The influence of ammonium and chloride on potassium and nitrate absorption by barley roots depends on time of exposure and cultivar.

Net uptakes of K(+) and NO(3) (-) were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K(+) and NO(3) (-) uptake in the absence of NH(4) (+) or Cl(-). Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH(4) (+), K(+), Cl(-), or NO(3) (-) affected the cultivars very differently: (a) the presence of NH(4) (+) as NH(4)Cl stimulated net NO(3) (-) uptake in Prato barley but inhibited net NO(3) (-) uptake in Olli barley; (b) Cl(-) inhibited net NO(3) (-) uptake in Prato but had little effect in Olli; and (c) NH(4) (+) as (NH(4))(2)SO(4) inhibited net K(+) uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl(-) initially inhibited net K(+) uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH(4) (+) and Cl(-) uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.