Exclusion, Amelioration, Tolerance: An investigation of the physiological basis for tolerance in serpentine Mimulus guttatus
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As sessile organisms, plants must cope with a variety of abiotic stress factors, including the mineral composition of soil. In serpentine soils, the low calcium-to-magnesium ratio has been shown to limit the growth and survival of many plant species. Some species are adapted to serpentine, employing a variety of physiological mechanisms. However the physiological basis for tolerance has not been clearly defined. Populations of <italic>Mimulus guttatus</italic> are found growing on and off of serpentine soil and were used here to identify a potential mechanism conferring tolerance to serpentine-adapted populations. Using a combination of soil and hydroponic culture reciprocal transplants, it was demonstrated that the serpentine and non-serpentine populations were differentially adapted to their native soils and thus ecotypes of <italic>M. guttatus</italic>. Biomass and photosynthetic rates of non-serpentine <italic>M. guttatus</italic> are dramatically reduced in low Ca:Mg conditions relative to the serpentine ecotype. Root and shoot concentrations of calcium (Ca) and magnesium (Mg) reflect external treatment conditions and indicate that tolerance to low Ca:Mg in the serpentine ecotype is not due to exclusion of Mg. Leaf expansion and photosynthetic rates of excised tissue from serpentine and non-serpentine plants are lower when exposed to low Ca:Mg conditions than in high Ca:Mg conditions. Recovery of the rates of both processes is observed in serpentine plants with continued exposure to low Ca:Mg, indicating that the tolerance to elevated Mg is through gradual acclimation. Uptake rates of Mg by serpentine <italic>M. guttatus</italic> roots are lower and the proportion of Mg contained in the vacuoles of leaf cells is consistent regardless of the treatment Ca:Mg. The results presented here indicate that the Ca:Mg ratio of the soil is the dominant factor affecting growth and that the tolerance of serpentine <italic>M. guttatus</italic> is through gradual acclimation of physiological processes that is likely through a combination of mechanisms of regulating uptake of Mg and the distribution of Mg throughout the tissues of serpentine <italic>Mimulus guttatus</italic>. These results, together with the recently sequenced genome of <italic>Mimulus guttatus</italic>, may provide a valuable framework for studying the transport mechanisms involved in stress response physiology and the genetic basis for tolerance to serpentine soil.
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