Physiological responses to heterogeneous cadmium exposure in the clonal fern Salvinia: Induced cadmium tolerance, phytochelatins and resource allocation

Three responses of a rhizomatous fern (Salvinia) to heterogeneous cadmium exposure were examined: (1) induction of Cd tolerance in "daughter" ramets as a result of Cd acclimation by their "parent" clones, (2) production of metal-binding peptides and their possible role in induced tolerance, and (3) changes in physiological integration and resource allocation between ramets.; Acclimation of parent ramets of Salvinia minima with 25 ug Cd.1{dollar}sp{lcub}-1{rcub}{dollar} increased the tolerance index (based on chlorophyll-a) of daughters by 13% and 17% over the control in two experiments, whereas 10 ug Cd.1{dollar}sp{lcub}-1{rcub}{dollar} had no such effect. Acclimation with 50 ug Cd.1{dollar}sp{lcub}-1{rcub}{dollar} had variable results. The daughter's tolerance to 15 ug Cd.1{dollar}sp{lcub}-1{rcub}{dollar} was 28% higher than the control in one experiment. In a second experiment, their tolerance to a higher Cd concentration (25 ug.1{dollar}sp{lcub}-1{rcub}{dollar}) was similar to that of the control. One explanation is that the more toxic Cd concentration in the latter test, combined with "carry-over" toxicity caused by acclimation, may have overwhelmed any induced tolerance. In both experiments, the "tolerant" daughters (i.e. based on chlorophyll-a) grew slower than control plants. This dichotomy may be explained if tolerance (maintenance of chlorophyll synthesis) is accompanied by a metabolic cost (glututhione depletion).; Acclimation of parents to Cd increased the levels of Cd-binding phytochelatins (PCs) and thiols in daughters, perhaps due to the translocation of thiol-compounds from parents to daughters through the rhizome. Despite the increases in Cd-binding compounds, and the coincidence of high thiol content with induced tolerance, Cd fractionation data did not support the hypothesis that quantitative Cd-binding by PCs or other thiol-compounds was responsible for tolerance. A qualitative role for PCs in induced tolerance is possible, however.; In Salvinia molesta, the pattern of integration (i.e. resource sharing between ramets) was unaffected by Cd acclimation. Clones were integrated in groups of three adjacent ramets, a pattern which matches the plant's phyllotaxy. Resource allocation from parent ramets to apically-produced daughters was increased by Cd pre-treatment. This increase was balanced by reductions in the biomass, ramet numbers and phosphorus content of laterally-produced daughters. The ecological implications of these physiological responses to heterogeneous Cd exposure for clonal plants in general are discussed.