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William S. Castle and James C. Baldwin

origin ( Frost and Soost, 1968 ). After 1 year, the occasional visible off-type was removed from each population of seedlings. Buds were taken from among ≈50 to 75 seedlings of each selection and used for propagation onto Swingle citrumelo rootstock

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Juan Carlos Melgar, Arnold W. Schumann and James P. Syvertsen

water application could alter both daily ET and root and shoot growth of potted Swingle citrumelo [ Citrus paradisi Macfad. × Poncirus trifoliata (L.) Raf.] rootstock seedlings under controlled conditions in a greenhouse. We tested the hypothesis that

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Catalina M. Anderson, William S. Castle and Gloria A. Moore

Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings. Most of the remaining zygotic plants in the rogued population were found among the smaller seedlings. Trees budded on zygotic rootstock seedlings were found in two of the three groves studied, and in some instances an apparent incompatibility was developing in young trees.

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L. Guazzelli, F.S. Davies and J.J. Ferguson

Two experiments were conducted with containerized `Hamlin' orange trees (Citrus sinensis [L.] Osb.) on `Swingle' citrumelo (C. paradisi Macf. × Poncirus trifoliata [L.] Raf.) rootstock to study the effects of N rate on growth of plants in the nursery. Treatments consisted of the following N rates: 12, 50, 100 and 200 mg·liter-1 applied once a week through drip irrigation. In Expt. 1, fertilization at the 200 mg·liter-1 rate resulted in greater scion growth, trunk diameter and total leaf dry weight as compared to the other rates. In Expt. 2, application of 100 and 200 mg·liter-1 rates resulted in greater scion growth and trunk diameter as compared to lower rates, but no differences were seen between the two highest rates. Trees receiving the 12 and 50 mg·liter-1 rates were stunted and leaves were chlorotic. Therefore, the optimum N rate for trees on `Swingle' citrumelo rootstock is between 100 and 200 mg·liter-1, although the 200 mg·liter-1 rate may not be economically justified. Moreover, the N rate for nursery plants growing on `Swingle' citrumelo rootstock in commercial medium may be higher than for other rootstocks, where rates less than 50 mg·liter-1 produce optimum growth.

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Laura Guazzelli, Frederick S. Davies, James J. Ferguson and William S. Castle

Two experiments were conducted with container-grown `Hamlin' orange trees [Citrus sinensis (L.) Osb.] on `Swingle' citrumelo [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock to study the effects of N rate on plant growth in the nursery. Treatments consisted of 12, 50, 100, or 200 mg N/liter per tree applied once a week by drip irrigation. Commercial media was used and soil water content was maintained at container capacity. In Expt. 1, fertilization at 200 mg·liter−1 resulted in greater scion growth, trunk diameter, and total leaf dry weight compared to the other rates. In Expt. 2, fertilization at 100 and 200 mg·liter−1 resulted in greater scion growth,” trunk diameter, and leaf and stem dry weights compared to lower rates, but no differences were observed between the two highest rates. Trees that received 12 and 50 mg·liter−1 were stunted and leaves were chlorotic. Therefore, the optimum calculated N rate for `Hamlin' nursery trees on `Swingle' citrumelo rootstock, based on critical level analysis, is 155 to 165 mg·liter-1.

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Mongi Zekri

Because of the many concerns about fruit quality and fruit production of `Ambersweet' cultivar, this study was conducted in Florida to evaluate the performance of this cultivar budded on two rootstocks and grown in three locations. The effects of Cleopatra mandarin (CM) rootstock on tree growth, yield, fruit quality, and leaf mineral concentration were compared to those of Swingle citrumelo (SC). Although tree shape differed with the rootstock, no consistent difference was found in tree growth between the two rootstocks. Significant differences in yield, fruit size, and fruit quality were found between the two rootstocks. Fruit produced on CM were large with a rough, thick peel and poor color. Swingle citrumelo rootstock promoted higher yield and better fruit and juice quality than CM. Earlier fruit maturity and higher soluble solids and juice content were obtained from trees grown on the Flatwoods compared to trees grown on the central ridge. With the exception of magnesium, no consistent difference in leaf mineral concentration was found between the two rootstocks.

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Kalyani M. Dias and Suzanne M.D. Rogers

This study reports some factors effecting rapid regeneration of Swingle Citrumelo (Citrus paradisi × Poncirus trifoliata).

Inter-nodal stem and root sections from in vitro grown seedlings were shocked with 10, 6, 2 or 0 mg/l BAP for 48 h and then transferred to hormone-free Murashige and Tucker (1961) medium gelled with 2 g/l Gelrite. Explants were cultured horizontally or vertically to study the effect of orientation on shoot initiation.

BAP shock had a pronounced effect on shoot regeneration by root, but not by stem explants. Root explants shocked with 10 mg/l BAP had the highest regeneration frequency. Only vertically placed root and stem explants produced shoots. Shoot buds were first observed in root explants about 10 days after BAP shock. Stem cuttings were slow in producing shoot buds which were first seen after 25 days. A total of 53 shoots were regenerated from 48 root explants while the same number of stem cuttings produced only 11 shoots. When subcultured onto the same medium, more than 85% of the shoots rooted, and were recovered as plants. Explant type, explant orientation and cytokinin shock all influenced regeneration.

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Concetta Licciardello, Biagio Torrisi, Maria Allegra, Fabiola Sciacca, Giancarlo Roccuzzo, Francesco Intrigliolo, Giuseppe Reforgiato Recupero, Paola Tononi, Massimo Delledonne and Vera Muccilli

., 1995 ), and the tolerance or sensitivity to Fe chlorosis varies in different rootstocks with a considerable phenotypic variability. For example, trifoliate orange ( Poncirus trifoliata ) and Swingle citrumelo are considered very susceptible, whereas

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William S. Castle, James C. Baldwin, Ronald P. Muraro and Ramon Littell

shortest trees at both locations were those on Swingle citrumelo (≈2.5 m) and the tallest ones were those on Volkamer lemon (≈3.3 m). After 11 years, mean tree heights had increased to ≈4 m, but again with little difference between locations. The shortest

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William S. Castle, James C. Baldwin and Ronald P. Muraro

elsewhere. The commercial rootstock situation has been relatively stable for many years in Florida and has changed little in recent years. The popular choices have been Swingle citrumelo [ Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] and Carrizo