Comparison of the amount of citric and malic acids in Al media of seven plant species and two cultivars each in five plant species

It was reported that differential Al tolerance between two cultivars of tare (Colocasia esculenta [L.] Schott) did not correspond to the amount of oxalic acid in Al medium (Ma and Miyasaka 1998: Plant Physiol., 118, 861-865). We compared the amount of organic acids, especially, citric and malic acids, in Al media of randomly selected seven plant species (Al tolerance order: Brachiaria brizantha, rice, tea > maize > pea, Cassia tora L. > barley) and of two Al-tolerant and Al-sensitive cultivars each in five plant species (rice, maize, wheat, pea, and sorghum). A larger amount of citric acid was found in the Al medium than in the medium without Al in all the plants except for Brachiaria brizantha and the Al-sensitive cultivar of pea. The largest amount of citric acid was detected in the Al medium of C. tora among the plant species, but its growth was inhibited considerably by both short- and long-term treatments with Al. In maize, wheat, and pea, a larger amount of citric and malic acids was detected in the Al media of Al-tolerant cultivars than in those of Al-sensitive cultivars, but the trends of cultivars of rice and sorghum were opposite. When protoplasts isolated from root-tips of several plant species were treated with Al. no correlation was detected between the amount of citric acid in the Al medium and their tolerance to Al. Thus, we did not observe any correlation of Al tolerance among some plant species or between two cultivars in some plant species with the amount of citric and malic acids in Al media. These results suggest the operation of more effective mechanism(s) in addition to or other than the exudation of citric and malic acids for Al tolerance of some of the plant species and cultivars used in the present investigation.