The high level of aluminum resistance in signalgrass is not associated with known mechanisms of external aluminum detoxification in root apices

Al resistance of signalgrass (Brachiaria decumbens Stayf cv Basilisk), a wi dely sown tropical forage grass, is outstanding compared with the closely r elated ruzigrass (Brachiaria ruziziensis Germain and Evrard cv Common) and Al-resistant genotypes of graminaceous crops such as wheat, triticale, and maize. Secretion of organic acids and phosphate by root apices and alkalniz ation of the apical rhizosphere are commonly believed to be important mecha nisms of Al resistance. However, root apices of signalgrass secreted only m oderately larger quantities of organic acids than did those of ruzigrass, a nd efflux from signalgrass apices was three to 30 times smaller than from a pices of Al-resistant genotypes of buckwheat, maize, and wheal tail much mo re sensitive to Al than signalgrass). Ln the presence, but not absence, of Al, root spices of signalgrass alkalinized the rhizosphere more than did th ose of ruzigrass. The latter was associated with a shortening of the alkali nizing zone in Al-intoxicated apices of ruzigrass, indicating that differen ces in alkalinizing power were a consequence, not a cause of, differential Al resistance. These data indicate that the main mechanism of Al resistance in signalgrass does not involve external detoxification of Al. Therefore, highly effective resistance mechanisms based on different physiological str ategies appear to operate in this species.