Transport of copper and manganese to the xylem exudate of sunflower

Abstract

Absorption of copper and manganese by sunflower roots from solution cultures of varying composition was followed by measuring the concentrations of the metals appearing in whole roots, root cell sap and xylem exudate. Total copper in the fibrous roots was linearly related to the concentration of copper in the external solution but the concentration of copper released to the xylem exudate was buffered somewhat against the changes made externally. No such buffering was observed for managenese. A copper-sensitive electrode, responsive only to free cupric ions was used in conjunction with total copper analysis by atomic absorption spectrophotometry to show that little of the copper (usually < 1%) existed as a free ion in any phase of the system. Copper in the xylem exudate may be strongly complexed. An electron paramagnetic resonance spectrum of the xylem exudate indicated that tnanganese probably was a free divalent ion. Calculation of the electrochemical potential gradient for free cupric ions showed that no special metabolically-linked mechanism need be postulated to account for absorption of copper (or manganese) other than that necessary to maintain the transmembrane potential.

Non-destructive determination of manganese state in rice leaf by X-ray absorption spectrometry

“…"Partially mobile" manganese, as defined by Bucovac and Wittwer (1957), is considered to correspond to a cationic Mn 2+ that may account for most of manganese contained in the sampled leaves with brown spots. Such findings are in agreement with observations showing that soluble manganese in ryegrass occurs in a single cationic, perhaps non-complexed form (Bremner and Knight 1970), and that manganese is absorbed mainly as Mn 2+ and translocated predominantly as free divalent cation in the xylem (Graham 1979). Clarkson and Hanson (1980) reported that manganese had the lowest complex stability constant and thus formed the weakest bonds among the micronutrient transition metals such as manganese, iron, copper, zinc, and molybdenum.…”

Toxicity and Tolerance in the Responses of Plants to Metals

“…The essential acidic L-amino acids give greatest enhancement of Cu uptake; once in the blood stream the Cu is rapidly transferred to its transport protein ceruloplasmin, though a small fraction persists in a ternary (Cu2+-histidine-threonine) complex. There is likewise no necessity to postulate any active mechanism for Cu uptake into plant cells and the evidence suggests transport through the plant in the form of organic complexes which remain to be characterised (GRAHAM 1979). It has frequently been noted in studies of clones of A. tenuis that the Cu requirement for optimal growth is higher for Cu-tolerant than for Cu-susceptible (1971) clones.…”

Metal micronutrients in xylem sap of iron-deficient barley as affected by plant-borne, microbial, and synthetic metal chelators

“…Several authors have used Strategy I plants for studies on mineral ions in the xylem sap and their possible relationship with the phenomenon of metal complexation and transport from roots to shoots (Tiffin 1967;Graham 1979;White et al 1981b). Translocation of nutrients in the xylem sap varies with the plant species.…”