Cortical cell fluxes of cobalt in roots and transport to the shoots of ryegrass seedlings

Abstract

Total uptake and transport of 58Co as a function of time were measured in seedlings of Lolium perenne L. cv. Premo, using nutrient solutions containing either 0.1 or 1.0 μM Co2+. After an initial shoulder, uptake was linear and about 15% of the Co absorbed was transported to the shoot after 72 h. Log total uptake and transport as a function of log Co concentration (0.01 to 1.0 μM) were also linear. Co uptake and transport markedly increased with increasing pH but were unaffected by water flux. Compartmental analysis of 58Co efflux data was used to estimate unidirectional fluxes and compartment al concentrations of Co in root cortex, cells. At both levels of external Co, influx to the cytoplasm was passive and cytoplasmic concentrations were comparable. In the 0.1 μM treatment, cytoplasm concentration was controlled by an efflux pump; fluxes across the tonoplast were passive and concentration in the vacuole was small. In the 0.1 μM treatment, the concentration of Co in the cytoplasm was regulated by both an efflux pump at the plasmalemma and an influx pump at the tonoplast. Stored Co in the vacuole was largely unavailable for transport. Factors limiting transport, and the significance of Co depletion in nutrient solutions due to uptake, were discussed. We also established that 0.1 μM Co was sufficient to provide adequate levels of ryegrass shoot Co for ruminant diets.