Nutrient Supply and the Growth of the Seminal Root System in Barley

Abstract

A method was devised by which different zones along a single seminal axis of an intact plant could be exposed for extended periods to contrasting concentrations of nitrate (either 0.01 or 1.0 mM) in continuous flow, the supply of all other nutrients being favourable throughout. The concentration of nitrate was found to exert a direct and strictly localized effect upon the growth of lateral roots which, depending upon the supply of assimilates from the shoot, resulted in marked modifications to the form of the root system. Zones receiving 1.0 mM nitrate showed an increase in the number and extension rate of both first- and second-order laterals, associated with a preferential accumulation of dry matter, compared with zones in 0.01 mM nitrate. The average number of laterals (both first and second order) per cm of parent root was 4.4 in the presence of 1.0 mM nitrate and 2.2 in 0.01 mM. The average extension rates of first-order laterals were 0.61 and 0.26 cm d⁻¹ and second-order laterals, 0.10 and 0.05 cm d⁻¹ for nitrate concentrations of 1.0 and 0.01 mM respectively. The precise numbers and extension rates of laterals in any one zone were affected, however, by the rate of growth of laterals in other parts of the root system. In contrast, the extension rates of axes were little affected by the concentration of nitrate to which their apical meristems were exposed and approached 2.0 cm d⁻¹ provided the plants were not nitrogen-starved. The significance of these results to the physiology of root growth and soil-plant relations is discussed.