The Al content was determined in roots, buds, and stems of dormant florists' hydrangeas [Hydrangea macrophylla subsp. macrophylla var. macrophylla (Thunb.) `Mathilda Gutges' and `Brestenburg'] that were or were not treated in the field with aluminum sulfate. During the greenhouse forcing stage, previously nontreated plants were subjected to four successive weekly subirrigated applications of aluminum sulfate totalling 4, 8, 12, or 16 g/pot. Applications were early (weeks 2, 3, 4, 5) or late (weeks 6, 7, 8, 9), using the start of forcing as week = 0. The Al contents in stems and buds of dormant plants were about five to six times higher in field-treated than in nontreated plants. Roots were the primary location of Al accumulation (≈70%). Aluminum sulfate applications of 12 to 16 g/pot during greenhouse forcing provided commercially acceptable blue plants. Maximum foliar Al concentration was 50% higher in early than in late-treated plants and calculated to occur with 14.5 and 12.2 g aluminum sulfate/pot for early and late-treated plants, respectively. There was a positive correlation (r = 0.74) between blueness ranking and the Al foliar concentration of the two uppermost expanded leaves taken from flowering plants.
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