Carbon Dioxide and pH in Relation to Salt Uptake by Beetroot Tissue

Abstract

The enhancement of cation uptake in beetroot disks with increasing pH is confirmed. In disks taken from growing beet, the presence of 2 per cent carbon dioxide in air results in a further promotion of cation uptake at pH 7.4, but not at lower pH levels. The ability to respond to added carbon dioxide is often absent in disks cut from dormant roots in winter storage, but may be partly restored by pre-washing the disks for a suitable period. The influence of various washing pre-treatrnents indicates that the carbon dioxide effect is most pronounced when excess cation uptake is at a maximum. The additional cation uptake at high pH with or without 2 per cent carbon dioxide, and part of the cation uptake at pH 5.6, is balanced by an equivalent synthesis of organic acids. It is concluded that cation uptake is not primarily determined by the external bicarbonate concentration, and that the pH effect is a genuine one. An electrochemical approach shows that the observed effects of high pH and of carbon dioxide can be largely accounted for in terms of passive diffusion of ions across the cell membrane, whilst cation accumulation at low pH may involve a hydrogen ion excretion pump.