Our studies show that needle necrosis caused by phytotoxic 

 gases can be differentiated microscopically from necrosis caused 

 by other abiotic agents. The controlled greenhouse study 

 showed that symptoms of gas-caused needle necrosis can be 

 distinguished from symptoms of selected nongaseous causes. 

 Phytotoxic gases caused extensive damage to parenchymatous 

 tissues, especially within the vascular system, but it was not 

 possible to distinguish among symptoms caused by various 

 gases. Winter drying and drought caused mesophyll damage, 

 but the tissues of the vascular system were not affected. En- 

 dodermal cells in contact with necrotic mesophyll became 

 necrotic when phytotoxic gas was the cause, but not when 

 damage was induced by winter drying or normal drought; 

 necrotic endodermal cells could result from acid hydrolysis. 

 The mechanisms of endodermis susceptibility to phytotoxic 

 gas, but resistance to winter damage, salt, and drought is 

 biologically interesting and should be investigated. The deep 

 staining of vascular tissue in the transition zone noted in the 

 gas fumigations could have been induced by cytoplasmic 

 dissolution. Evans and Miller (1975) noted that sulfur dioxide 

 decreased the intensity of stain where cytoplasmic dissolution 

 was extensive. We observed the same in the necrotic areas. 

 However, in the chlorotic part of the transition zone, leakage 

 of cytoplasmic contents from lysed cells into the intercellular 

 spaces could account for the general deep staining observed in 

 our study. 



The histological interpretations of field-collected specimens 

 were similar to those of the laboratory investigation; specimens 

 collected from Switzerland, Montana, and California near 

 known sources of phytotoxic gases exhibited the pollutant syn- 

 drome, but it was not possible to distinguish between pollutants. 

 Winter-induced needle necrosis collected from areas of known 

 winter drying was similar histologically to the laboratory-induced 

 winter drying. Studies by EPA (1971), Gordon (1972), Gordon 

 and Tourangeau (1975), Carlson and Dewey (1971), Carlson and 

 others (1974), and Gordon and others (1976) support these find- 

 ings. In studies with tip-burned field-collected Austrian pine 

 needles, Maiello and others (1972) determined that sulfur dioxide 

 caused distortion of endodermis and transfusion tissues and that 

 this disruption of vascular tissues extended about 0.02 in 

 (0.5 mm) into the region of healthy mesophyll. Results of our 

 studies agree. 



Solberg and Adams (1956), Gordon (1972), and Gordon and 

 Tourangeau (1975) believed they could distinguish between in- 

 jury caused by sulfur oxides and fluoride. Vascular tissues were 

 disrupted in the area of healthy and necrotic mesophyll when 

 fluoride was causal, whereas vascular tissues appeared normal 

 even in the region of necrotic mesophyll when sulfur dioxide 

 was the agent. We did not observe this either in the controlled 

 fumigation or in the field-collected specimens. It seems unlikely 

 that plant tissues would have separate response syndromes to 

 individual gases. However, it is plausible that responses would 

 differ between such different agents as gases versus drought or 

 salt, as suggested by our work. 



The simulated winter injury treatment subjected physiologi- 

 cally active, succulent young needle tissue to relatively severe 

 stress. Natural winter drying occurs to older foliage (8 to 10 

 months) that likely is in a subdued physiological state. Thus, the 

 simulated treatment can be interpreted as severe. However, not- 

 withstanding these treatment differences, the histological effects 

 of field- and laboratory-induced stress were similar, indicating 

 similarity between physiological and morphological responses. 



The gases were administered at high concentrations, pre- 

 sumably much higher than one would expect to find in field situ- 

 ations. Also, the gas concentrations were not monitored in the 

 exposure chamber, but were inferred from flow input. We were 

 not interested in testing effects of differential gas concentrations; 

 rather, we wished to assure the development of needle necrosis 

 and to compare between treatments. Histological reactions to 

 our dosages may represent responses only to short-term acute 

 fumigations and may not be representative of actual field situa- 

 tions. However, histological symptoms in field-collected 

 specimens were similar within treatment to those in needles with 

 experimentally induced necrosis. It is likely, but not known, that 

 necrosis on needles from field locations was caused by longer- 

 term, lower concentration of pollutant. If so, the continuity be- 

 tween symptoms observed in field and laboratory specimens 

 suggests that the histological reactions represent a biochemical- 

 morphological reaction not entirely dependent on gas 

 concentration. 



Our field study did not include either drought- or salt-caused 

 needle necrosis. However, there is little reason to believe that 

 field-induced symptoms would differ significantly from our 

 greenhouse work. 



Finally, the concept that needle necrosis caused by phytotoxic 

 gases can be separated from other selected abiotic agents is well 

 supported in that differences observed in the laboratory experi- 

 ment also were observed in corresponding field collections. 



SUMMARY AND CONCLUSIONS 



Phytotoxic gases cause histological symptoms in Douglas-fir 

 and ponderosa pine needles distinct from those induced by 

 winter damage, drought, or salt. The symptoms caused by gas- 

 eous pollutants under controlled conditions are: 



1 . Collapse of endodermis in contact with collapsed 

 mesophyll. 



2. Hypertrophy and hyperplasia of vascular parenchyma. 



3. Deep staining of vascular tissue, extending basipetally into 

 the region of healthy mesophyll. 



Winter damage, drought, and salt induce the following 

 symptoms: 



1 . Mesophyll cells collapse, but endodermis is not affected, 

 even when in contact with necrotic mesophyll. 



2. Vascular parenchyma collapse; hypertrophy and 

 hyperplasia are not evident. 



3. Deep staining of vascular tissues does not occur. 

 Symptoms in field-collected specimens representative of a 



variety of conifers were similar for similar causal agents. 



Identification of the cause of conifer injury and related forest 

 damage near sources of phytotoxic air emissions may be con- 

 founded by insects, disease, weather, or other abiotic agents. 

 Affected trees usually exhibit foliar chlorosis and necrosis in re- 

 sponse to causal factors. This study demonstrates that through 

 histological procedures these causal factors may be distin- 

 guished. When used in conjunction with ambient air quality 

 data, emission data, foliar chemical analyses, and observations 

 of the status of biotic plant pathogens and foliage-feeding in- 

 sects, histological observations should strengthen the diagnosis 

 of cause in damage surveys done near polluting industries. 



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