Abstract
ATP applied in the external medium of leaf slices and bundle-sheath preparations at concentrations between 1.0 and 2.5 mM apparently enhances K+ uptake. However, this effect depends on the presence of Ca2+ or Mg2+ in the medium. ATP is shown to release a Ca2+ or Mg2+ inhibition of K+ uptake. If the Ca2+ or Mg2+ content of the medium is varied and EDTA used to mimic the effect of ATP, it can be demonstrated that the observed effect of ATP is entirely explicable by its chelation of divalent cations. Externally applied ATP has similar relative effects on K+ uptake under control conditions and under conditions where metabolism is inhibited, and although it apparently fully releases metabolic inhibition, these experiments reveal ambiguities which are discussed. It is con- cluded that apparent effects of externally applied ATP may often be indirect. The possibility of specific, i.e. energy-providing, effects can nevertheless not be ruled out in principle, but more sophisti- cated experimentation than found hitherto in the literature is required to demonstrate such effects.
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