WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



91 



and measuring specific air pollutants must be im- 

 proved. Physical and biological data must be so 

 accrued as to be amenable to sound economic 

 analysis. 



The potential exists for improving the design 

 of physical facilities towards improving assess- 

 ment of losses to agriculture, including the relative 

 significance of each specific pollutant. Parallel 

 field studies will be needed to check the validity of 

 laboratory findings for field conditions. 



Effects of Airborne Toxicants on Plants, Including 

 Interactive Effects 



That air-polluting chemicals damage leaves and 

 blooms of salable plants is an established fact. But 

 little is known of the actual physiological mech- 

 anisms by which these harmful effects take place. 

 A better knowledge of how these toxicants alter 

 the biochemistry of leaves is a prerequisite for the 

 development of counteractive measures. 



The potential for improving procedures to 

 ameliorate air pollution damage has not been ade- 

 quately developed. Ozone fleck in tobacco has been 

 partially diminished by ozone-resistant varieties. 

 However, plant resistance is relative. The floor 

 for injury of a resistant variety may be exceeded if 

 pollutant concentration increases. Cultural prac- 

 tices and use of chemical protectants promise some 

 control of air toxicant damage. 



The best way to alleviate air pollution damage 

 to agriculture and forestry would be the use of 

 technology to eliminate industrial and automotive 

 sources. The prospects for this happening are dim. 

 Therefore, the highest promise for alleviating the 

 damage now being inflicted on crops, ornamentals, 

 and trees is to foster adequate research for resist- 

 ant varieties as -trains, and for effective chemical 

 protectants from the adverse ph}'siological changes 

 that air toxicants induce. 



Development of Tolerant Species, Varieties, and 

 Breeds of Plants 

 There is an urgent need for information on tol- 

 erance of species, varieties, and breeds of plants, if 

 profitable production of sensitive species, such as 

 pine-, citrus, tobacco, and leafy vegetables, is to be 

 maintained in certain areas of the country. The 

 situation is already acute in southern California. 

 It is becoming increasingly serious in the densely 

 populated stretch from Boston, Mass., to Rich- 

 mond, Va. Information on tolerance, of orna- 



mentals is especially needed by city planners and 

 landscape architects. 



The apparently unlimited potential for develop- 

 ing plants more tolerant to air toxicants should be 

 exploited more fully. Systematic studies to develop 

 tolerant vegetation have just tiegun. Evidence 

 shows that white pine trees can be selected that are 

 resistant to ozone and other toxicant s. 



The effort needed to develop tolerant plants is 

 large because of the tremendous number of plant 

 species involved, and the need to genetically com- 

 bine yielding ability, quality characteristics, and 

 resistance to diseases with tolerance to airborne 

 toxicants. 



Adequate physical facilities will need to be pro- 

 vided for maintaining a controlled level of toxi- 

 cant in progeny evaluation studies. 



Evaluation of Air Cleansing of Toxicants by 

 Foliage 



Plant species vary considerably in their reaction 

 to toxic substances in air. For example, a few parts 

 per billion of fluoride in the air are extremely toxic 

 to gladiolus, corn, sweet potatoes, prunes, and apri- 

 cots; but other species such as alfalfa are tolerant 

 and may accumulate levels about 500 p. p.m. with- 

 out injury. Other toxicants such as sulfur dioxide 

 cause injury to pines, alfalfa, cotton, and grains 

 at relatively low concentrations; but citrus, privet, 

 and apple are very resistant. 



"When sulfur dioxide enters leaves, it is con- 

 verted to sulfite. Then sulfite is slowly oxidized 

 to sulfate, which reduce- the sulfite toxicity by a 

 factor of about 30. Consequently, if sulfur di- 

 oxide is not added to the system too rapidly, a large 

 amount may be added without resultant toxicity. 

 .More information is needed on the extent to which 

 sulfur dioxide may be a source of sulfur nutrition 

 of crops on sulfur-deficient soils. 



It is known that plants exposed for several days 

 to about 1 p. p.m. of nitrogen oxide become greener, 

 although somewhat stunted in growth. This sug- 

 gests that some plants may be able to use nitrogen 

 from this toxicant. 



Coni fers appear to be more efficient in aircleans- 

 ing than maples because they have greater leaf 

 surfaces and are not deciduous. 



Much more in format ion is needed about the rela- 

 tive rate at which plants rem cants from 

 air during different seasons. It is important to 



