Figure 4.

Efflux of oxalate, citrate, and malate from root apices (means ± se of three to five groups of 30 apices each). Ruzigrass (upper row) and signalgrass (lower row) were cultivated as in Figure 2. Organic acids secreted by root apices were collected during a 2-h incubation of excised apices in 200 μm CaCl₂ (pH 4.2), containing a varying level of Al. Dotted lines indicate approximate detection limits. Efflux of citrate from apices of plants cultivated in the absence of Al, and efflux of malate from apices of ruzigrass, were below the limit of detection.