Electronic theory for materials science

Some of the important ingredients used in theoretical investigations of the electronic properties of materials are delineated by means of examples. The importance of examining the physical behavior of related materials is emphasized as a means of establishing the credibility of theoretical explanations. Since the ability to deal realistically with complex technologically important materials is limited by analytic and computational capabilities, theories for materials science must aim at the development of comprehensive and predictive models based on physically motivated approximations. The emergence of such models and their ability to describe a broad range of properties foreshadows an increasingly important role of theoretical analysis in the future development of the field.     

Growth and the composition and transport of carbohydrate in compatible and incompatible peach/plum grafts

The growth of scions and rootstocks of compatible (Prunus persica L. Batsch cv. Springtime/Prunus cerasifera L. Ehrh. cv. myrobolan P2032) and incompatible (Prunus persica L. Batsch cv. Springtime/Prunus cerasifera L. Ehrh. cv. myrobolan P18) peach/plum grafts were compared. The composition of soluble carbohydrates in phloem and cortical tissues of both peach/plum grafts and ungrafted plums and the translocation of these compounds across the union of grafted plants were examined. Sorbitol and sucrose were the dominant sugars in the phloem and cortical tissues of plum. A cyanogenic glycoside, prunasin, was present in peach tissues in amounts equivalent to those of sorbitol or sucrose, whereas only small amounts of prunasin were detected in plum tissues. The concentration of prunasin was significantly higher in the phloem of the P18 rootstock of the incompatible graft. Sorbitol was the only sugar significantly depleted in rootstock tissues of the incompatible graft when the first foliar symptoms of graft incompatibility became evident. Translocation studies with 1-(14)C-deoxyglucose showed that the relative distribution of radioactivity across the graft union was similar in both compatible and incompatible grafts. However, the total amount of radioactivity translocated across the incompatible graft was less than one-third of that translocated across the compatible graft. The results are consistent with the hypothesis of a progressive poisoning of the root system in the incompatible graft by a compound synthesized in peach foliage. The role of prunasin as a possible candidate is discussed.     

A low-threshold test ban is feasible

At present, nuclear explosions are limited by treaty to underground testing with yields of no more than 150 kilotons, and recently there have been renewed calls for further test restrictions. As part of these discussions, the U.S. Congress is considering bills that would legislate new limits to testing,whereas the Reagan Administration opposes such constraints. The editors of Science have asked two groups of participants in the debate to present their arguments for or against new limits to testing. Feiveson, Paine, and von Hippel argue for a treaty of indefinite duration between the United States and the Soviet Union, which includes the following provisions: (i) a ban on all testing outside a desiqnated site having known seismic properties; (ii)verification by means of on-site inspection and in-country seismic monitoring; (iii) unlimited testing below 1 kiloton at the special site; and (iv) an average of one test per year with a yield of up to 15 kilotons for ensuring reliability of the nuclear stockpile. MiUer, Brown, and Nordyke argue that a lowering of the present 150-kiloton threshold would be undesirable, and that new test bans would divert attention from a comprehensive approach to negotiated reductions in the nuclear and conventional arsenals of the United States and the Soviet Union.