Tolerance to ultramafic soils is mediated by control of Ca:Mg ratio in Santolina semidentata Hoffmanns. & Link. and allied taxa within the Santolina rosmarinifolia complex

Santolina semidentata is a Mediterranean shrub which grows on different substrates including ultramafics, but there is no information about the role of phenotypic plasticity or local adaptation in the colonisation of ultramafic soils. We conducted a greenhouse trial in which seeds from three populations of S. semidentata, one population of S. rosmarinifolia, and a population of the ultramafic endemic S. melidensis were cultivated for 17 weeks in three different soils (derived from limestones, granodiorites, and serpentinites). At harvest, we estimated differences between populations in plant survival, plant biomass and leaf ionomes. The five populations had a similar survival on limestone and granodiorite-derived soils, whereas the ultramafic endemic S. melidensis and the ultramafic population of S. semidentata were the only ones able to survive in serpentinites. The two ultramafic populations had lower biomass and phenotypic plasticity than the non-ultramafic ones. Soil type had a major effect on leaf ionomes of the five populations. The ultramafic populations showed consistently higher Ca concentrations and lower Mg concentrations than the non-ultramafic populations. S. melidensis showed unusually high sodium foliar concentrations (around 1%) across the three soils. Our results suggest that the colonisation of ultramafic soils has involved the evolution of a locally adapted ecotype in S. semidentata. Ultramafic tolerance seems to be mediated by efficient mechanisms of regulation of the foliar Ca:Mg ratio. High accumulation of sodium in the leaves of the ultramafic-endemic S. melidensis may be related to mechanisms of tolerance to drought stress in ultramafic areas.