Nutritional differentiation among populations of the mediterranean shrub Dittrichia viscosa (Asteraceae) in siliceous and calcareous habitats.

Dittrichia (ex Inula) viscosa is a ruderal species that has recently become an invading plant in the northwest Mediterranean basin. A previous study failed to demonstrate the occurrence of morphologically differentiated ecotypes among populations of the species but suggested the existence of nutritional ecotypes. This latter possibility is examined here by comparing the ability of plants from contrasting habitats to control cation accumulation balance. Dittrichia viscosa plants, from eight siliceous habitats and nine calcareous habitats of southern France and neighbouring Spain and Italy, were cloned and grown together hydroponically with a solution simulating an acid soil with an aluminium constraint. Two independent hydroponic units containing solution supplemented with two levels of Al were used (2 Al levels x17 populations x3 genotypes x3 replicates). The growth and cation content (K, Ca, Mg and Na) of plant shoots and the chemical composition of the soil of each habitat were analysed. At the high Al level (1.1MM), populations differed in K, Ca and Mg plant proportions. Two groups could be distinguished: one containing all but one siliceous populations and the other containing all but one calcareous populations. Plants of the siliceous group accumulated proportionally more K and less Ca, and had better growth, than plants of the calcareous group, in the same way as calcifuge and calcicole species when grown on acid soil. At the lowest Al level (0.37MM), differences between siliceous and calcareous populations were less marked. The results suggest that differences in the ability of plants to control K and Ca balance, which appear to be of adaptive significance, could have arisen through selection, and that Dittrichia viscosa has evolved calcifuge and calcicole nutritional ecotypes in siliceous and calcareous habitats respectively. Various degrees of calcifugy, and to a lesser extent of calcicoly, can thus be suggested to occur among the studied populations, some in relation to the intensity of mineral stress in the natural habitats. So far, only functional traits have provided evidence of ecotypic differentiations within Dittrichia viscosa.