SPATIAL VARIATION IN THE KINETICS OF ALUMINUM (AL) UPTAKE IN ROOTS OFWHEAT (TRITICUM-AESTIVUM L.) EXHIBITING DIFFERENTIAL RESISTANCE TO AL- EVIDENCE FOR METABOLISM-DEPENDENT EXCLUSION OF AL

We have investigated the kinetics of-Al uptake into various root segme nts and have compared uptake between two wheat (Triticum aestivum L.) cultivars that differ in their ability to resist Al toxicity. Followin g 60 and 180 min of exposure to Al, we found no differences in total A l accumulation between root tips (0-5 mm) and more mature root regions (5-10 and 10-20 mm). However, when roots were exposed to Al and subse quently desorbed in citric acid to remove exchangeable Al in the cell wall and in NH4Cl to remove the mucilage, non-exchangeable Al levels i n root tips were 33 to 37% greater than in mature root regions. When t he rate of accumulation of non-exchangeable Al in 20 mm root tips betw een an Al-resistant cultivar (PT 741) and an Al-sensitive cultivar (Ne epawa) was compared, no differences were observed. When the same compa rison was made in 5 mm root tips, the rate of Al accumulation in the A L-sensitive cultivar was nearly twice that in the Al-resistant cultiva r. Furthermore, the rate of Al uptake in the Al-resistant cultivar inc reased dramatically in the presence of the metabolic inhibitor and pro tonophore, DNP, while DNP had little effect on uptake in the sensitive cultivar. This effect of-DNP was also specific to root tip tissue. Ou r results support the hypothesis that DNP inhibits an AL exclusion mec hanism operating at the plasma membrane in Al-resistant cultivars.