INFLUENCE OF PLANT EXTRACTS, PROTEINS, AND AMINO ACIDS ON THE ACCUMULATION OF SELENIUM IN PLANTS

THE AMOUNT of selenium available in the soil for absorption by plants is increased by the decomposition of dead plant tissues that yield water-soluble organic selenium compounds. It has been shown by field-plot experiments that plants accumulate selenium from such compounds more readily than from inorganic salts of selenium (Beath, 1937; Beath, Eppson, and Gilbert, 1935, 1937). A previous paper compared the effects of sodium selenite and a variety of Astragalus extracts on growth and selenium accumulation in young corn plants grown in sand cultures (Trelease and Di Somma, 1944). Plants given selenium in the form of a suitable extract of seleniferous Astragalus accumulated far greater amounts than those that received an equal concentration of selenium as sodium selenite. It was also found that the addition of an aqueous extract of non-seleniferous string-bean pods, or of nearly non-seleniferous Astragalus, to a culture solution containing sodium selenite increased the amount of selenium accumulated by the young corn plants. The present paper reports the results of experiments planned to compare the effects of Astragalus extract and sodium selenite on corn (Zea Mays) and Astragalus racemosus. It also presents the results of a further study of the influence of non-seleniferous organic materials in the culture solution. Since selenium is thought to be involved in the protein metabolism of the plant, an investigation was made of the effects of proteins and their derivatives on the absorption and accumulation of selenium from culture solutions containing sodium selenite. These experiments were performed with plants grown in solution culture. A preliminary account of some of this work has been published (Trelease, Greenfield, and Di Somma, 1942). MATERIALS AND METHODS.-Pioneer hybrid corn No. 307 was germinated in quartz sand watered daily with mineral nutrient solution. When the seedlings were about 8 cm. high, they were washed free of the sand and selected for uniformity. Three seedlings were mounted in holes in a cork stopper that fitted into a jacketed one-quart Mason jar containing the culture solution. In most of the experiments three jars, or nine plants, constituted the unit for each treatment. A similar method was used for cultures of Astragalus racemosus. The seeds were treated with concentrated sulfuric acid for twenty minutes, soaked for three hours in water, and then allowed to germinate in quartz sand that was watered daily with culture solution (Trelease and Trelease, 1938). Five seedlings about 30 mm. high were mounted in the cork stopper of a one-quart Mason jar. Experimental treatments began after allowing a week for 1 Received for publication June 16, 1944. adaptation to the solution-culture conditions. Each group consisted of five jars, or 25 plants. All culture solutions had the following salt concentrations: .00160 M KNO3, .00072 M KH2PO4, .00048 M K2HPO4, .00400 M Ca(NO3)2, .00053. M (NH4)2SO4, .00160 M MgSO4, .00008 M FeSO4, .00008 M K-citrate, and the following amounts, in parts per million, of the microtrophic elements (Trelease and Trelease, 1935): .44 Mn, .05 B, .07 Zn, .008 Cu, .02 Mo, .09 Al, .06 Si, .01 I, .12 Na, .005 Li, .01 As, .01 Co, and .01 Ni, of which only the first five are generally regarded as essential for the development of higher plants. Sulfur, as sulfate, was uniformly present in a concentration of .0022 gramatom per liter (or 71 ppm. of sulfur). The pH value was 6.3, and this was stabilized by a NO3-/NH4+ ratio of 90/10 (Trelease and Trelease, 1935). Solutions were used selenium-free, or prepared to contain various concentrations of sodium selenite (Na2SeO3) or of Astragalus extract. The extract was made from finely ground seleniferous Astragalus bisulcatus, collected during the summer of 1941 near the Twin Buttes, about 15 miles southwest of Laramie, Wyoming. The dry po'wder, containing 1900 parts per million of selenium, was mixed with culture solution (1 g. per 40 ml.) and allowed to stand for 16 hours at room temperature. Filtration with suction yielded a clear brownish extract containing 42 ppm. of selenium. The extract was freshly prepared for each renewal of culture solution. This extract contained twice as much selenium as that obtained from plants collected at the same location a year earlier, and used in previous experiments (Trelease and Di Somma, 1944). Selenium is present in the Astragalus extract in some organic form, not as inorganic selenite or selenate (Beath, Eppson, and Gilbert, 1937; Byers, Miller, Williams, and Lakin, 1938; Painter, 1941). Although the culture solutions were not maintained free from microorganisms, it seems unlikely that these had an important influence on the results obtained. The use of freshly prepared solutions and frequent renewal tended to reduce possible effects of microorganisms that may have been present. Consistent results were obtained from triplicate cultures and from repetition of experiments, with both solution and sand cultures. All cultures were kept on rotating tables in the greenhouse, and solutions were renewed twice a week during a growth period of three weeks for the corn cultures and of four months for the Astragalus cultures. At the end of this period the plants were photographed, then harvested and dried at 750C. in an electrically heated oven with forced draft ventilation. Dry weights were recorded, and the selenium contents of tops of the plants were determined.