Significance and technique of short-term experiments

The significance of short-term absorption periods in experiments on solute absorption by plant tissues is discussed. A technique is described for such short-term experiments. As applied in experiments on ion absorption by barley roots, the technique permits accurate absorption rate determinations to be made in absorption periods of ten minutes or even less. The heterogeneity of cation uptake by this tissue is demonstrated. A readily exchangeable fraction is often present which must be accounted for if rates of metabolically active transport are to be determined. If the results of experiments on the absorption of ions and other solutes by plant tissues are to be amenable to quantitative kinetic inter-pretation, it is preferable, as a rule, to conduct them in such a way as to yield rates or velocities of absorption. The observed rates can then be inserted into kinetic equations or graphs to provide tests of the goodness of fit of the data with theoretical models of solute transport, and to yield values for theoretical maximal velocities at " infinite " substrate concentra-tion, MICHAELIS constants, and others (EPSTEIN and HAGEN, 1). For a number of reasons it is desirable that the actual absorption periods be as brief as possible. These reasons are as follows. 1) During prolonged periods of absorption, rates of absorption will often not remain constant. 2) Respiratory and other metabolic patterns may change as a result of exposure of the tissue to the experimental solutions for long periods. This may make for quantitative and qualitative differences in its absorption behavior. 3) Inclusion of buffers in the experimental solutions is often inadvis-able since the solutes used as buffers may have independent