The Physiology of Flowering Plants - KHAM PHA MOI

ION UPTAKE AND TRANSPORT IN THE PLANT 109natural state, for cells other than the outer root cells, the environmentfor ion uptake is the apoplast.Compartmentation of the plant cell and the concept of freespaceThroughout discussions of ion uptake reference is made to ‘inside’and ‘outside’. Plant cells are, however, more complicated than asac surrounded by a membrane. When plant tissue is thoroughlyprewashed and then immersed in a solution containing mineralions, there is first a very rapid uptake, followed by a much slowerrate (Fig. 4.2). When the immersion takes place at low temperature,around 4 8C, or under anaerobic conditions, only the initial rapiduptake occurs, suggesting that the initial uptake is a physicochemicalprocess not requiring metabolic energy. When the tissue is washed indistilled water after a period of uptake, some of the ions taken up arequickly washed out. These ions are said to occupy the ‘water freespace’ which seems to correspond to the cell walls of the tissue. Somemore cations can be removed from the tissue by immersion in asolution of cations, e.g. K + might be washed out in exchange forexcess NH 4 + . The exchangeable cations are again present in the cellwalls, but they are associated with the negative charges on the cellwall polysaccharides and at the plasma membrane surface; these ionsare described as being in the ‘Donnan free space’, named afterDonnan, an investigator in this field. A certain fraction of ions is,however, retained firmly; these are the ions which have passedFig: 4:2 Diagrammaticrepresentation of a typical timecourse of ion uptake by plant tissue.Curve (A) under aerobic conditionsand at a relatively high temperature,say 15–25 8C; curve (B) underanaerobiosis, or in the presence ofrespiratory inhibitors, or at a lowtemperature, say 0–5 8C. The greycurves indicate what happens whenthe tissue is placed in distilled waterat the time indicated by the arrow.The initial rapid uptake lasts 10–20minutes and represents uptakeinto the free space; in water, thefree-space ions are washed out andx 1 = x 2 .