Organic acid exudation by wild herbs in response to elevated Al concentrations

In acidic soils, monomeric aluminium (Al3+) can reach levels that are toxic to plants, thus preventing many species from growing there. Organic acids chelate Al and render it non-toxic. It has been shown that exudation of organic acids by Al-tolerant crops increases their tolerance to Al. We have extended this observation to wild plants by comparing the ability of ten herbs to exude organic acids in response to elevated Al levels. We hypothesized that exudation of organic acids was related to the ability of plants to grow on Al-rich soils. Two grasses were grown in rhizotrons in soils with 41 and 63 muM reactive Al. Organic acids were sampled from root tips connected to an intact plant-root system. Deschampsia flexuosa (L.) Trin. exuded more malic acid when grown in the soil with the highest Al content. Five forbs and five grasses were also exposed to three Al levels (0, 25 and 75 muM) in a hydroponic system. Rumex acetosella L, and Viscaria vulgaris Bernh. increased exudation of oxalic acid and Galium saxatile auct. non L. and Veronica officinalis L, increased exudation of citric acid in response to elevated Al. The distribution of the forbs in the field as described by soil pH was negatively related to the amount of organic acids exuded in response to Al. In contrast, none of the grasses exuded higher amounts of organic acids with increasing Al concentration in the hydroponic experiment. (C) 2001 Annals of Botany Company.