INDUCTION OF CADMIUM TOLERANCE BY ACCLIMATION TRANSFERRED BETWEEN RAMETS OF THE CLONAL FERN SALVINIA-MINIMA BAKER

Changes in the cadmium tolerance of 'daughter' ramets resulting from prior acclimation to Cd by 'parent' plants were examined in the clonal fern Salvinia minima Baker. In two experiments, parents were acclimated to 0, 10, 25 or 50-mu-g Cd l-1, and subsequently produced daughter ramets in a Cd-free medium. Parent acclimation to 25-mu-g Cd l-1 increased the daughter ramets' tolerance index by 13-17% (based on chlorophyll-a content), while 10-mu-g Cd l-1 had no effect. Acclimation to 50-mu-g Cd l-1 produced 'carry-over' toxicity (i.e. reduced growth and chlorosis) in daughter ramets in Cd-free solution. Their tolerance index was 28% higher than the control when tested with 15-mu-g Cd l-1 in one experiment. In contrast, there was no such increase when they were tested with 25-mu-g Cd l-1 in a second experiment. This may be due to the higher Cd concentration in the latter test, combined with carry-over toxicity, overwhelming induced tolerance. 'Tolerant' daughter ramets, which maintained higher chlorophyll concentrations during Cd exposure than control ramets, also grew slower, a seeming contradiction. A possible explanation is that induced tolerance, involving protection of chlorophyll synthesis by phytochelatin binding with Cd, has an accompanying metabolic cost due to glutathione depletion. These data may be the first evidence of (a) induced metal tolerance in a vascular plant other than grasses, and (b) increased metal tolerance of daughter ramets due to acclimation of their parent clones. In patchily metal-contaminated habitats, the latter phenomenon may benefit rhizomatous clonal plants over non-integrated clones or annual plants. Daughter ramets connected to acclimated parents may be more likely to survive in other contaminated patches, increasing the possibility that the clone will grow into uncontaminated areas.