ALUMINUM EFFECTS ON CALCIUM (CA-45(2-TOLERANT AND ALUMINUM-SENSITIVE WHEAT (TRITICUM-AESTIVUM L) CULTIVARS - DIFFERENTIAL RESPONSES OF THE ROOT APEX VERSUS MATURE ROOT REGIONS()) TRANSLOCATION IN ALUMINUM)

The influence of Al exposure on long-distance Ca2+ translocation from specific root zones (root apex or mature root) to the shoot was studie d in intact seedlings of winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). Seedlings were grow n in 100 mum CaCl2 solution (pH 4.5) for 3 d. Subsequently, a divided chamber technique using Ca-45(2+)-labeled solutions (100 mum CaCl2 Wit h or without 5 or 20 mum AlCl3, pH 4.5) was used to study Ca2+ translo cation from either the terminal 5 to 10 mm of the root or a 10-mm regi on of intact root approximately 50 mm behind the root apex. The Al con centrations used, which were toxic to Scout 66, caused a significant i nhibition of Ca2+ translocation from the apical region of Scout 66 roo ts. The same Al exposures had a much smaller effect on root apical Ca2 + translocation in Atlas 66. When a 10-mm region of the mature root wa s exposed to Ca-45(2+), smaller genotypic differences in the Al effect s on Ca2+ translocation were observed, because the degree of Al-induce d inhibition of Ca2+ translocation was less than that at the root apex . Exposure of the root apex to Al inhibited root elongation by 70 to 9 9% in Scout 66 but had a lesser effect (less than 40% inhibition) in A tlas 66. When a mature root region was exposed to Al, root elongation was not significantly affected in either cultivar. These results demon strate that genotypic differences in Al-induced inhibition of Ca2+ tra nslocation and root growth are localized primarily in the root apex. T he pattern of Ca2+ translocation within the intact root was mainly bas ipetal, with most of the absorbed Ca2+ translocated toward the shoot. A small amount of acropetal Ca2+ translocation from the mature root re gions to the apex was also observed, which accounted for less than 5% of the total Ca2+ translocation within the entire root. Because Ca2+ t ranslocation toward the root apex is limited, most of the Ca2+ needed for normal cellular function in the apex must be absorbed from the ext ernal solution. Thus, continuous Al disruption of Ca2+ absorption into cells of the root apex could alter Ca2+ nutrition and homeostasis in these cells and could play a pivotal role in the mechanisms of Al toxi city in Al-sensitive wheat cultivars.