RESEARCH
the precipitation of strontium nitrate. > Scavenging Removes Contamin a n t s . Barium-chromate-ferric hydroxide scavenging removes radiochemical contaminants, such a radium, lead-210, a n d t h e actinides. T h e strontium-90 present is measured indirectly by isolating a n d counting its daughter n u clide, 64-hour yttrium-90. Cesium gets freed of most of the potassium and sodium in the mixed cobaltinitrite precipitated by precipitation of cesium silicotungstate. After converting the latter to the perchlorate, a phenol-sulfonic cation exchanger absorbs it. Potassium-40 and rubidium-87 are the chief radiochemical contaminants. After washing with 0.3M hydrochloric acid, rubidium and lighter alkali metals show up on bands well apart from t h a t of cesium. In the case of the rare-earth fraction, 10 mg. each of cerium ( I I I ) , neodymium, a n d samarium are added to a solution of the ferric hydroxide precipitate. Iron stays in solution by precipitating the rare earth fluorides. Scavenging with barium sulfate removes radium and lead-210. Anion exchange on Dowex-1 removes uranium and protactinium in concentrated hydrochloric acid; thorium in 7.5M nitric acid, Cerium is finally isolated in pure form as ceric iodate. Promethium is obtained by a cationexchange separation on Dowex-50. Positions of the samarium and neodymium bands define the promethium band. Promethium is carried on neodymium for counting purposes. Sugihara lists typical yields from these procedures: strontium, 7 0 % ; cesium and cerium, 60%; and p r o methium, 8 0 % . T h e promethium yield is estimated by averaging the yields of neodymium and samarium. A cylindrical, flow Geiger counter, V 4 inch in diameter and 2l/2 inches long, does the counting for the tests. Its background inside 7 inches of iron and with anticoincidence shielding is about 0.22 count a minute. Yttrium-90 is identified by its characteristic halflife, and the others can be positively identified with a beta absorption curve. Sugihara reports t h a t t h e strontium-90 level in Atlantic surface waters is about 0.1 disintegration per minute per liter. And the corresponding cerium-144 activity measures about t h e same. T h e cerium-144 to promethium147 ratio is about three. Cesium samples have not yet been counted.
Pectin Plugs Cause Wilts Wisconsin researchers shed light on mechanism o f v a r i ous fungus diseases of tomatoes / b a n a n a s By producing wilt diseases in ACS NATIONAL p l a n t s , f u n g i MEETING cause millions of dollars worth of damage a year. Food Chemistry These losses could be reduced if there were a basic understanding of how fungi operate. At stake is the possibility of eradicating tomato wilt, a severe problem in many sections of the U. S. and Europe. Also possible is the prevention of oak wilt, a destructive disease throughout the Midwest, and Panama disease, one of the greatest hazards in the growing of bananas. Insights into how these disease-producing fungi behave in plants are provided by Mark A. Stahmann and John C. Walker of the University of Wisconsin. As Stahmann told the ACS Division of Agricultural a n d Food Chemistry, their research so far has been focused mainly on tomato wilt. In this fungus disease, the lower leaves of the plant gradually turn yellow, wilt, and die. The plant is stunted and becomes permanently wilted. T h e Fusarium fungus, a cause of wilt, enters the plant through the roots and passes into the vascular system. There, the fungus produces two enzymes, pectin methylesterase and pectic depolymerase. These enzymes, says Stahmann, hydrolyze some of the pectin in the vascular cell walls. The pectin is d e g r a d e d to a point where soluble fragments go into the vascular stream. I n this stream, the pectin fragments react with calcium or other ions to form pectin gels. These gels accumulate sufficiently to block the transport of water and nutrients to the upper parts of t h e plant. This blockage, Stahmann explains, causes the plant to wilt. Of the two types of enzymes responsible for t h e partial breakdown of pectin, the most important is pectic depolymerase. Pectin methylesterase alone causes very little vascular plugging and thus very little wilting. Fungi-produced enzymes capable of degrading pectin may be key factors in the biochemistry of Fusarium wilt of
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FUNGI DID IT. Tomato w i l t is caused by pectin gels that plug t h e conducting vessels of t h e plant and block off water and nutrients, says Mark A. Stahmann, University of Wisconsin. Cutting he holds at left has been wilted by fungusproduced pectic enzymes cotton, Fusarium and Verticillium wilt of tomatoes, Dutch elm disease, oak wilt, and other fungus diseases, Stahmann says. • Mechanism of Resistance. But why are certain plants resistant to wilt diseases and others are not? H o w is it possible, for example, that some tomato plants are immune to Fusarium wilt? T h e answer, Stahmann believes, is that resistant plants are able to reduce the amount of pectin-splitting enzymes formed by the attacking fungi. In a resistant t o m a t o plant, the amount of depolymerase secreted by t h e fungus is only one third that produced in a susceptible plant. In a resistant plant, some substance may be continuously produced t h a t is toxic to t h e fungus or may reduce enzyme formation. This resistance mechanism, Stahmann says, is closely related to the respiratory system of the plant. When the system is blocked by a respiratory inhibitor, such as 2,4-dinitrophenol, a resistant plant loses its immunity. Recent work at t h e Wisconsin laboratory suggests that P a n a m a disease of bananas is also caused by the forming of pectin plugs in the vascular system. Control of this highly destructive disease of bananas can best b e achieved, Stahmann says, by selecting and breeding resistant varieties, as has already been done successfully with tomatoes and other plants. • APRIL
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