Effect of Stable Boron Isotopes on Growth of Tobacco in Solution Cultures

It has been stated and is generally accepted that the isotopes whetlher stable or radioactive behave relative to biological functions as do the commonly occurring forms. It appears to have been assumed that they will function in all phases in the physiology of a green plant in the same way. However, lethal effects (7) and other effects (1) on the plants have been reported but appear to be associated with the level of radioactive material to which the plant is exposed. There were some reductions in growth even when the concentration was below what was considered the critical level. These findings have been supported by recent work (2). The extensive studies (3) now under way with radioactive isotopes of the elements essential to plant growth including carbon, calcium, nitrogen and phosphorus, would appear to be intimately related to the assumption that they function biologically as do the commonly occurring forms. The micronutrient elements have not apparently received any considerable attention. One paper by STOUT and M1EAGHER (8) deals with radioactive molybdenum effects in plant growth. No publication dealing with stable or radioactive isotopes of boron effects on plant growth has been noted. The stable isotope of boron, B10, recently made available by the Atomic Energy Commission appeared to offer an interesting approach to study any differences in growth from stable boron isotopes. Comparisons using boric acid commonly available as the reagent grade and boric acid prepared from the complex supplied by the commission in which 96% of the boron occurred as B'0 were carried out in solution cultures using tobacco as the test plant. The cultures were located on three adjacent tables with the B" groups on one table and the B'0 groups on another with the groups with no boron on a third table placed between the two. The tables were 13.5 feet long and one foot seven inches wide and were located one foot 10.5 inches apart. The plants were thus grown in an area 8.5 feet by 13.5 feet so that if there were a position effect on growth it should not be of a serious nature. The procedures used in culturing the plants were essentially those described previously (5, 6). The solutions used in these studies were of the same composition as those previously used (6). They contained the following voluine molecular concentrations: Ca (NO3) 2 4H20, 0.00617; KNO3, 0.00108; Mg(NO3)2 6H20, 0.00062; KH2PO4, 0.00211; and NH4Cl, 0.00140. The parts per million of the various elements supplied were: Nitrogen, 225; phosphorus, 66; potassium, 125; calcium, 247; magnesium, 30; sulphur, 20; and chlorine, 50. The following elements were supplied in parts per million