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Barbara Biernacka and Mary Ann Rose

Seasonal patterns of N uptake and allocation in Buxus microphylla, Acer × freemanii and Fothergilla gardenii were investigated for improving fertilization strategies. Rooted cuttings were planted to 3.5-L containers 25 May 1995. Plants were drip-irrigated on an as-needed basis with N at 50, 100, or 200 mg·L–1 solutions. Leaves, stems, and roots were destructively harvested every 6 weeks, starting 24 June. Net changes in dry weight and N uptake were determined for each of four, 6-week periods. Increasing N rate increased tissue N concentration in all species, and reduced root: shoot ratios in Acer and Fothergilla. Dry weights of Acer increased with N rate; whereas other species gave no positive response in dry weight beyond N at 100 mg·L–1. Nitrogen at the 200-mg·L–1 rate caused severe injury to Fothergilla. Nitrogen uptake of the deciduous species increased in the first three periods, with greatest N uptake between 1 Sept. and 12 Oct. Greatest N uptake in Buxus occurred between 15 July and 1 Sept. Total N content in Buxus increased between 15 Oct. an 1 Dec. with a large proportion of N appearing to shift from leaf to stem tissue. In the other species, leaf abscission caused a net reduction in total N contents in the 100 and 200 mg·L–1 rates, although stem and root N contents increased. Increasing N rate in Acer delayed fall coloration but hastened leaf abscission. End-of-season N recovery (N taken up/N applied) was extremely low, and decreased with increasing rate of N. Acer had the highest recoveries (4.1%, 2.6%, and 2.0%) for low, medium, and high fertilizer rates, followed by Buxus (2.6%, 2.2%, 1.0%) and Fothergilla (1.7%, 1.8%, 0.4%).

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Mary Ann Rose and Barbara Biernacka

Seasonal patterns of N, P, and K accumulation and remobilization in Freeman maple (Acer× freemanii E. Murr. `Jeffersred') were studied to guide future improvements in fertilization efficiency. Leaves, stems, and roots of container-grown trees were harvested over a 12-month period (June to June) in each of two experiments. Plants were fertilized from June to October with three rates of soluble fertilizer (50, 100, and 200 mg·L-1 N). Fertilizer rate had linear and quadratic effects on dry weight and nutrient contents, but did not affect seasonal accumulation patterns. Whole-plant nutrient contents and dry weights increased until mid-October, prior to leaf abscission. The largest fractions of nutrients and dry weight were allocated to leaves until early September. Between September and October, the most rapid accumulation of N, P, and dry weight occurred in root tissue. Highest nitrogen recovery efficiency occurred in late summer (Expt. 2) or early fall (Expt. 1). There was no statistically significant evidence for N, P, or K resorption in the fall, but evidence of N (not of P or K) remobilization in the spring was very strong. Whole plant dry weight doubled between April and June, while ≈50% of the N stored in woody tissues was translocated to new shoots.

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Ursula K. Schuch and Barbara Biernacka

Four azalea cultivars [Rhododendron × `White Lace' (WT), `Southern Charm' (SC), `Formosa' (F), and `George Tabor' (GT)] with different growth and flowering habits were treated with a foliar spray of uniconazole (U) at 0, 10, or 15 mg·liter–1 with or without a surfactant. GA was applied at 0 or 15 mg·liter–1 as a foliar spray to half of the plants on 23 Sept. 1993, 53 days after the uniconazole application. U reduced number, length, and dry weight of bypass shoots, and increased number of flower buds for all cultivars by Dec. 1993. Application of GA after U further increased the number of flower buds on SC and GT, which otherwise had few flowers. At the final evaluation in Mar. 1994, time to anthesis for cultivars F and GT was not affected by any treatment. Anthesis of SC and WL treated with 15 mg U and GA/liter started 6 days earlier than those treated with 15 mg U/liter. Number of flowers at anthesis and number of flower buds was increased two to four times on U-treated vs. nontreated plants. U decreased plant height, size, leaf area, and shoot dry weight of all cultivars. Shoot elongation of F and GT was further reduced with the 15 vs. 10 mg U/liter treatment. Application of GA increased the retarding effects of U on plant height for WL, SC, and GT, and on leaf area and shoot dry weight for WL.