DIFFERING ORGANIC-ACID EXUDATION PATTERN EXPLAINS CALCIFUGE AND ACIDIFUGE BEHAVIOR OF PLANTS

Many vascular plant species are unable to colonize calcareous sites. T hus, the floristic composition of adjacent limestone and acid silicate soils differs greatly. The inability of calcifuge plants to establish in limestone sites seems related to a low capacity of such plants to solubilize and absorb Fe or phosphate from these soils. Until now, mec hanisms regulating this differing ability of plants to colonize limest one sites have not been elucidated. We propose that contrasting exudat ion of low-molecular organic acids is a major mechanism involved and s how that germinating seeds and young seedlings of limestone plants exu de considerably more di- and tricarboxylic acids than calcifuges, whic h mainly exude monocarboxylic acids. The tricarboxylic citric acid is a powerful extractor of Fe, and the dicarboxylic oxalic acid a very ef fective extractor of phosphate from limestone soils. Monocarboxylic ac ids are very weak in these respects. The study is based on ten species from limestone soils and ten species from acid silicate soils.