The effect of carboxylates (organic acid anions) on NH 4 uptake was studied by changing the carboxylate level of roots prior to uptake experi ments. Succinate was the most effective stimulator of ammonium uptake. The oxocarboxylates (α-oxoglutarate, oxaloacetate and pyruvate) and malate also promoted NH 4 entry. Preloading of roots with citrate, acetate, oxalate, glyoxylate or malonate reduced subsequent ammonium uptake. It is concluded that oxocarboxylates are important compounds in both the assimilation and the uptake process of ammonium. All carboxylates tested enhanced K uptake, but repressed NO 3 uptake, except citrate which increased nitrate absorption. Carboxylate stimulation of NH 4 entry showed metabolic as well as non-metabolic aspects.
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