Studies with Radioactive Tracers in Plant Nutrition

Abstract

The effect of radiation from P³² on the growth and phosphate absorption of barley seedlings was studied in water culture. Concentrations of carrier phosphate ranging from less than 0·01 to 31 p.p.m. P were employed for varying periods. Between experiments wide variations occurred in the nature and extent of the effects of radiation. An increase in the proportion of the absorbed phosphate translocated to the shoots, and the reduction of root weight, were the most consistent symptoms. While severe injury was characterized by a reduction in both growth and phosphate absorption, intermediate levels of P³² on occasion caused increases in dry weight, in nutrient absorption, and in the development of lateral meristems. The interrelation of the observed effects of radiation is discussed; the reduced metabolic activity of root meristems is considered to be the primary effect in the syndrome of radiation injury. Although injury is due to the accumulation of P³² in meristematic regions, no close correlation can be shown between its extent and the level of radiation received by the meristems. The inaccuracy of present methods for estimating the levels of radiation to which tissues are exposed has complicated the investigations of this subject. It appears, however, that the injurious level of radiation may vary from under 10 to over 40 rep. per day when the treatment period is 5 to 7 days. If active accumulation of phosphate occurs, the meristems of plant roots may receive this dosage when treated with more than 5 μ C. of P³² per litre for periods of 6 days. The extent to which the effects of radiation may complicate the interpretation of the results of plant physiological investigations is considered.