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

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 μ M 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 μ M ) 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 μ M 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 μ M 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 μ M Co was sufficient to provide adequate levels of ryegrass shoot Co for ruminant diets.     

Physiological Gradients in the Potato Tuber

The distribution of the major cations and anions, together with citric acid, malic acid and iron, has been studied in tubers of Solanum tuberosum cv. Golden Wonder, by analysis of longitudinal cores divided into 16 equal cylindrical sections. In immature daughter tubers the constituents studied were evenly distributed along the tuber, but in mature tubers a considerable polarity had developed. The concentrations of potassium, phosphorus and citric acid each rose from the heel to the rose end of the tuber. Iron showed a gradient in the opposite direction. These gradients were maintained in sprouting tubers, and although after 25 days the levels of these constituents were reduced, the patterns of distribution were unchanged. In the exhausted mother tubers all these constituents were much depleted and only potassium exhibited any pattern of polar distribution. Malic acid was barely detectable at any stage, and calcium, sodium, magnesium, chloride, sulphate and nitrate were always at low levels and showed no polarity of distribution. At each developmental stage potassium was correlated with the difference in electrical potential measured between cells at points across the tuber. A close correlation was apparent between potassium and citric acid content in young, mature and sprouting tubers. In all stages a considerable discrepancy existed between total anions and total cations and this was attributed to the presence of organic acids other than citric acid.