0 2 disrupted vascular 

 tissues and caused hypertrophy and hyperplasia of vascular 

 parenchyma. They indicated that HF injury could be distin- 

 guished from S0 2 . Evans and Miller (1972, 1975) stated that 

 S0 2 , suspected winter injury, and ozone injury could be 

 distinguished histologically in that SO disrupted and dissolved 

 cytoplasmic constituents of all needle tissues, whereas ozone 

 injured only the plicate parenchyma and winter fleck caused 

 abnormalities within the phloem and transfusion cells. Their 

 sections were taken adjacent to necrotic lesions and not within 

 a transition zone as described in this paper. Also, their winter 

 injury was suspected to be direct cold injury and not of the 

 drying type. Stewart and others (1973) were not able to 

 distinguish between symptoms of SO->, HF, and winter injury. 

 They observed hypertrophy of phloem cells and mesophyll 

 parenchyma cells when natural senescence, drought, and 

 fluoride were causes of necrosis. Consequently, Stewart and 

 others (1973) saw little value in the use of histological inter- 

 pretations to differentiate among various environmental 

 stresses. Conversely, biotic causes usually are easily diagnosed 

 histologically by signs of the causal organism. 



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