Absorption and Transport of Sulfate by Wheat at Varying Mannitol Concentration in the Medium

Uptake and transport of sulfate labelled with 35 S have been determined in wheat at varying mannitol concentration in the medium. The magnitude of the apparent free space (AFS) of the roots from decapitated plants was constant up to incipient plasmolysis where the AFS values increased abruptly due to the entrance of the medium into the cell lumen. The sulfate retention of the roots from decapitated plants and from intact plants at a low water uptake increased at non‐plasmolysing mannitol concentrations but fell abruptly at plasmolysis. The retention including the increase at mannitol addition seemed to be effected by an active process. The sulfate transport to the shoot was also increased at a low, practically constant water uptake at the addition of non‐plasmolysing mannitol concentrations. At transpiration‐stimulating conditions the sulfate transport was almost constant while the water uptake was sinking. This apparent constancy depended on a composed effect of the mannitol addition. A non‐plasmolysing concentration in itself tended to increase the sulfate transport and the simultaneously lowered water uptake diminished it. The increase of the sulfate transport to the shoot before plasmolysis in the root seemed to involve stimulation of an active process. A passive mass flow directly from the medium to the shoot could be shown to occur only at a very limited extent or not at all. The concentration of such a flow as compared with unity concentration of the medium was calculated to be maximally 0.03. After plasmolysis in the root the transport seemed to be entirely a mass flow indicating that a diffusion barrier could now be passed by the sulfate. Endodermis is suggested to be this barrier. The concentration of the mass flow was lower than the medium concentration indicating that the endodermis was only partly broken. The effect of a non‐plasmolysing mannitol concentration increasing both the sulfate retention of the root and the sulfate transfer to the shoot cannot be explained but some possibilities are discussed. An explanation by a structural change of the free space is favoured.