INTERACTION BETWEEN ALUMINUM TOXICITY AND CALCIUM-UPTAKE AT THE ROOT APEX IN NEAR-ISOGENIC LINES OF WHEAT (TRITICUM-AESTIVUM L) DIFFERING IN ALUMINUM TOLERANCE

Aluminum (Al) is toxic to plants at pH < 5.0 and can begin to inhibit root growth within 3 h in solution experiments. The mechanism by which this occurs is unclear. Disruption of calcium (Ca) uptake by Al has l ong been considered a possible cause of toxicity, and recent work with wheat (Triticum aestivum L. Thell) has demonstrated that Ca uptake at the root apex in an Al-sensitive cultivar (Scout 66) was inhibited mo re than in a tolerant cultivar (Atlas 66) (J.W. Huang, J.E. Shaff, D.L . Grunes, L.V. Kochian [1992] Plant Physiol 98: 230-237). We investiga ted this interaction further in wheat by measuring root growth and Ca uptake in three separate pairs of near-isogenic lines within which pla nts exhibit differential sensitivity-to Al. The vibrating calcium-sele ctive microelectrode technique was used to estimate net Ca uptake at t he root apex of 6-d-old seedlings. Following the addition of 20 or 50 muM AlC13, exchange of Ca for Al in the root apoplasm caused a net Ca efflux from the root for up to 10 min. After 40 min of exposure to 50 muM Al, cell wall exchange had ceased, and Ca uptake in the Al-sensiti ve plants of the near-isogenic lines was inhibited, whereas in the tol erant plants it was either unaffected or stimulated. This provides a g eneral correlation between the inhibition of growth by Al and the redu ction in Ca influx and adds some support to the hypothesis that a Ca/A l interaction may be involved in the primary mechanism of Al toxicity in roots. In some treatments, however, Al was able to inhibit root gro wth significantly without affecting net Ca influx. This suggests that the correlation between inhibition of Ca uptake and the reduction in r oot growth may not be a mechanistic association. The inhibition of Ca uptake by Al is discussed, and we speculate about possible mechanisms of tolerance.