Estimation of the Flux of Ions into and out of the Vacuole of a Plant Cell

When considering the results of experiments on uptake of salts by plant tissues the possibility of simultaneous outflux is usually ignored except some times in relation with the early stages after immersion of the tissue in the solution. It is usual, for example, to consider net uptake of a substance as a function of factors such as its external concentration, 'salt-respiration', and substances and conditions which affect the rate of this process. Perhaps this neglect is intentional with some writers and based on the conclusions which have been drawn from experiments on uptake in which one or both of the ions in the solution have been labelled with a radioactive isotope. Davies and Wilkins (1951) found that after 5 days 10 g. of carrot disks had reduced the concentration of potassium in 170 ml. of 1-05 x1er3 M. KBr containing 42K to 0-05 Xio~3 and the radioactivity of the solution from 3-05 units to 1-96. The potassium originally in the tissue was 26 mM./kg. and they calculate that if isotopic equilibrium had been reached the radio activity would have fallen to 1-22 and state 'therefore K is only 38 per cent, of way to complete isotopic equilibration'. Accepting the figure 1-22, which should on the data be 1-24, this is a curious conclusion. Since the fall in activity is 1 -09 and the maximum fall is 1 -83 it would be right to say 60 per cent, of the way. Apparently they have divided 0-74, the amount the specific activity is short of the equilibrium value, by 1-96, which happens to be the number on the milestone after 5 days and is no measure of the total distance or the distance already covered. Sutcliffe (1954) found that o-8 g. of beet disks in 4 ml. of 0-02 M. KBr containing 42K absorbed 81-5 fi equiv./g. in 24 hours on the basis of chemical analysis and 85-3 on the basis of radioactivity measurements and concludes 'it is certain that at the end of 24 hours most of the potassium originally in the tissue remained unchanged'. His other results show that the potassium in the tissue originally was 50 /a equiv./g. Hence the specific activity of the potassium in the tissue after 24 hours was 85-3/131-5 or 0-647 °f that of the original solution which had thus fallen to 0-795. ^ isotopic equilibrium had been reached then the specific activity throughout would have been 0-667 °f the original and the absorption as indicated by radioactivity would have been