Kinetic analysis of Cd uptake in Cd‐tolerant and intolerant populations of the moss Rhytidiadelphus squarrosus (Hedw.) Warnst and the lichen Peltigera membranacea (Ach.) Nyl.

Abstract

SUMMARYCadmium uptake by photosynthetically Cd‐tolerant and intolerant populations of the moss Rhytidiadelphus squarrosus (Hedw.) Warnst and the lichen Peltigera membranacea (Ach.) Nyl. was investigated. Intolerant plants showed increased intra cellular Cd uptake compared to Cd‐tolerant material. Kinetic analysis indicated that reduced intracellular Cd uptake by Cd‐tolerant plant material was reflected in an increased Km or decreased Vmax for Cd. Pretreatment with KNO3 modified the extra cellular ion content of plant material, significantly reduced the apparent Km and could increase or decrease the Vmax for intracellular Cd uptake. Changes in kinetic constants caused by K pretreatment could be related to altered chemical equilibria when plant material was incubated in Cd solutions. Selective exclusion of Cd from intracellular locations was excluded as a tolerance mechanism in R. squarrosus and P. membranacea. Contrasting kinetic constants for intracellular Cd uptake in intolerant and Cd‐tolerant plant material were due to the prevailing extra cellular ion content when collected from the field. In R. squarrosus, K pretreatment resulted in increased uptake capacities and cell‐wall binding affinities for Cd in field‐grown moss apices. Laboratory‐grown apices, following K pretreatment, displayed increased uptake capacities but reduced affinities for Cd. Increased extra cellular Cd uptake in K‐pretreated moss apices was interpreted as additional weak electrostatic binding. Kinetic analysis indicated that electrostatic binding by extra cellular sites was less influential than the available supply of wall‐bounds ions on intracellular Cd uptake by moss apices.