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© 1974 Oxford University Press

RESEARCH-ARTICLE

The Transport of Potassium to the Xylem Exudate of Ryegrass

II. EXUDATION

JAMES DUNLOP

Grasslands Division, D.S.I.R. Palmerston North, New Zealand

The membrane potentials of ryegrass root cells (Ev0) were found to be linearly related to the logarithm of the external KCl concentration ([KCl]o), over the range 0.1 to 20.0 mM. Exudation was studied over the same concentration range. The concentration of potassium in the exudate did not vary significantly with [KCl]0 but the rates of movement of water and potassium to the exudate (fH2O and fK respectively) and the electrical potential and electrochemical potential for potassium in the exudate (Exo and {Delta}x0,K respectively) all tended to decrease as [KCl]0 increased. There was a very highly significant correlation between fK and fH2O.

By rapidly increasing [KCl]E0 and following the depolarization, two components of Ex0 were observed. The first of these was instantaneous and was attributed to Ev0 of the epidermal cells. The second component, a gradual repolarisation which commenced about 9 min later, was attributed to Ev0 of the stelar cells. With an additional contribution from electro-osmosis, these two components quantitatively account for Ex0.

The implications of these data for the mechanism of radial ion transport in roots are discussed and it is concluded that the stelar cells are not exclusively specialized for transporting potassium into the xylem vessels.


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