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

The development of the seminal root system, its ability to absorb nitrate, and effects on shoot growth were studied in barley plants in nutrient solution. The roots received either a uniform supply of 1.0 mM nitrate (controls), or a supply of the same solution restricted to a 4-cm length of only one of the main seminal roots (axes) on each plant, the remainder of the root system receiving a solution containing a low concentration (0.01 mM). Marked increases took place in both the growth of lateral roots and the absorption and transport of 15N-labelled nitrate (per unit root weight) from the zone locally supplied with 1.0 mM nitrate. These effects appear largely to compensate for the deficient supply of nitrate to the remainder of the root system, since after 14 d the relative growth rate (g g−1d−1) of the total plant equals that of the controls. Rates of 15N-nitrate uptake (per unit root weight) remain relatively uniform throughout the 29-d experiment, during which root axes develop from their initial unbranched form to a complex system of laterals. The results are discussed in relation to possible mechanisms by which coordination is maintained between root growth, ion uptake, and shoot growth.