3.6 XZ29 shows increased Pi efflux for less Al accumulation in the root elongation zone
Initial accumulation of Al in root cells, indicated by Morin staining, occurred largely in the root epidermal cells of the meristem and elongation zone after 2 h, and this increased greatly after 6 h (Figure S10a-d). The intensity of Al fluorescence in the epidermal cells of the elongation zone in Al-sensitive XZ9 was much higher than for the same tissues in XZ29 (Figure 7a, 7b). After 24 h, Al accumulation decreased in the epidermal layer and stele of the elongation zone but increased in the cortex (Figure S10e,f). In the mature zone, Al accumulation clearly damaged epidermal cells of both genotypes but this occurred much earlier in XZ9 (after 24 h) than that in XZ29 (after 72 h) (Figure S10e-h). The fluxes of HPO42- from the elongation zone showed significant differences between XZ29 and XZ9. In the control, HPO42- efflux in XZ29 was larger than that in XZ9 (Figure 7c). The addition of Al transiently increased HPO42- efflux in XZ29 by 60% whereas no changes occurred in XZ9. HPO42- efflux then decreased over time in both accessions.