Serpentine tolerance in Mimulus guttatus does not rely on exclusion of magnesium

The effect of serpentine soil-like low Ca:Mg ratios on growth was investigated in serpentine-adapted and nonadapted populations of Mimulus guttatus Fischer ex DC through soil and hydroponic reciprocal transplants. Adaptation to Ca:Mg ratios in M. guttatus was measured as differences in biomass accumulation, uptake of Ca and Mg, and photosynthetic rates. Serpentine-adapted plants persisted on both serpentine and nonserpentine soils, but nonadapted plants survived only on nonserpentine soil. When grown hydroponically, a low Ca:Mg ratio decreased the biomass of nonadapted plants but serpentine-adapted plants increased in biomass relative to their growth on high Ca:Mg. Internal concentrations of Ca and Mg mirrored those of the growth solution in both populations; however, serpentine-adapted M. guttatus had a higher shoot:root ratio of Mg when grown in low Ca:Mg solutions. Elevated Mg reduced photosynthetic rates in nonadapted plants without changes in chlorophyll concentration or photosystem efficiency. Hydroponic culture isolated the Ca:Mg ratio from other soil characteristics as the dominant factor affecting growth. Differences in the growth of plants from these populations in reciprocal transplant experiments indicate a genetic basis for a tolerance mechanism to low Ca:Mg, but one that is not based on the exclusion of Mg.