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

Abstract

Responses to a localized supply of phosphate were studied in barley grown in continuous flow solution culture. Root systems were either uniformly supplied with 50 μM phosphate (controls) or the same solution was supplied to only a 4 cm or 2 cm length of a seminal root (localized supply), the remainder of the root system receiving a nutrient solution lacking phosphate. Little development of laterals occurred on those parts of the root system receiving no phosphate from the external solution, while an increase in the number and extension of laterals took place in the 4 cm zone enriched with phosphate. Compared with similar zones on controls, the total length of laterals was increased 15-fold in 21 d plants. In addition, rates of ³²P-phosphate uptake and translocation to shoots per unit root weight were higher than in controls by a factor of 2·5–5·0. Further increases in the growth of lateral roots, and rates of phosphate uptake, were induced when the segment initially supplied with phosphate was restricted to only 2 cm. These localized modifications to root growth and uptake of phosphate largely compensated for the deficient supply of phosphate to the remainder of the root system. After an early period of retarded growth and phosphate stress, the relative growth rate of plants and the concentration of phosphate in shoots were restored to levels similar to that of the controls. The manner in which the supply of phosphate may control root development, and the nature of the co-ordination between root growth, phosphate uptake, and shoot growth, are discussed.