RESUMO
Al resistance of signalgrass (Brachiaria decumbens Stapf cv Basilisk), a widely sown tropical forage grass, is outstanding compared with the closely related 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 alkalinization of the apical rhizosphere are commonly believed to be important mechanisms of Al resistance. However, root apices of signalgrass secreted only moderately larger quantities of organic acids than did those of ruzigrass, and efflux from signalgrass apices was three to 30 times smaller than from apices of Al-resistant genotypes of buckwheat, maize, and wheat (all much more sensitive to Al than signalgrass). In the presence, but not absence, of Al, root apices of signalgrass alkalinized the rhizosphere more than did those of ruzigrass. The latter was associated with a shortening of the alkalinizing zone in Al-intoxicated apices of ruzigrass, indicating that differences 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 strategies appear to operate in this species.
Assuntos
Alumínio/toxicidade , Raízes de Plantas/metabolismo , Poaceae/efeitos dos fármacos , Alumínio/farmacocinética , Resistência a Medicamentos/genética , Genótipo , Inativação Metabólica , Poaceae/fisiologia , Especificidade da EspécieRESUMO
A novel di-hydroxycinnamoylquinic acid ester, 1,3-di-O-trans-feruloylquinic acid (DFQA), was isolated from roots of nutrient-deprived Brachiaria species--the most widely sown tropical forage grasses in South America. In contrast to other so far characterized quinic-acid esters, DFQA exists in a chair conformation with the carboxylic group in the axial orientation. It accumulates in older parts of the root system, but not in root apices or shoots. Higher levels were found in B. ruziziensis, which is poorly adapted to infertile acid soils, than in well adapted B. decumbens. DFQA was also found in the soil, most likely as a result of root decay, because it was not detected in root exudates of plants cultivated in solution culture. Nitrogen and phosphorus deficiency--but not aluminum toxicity or deprivation of other nutrients--stimulated its synthesis in roots. Its accumulation was correlated with a shift in biomass partitioning toward the root system.