Organic Acid Exudation by Wild Herbs in Response to Elevated Al Concentrations

In acidic soils, monomeric aluminium (Al³⁺) can reach levelsthat are toxic to plants, thus preventing many species fromgrowing there. Organic acids chelate Al and render it non-toxic.It has been shown that exudation of organic acids by Al-tolerantcrops increases their tolerance to Al. We have extended thisobservation to wild plants by comparing the ability of ten herbsto exude organic acids in response to elevated Al levels. Wehypothesized that exudation of organic acids was related tothe ability of plants to grow on Al-rich soils. Two grasseswere grown in rhizotrons in soils with 41 and 63 µM reactiveAl. Organic acids were sampled from root tips connected to anintact plant-root system.Deschampsia flexuosa (L.) Trin. exudedmore 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 µM) in a hydroponic system.Rumex acetosellaL. and Viscaria vulgaris Bernh. increased exudation of oxalicacid and Galium saxatile auct. non L. and Veronica officinalisL. increased exudation of citric acid in response to elevatedAl. The distribution of the forbs in the field as describedby soil pH was negatively related to the amount of organic acidsexuded in response to Al. In contrast, none of the grasses exudedhigher amounts of organic acids with increasing Al concentrationin the hydroponic experiment. Copyright 2001 Annals of BotanyCompany Citrate, malate, aluminium detoxification, rhizotron, hydroponics, rhizosphere, Carex pilulifera,Deschampsia flexuosa , Festuca gigantea, Galium saxatile, Geum urbanum, Holcus mollis,Milium effusum , Rumex acetosella, Veronica officinalis, Viscaria vulgaris