The Response of Serpentine and Non-Serpentine Agrostis Stolonifera to Magnesium and Calcium

Abstract

Serpentine soils often support a sparse and unusual vegetation, the cause of which has been variously attributed to low nutrient levels, high concentrations of heavy metals and an unusually low Ca/Mg ratio. Proctor & Woodell (1975) have provided a recent review of the literature on the ecology of these soils. The present paper is concerned with the adaptations of two races of Agrostis stolonifera L. to a low Ca/Mg ratio in two European serpentine soils. American workers, most recently Madhok & Walker (1969) and Main (1970, 1974), have observed the growth of plants in culture media in which the levels of Ca and Mg have been varied. Madhok & Walker showed that a species endemic to serpentine sites, Helianthus bolanderi Gray ssp. exilis Heiser, could tolerate higher levels of Mg in the culture solutions than a non-serpentine species Helianthus annuus L. The non-serpentine species tended to absorb much more Mg than the serpentine species and they postulated that this 'luxury consumption' of Mg was toxic to the plant. They also showed that the serpentine species contained more Ca in the shoots than the non-serpentine species. Main (1970, 1974) obtained similar results and demonstrated 'luxury consumption' of Mg in a non-serpentine race of Agropyron spicatum (Pursh) Scribn. and a tolerance to high Mg levels in a serpentine race of the same species and also in Poa curtifolia Scribn., a species endemic to serpentine sites. The serpentine race and endemic species also showed a higher Ca uptake. Both Madhok & Walker and Main showed that the serpentine plants required a higher level of Mg in the culture solutions to obtain maximum growth than non-serpentine control plants. For example Madhok & Walker found that the nonserpentine species, H. annuus, reached maximum growth when a 2 mM Mg solution was supplied compared to the serpentine endemic species H. bolanderi ssp. exilis which required a 10 mM solution of Mg. Moreover the serpentine species had a higher Mg concentration in its tissues at the point of maximum growth (17-0-27-0 m-equiv./100 g) compared to that of the non-serpentine species (9-0-15-0 m-equiv./100 g). There is a shortage of similar work on European serpentine plants. Recent analyses of plants and soils from European serpentine sites by Johnston (1974) and Shewry & Peterson (1975) have shown that species differ in the amounts of Mg and Ca taken up from the soil. This may indicate that different tolerance or exclusion mechanisms occur in different species, although the authors attempted no experimental work on the species which they analysed. We have investigated the Ca and Mg nutrition of races of Agrostis stolonifera from two serpentine soils, at Kittelfjall, Sweden (Latitude c. 65?N, Longitude 15?45'E) and the Keen of Hamar, Unst, Shetland (Latitude c. 60?50'N,