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Victor Medina-Urrutia, Karla Fabiola, Lopez Madera, Patricia Serrano, G. Ananthakrishnan, Jude W. Grosser and Wenwu Guo

No presently available rootstock combines all the available rootstock attributes necessary for efficient long-term citriculture (production and harvesting) of Mexican limes and other commercially important scions. In the present study, somatic hybridization techniques were used to combine the widely adapted Amblycarpa mandarin (also known as Nasnaran mandarin) with six different trifoliate/trifoliate hybrid selections: Benton, Carrizo, and C-35 citranges; Flying Dragon and Rubidoux trifoliate oranges; and a somatic hybrid of sour orange + Flying Dragon. The ultimate goal of this research is to generate polyploid somatic hybrids that express the complementary horticultural and disease resistance attributes of the corresponding parents, and have direct potential as improved tree-size controlling rootstocks. Somatic hybrids from all six parental combinations were confirmed by a combination of leaf morphology, flow cytometry, and randomly amplified polymorphic DNA (RAPD) (for nuclear hybridity) and cleaved amplified polymorphic sequence (CAPS) analyses (for mtDNA and cpDNA). This is the first report of citrus somatic hybridization using Amblycarpa mandarin. Unexpected hexaploid somatic hybrid plants were recovered from the fusion of Amblycarpa mandarin + C-35 citrange. Hexaploid hybrids should be very dwarfing and may have potential for producing potted ornamental citrus. Resulting somatic hybrid plants from all six combinations have been propagated by tissue culture and/or rooted cuttings and are being prepared for commercial field evaluation for their potential as improved rootstocks for Mexican lime and other important scions.

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T.A. Wheaton, J.D. Whitney, W.S. Castle, R.P. Muraro, H.W. Browning and D.P.H. Tucker

A factorial experiment begun in 1980 included `Hamlin' and `Valencia' sweet-orange scions [Citrus sinensis (L.) Osb.], and Milam lemon (C. jambhiri Lush) and Rusk citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] rootstocks, tree topping heights of 3.7 and 5.5 m, between-row spacings of 4.5 and 6.0 m, and in-row spacings of 2.5 and 4.5 m. The spacing combinations provided tree densities of 370, 494, 667, and 889 trees ha. Yield increased with increasing tree density during the early years of production. For tree ages 9 to 13 years, however, there was no consistent relationship between yield and tree density. Rusk citrange, a rootstock of moderate vigor, produced smaller trees and better yield, fruit quality, and economic returns than Milam lemon, a vigorous rootstock. After filling their allocated space, yield and fruit quality of trees on Milam rootstock declined with increasing tree density at the lower topping height. Cumulative economic returns at year 13 were not related to tree density.

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Gregory L. Reighard

New foreign rootstocks for peaches [Prunus persica (L.) Batsch] are now being introduced into the United States through commercial nurseries for future sales to stone fruit growers. Almost all of these rootstocks are complex Prunus L. hybrids that are propagated vegetatively. Past experience with foreign Prunus rootstocks has shown that extensive testing is critical to avoid potential problems in commercial situations due to nonadaptation of some rootstocks to North American climatic and edaphic conditions. In addition, putative resistance of introduced rootstocks to common soil diseases and other pathogens has not always carried over to orchard sites in the United States. To ensure widespread horticultural testing of new rootstocks, the NC-140 regional research group continues to serve as an unbiased tester in many different geographic and production areas of the United States and Canada.

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J.W. Grosser, J. Jiang, E.S. Louzada, J.L. Chandler and F.G. Gmitter Jr.

Production of tetraploid somatic hybrids that combine complementary diploid rootstock germplasm via protoplast fusion has become a practical strategy for citrus rootstock improvement, with the overall objective of packaging necessary disease and pest resistance into horticulturally desirable, widely adapted rootstocks. Citrus somatic hybridization techniques have been advanced to the point where numerous somatic hybrid rootstocks can now be produced and propagated for evaluation on a timely basis. Herein we report the production of 11 new somatic hybrid rootstock candidates from 12 different parents, including Milam lemon hybrid (Citrus jambhiri Lush.), Cleopatra mandarin (C. reticulata Blanco), sour orange (C. aurantium L.), `Succari' sweet orange [C. sinensis (L.) Osbeck], `Redblush' grapefruit (C. paradisi Macf.), `Nova' tangelo [C. reticulata × (C. paradisi × C. reticulata)], `Kinkoji' (C. obovoidea Hort. Ex Takahashi), Swingle citrumelo [C. paradisi × Poncirus trifoliata (L.) Raf.], Carrizo citrange (C. sinensis × P. trifoliata), rough lemon 8166 (C. jambhiri), and Palestine sweet lime (C. limettoides Tan.). All hybrids were confirmed by cytological and VNTR-PCR analyses, and have been propagated, budded with a commercial scion, and field-planted for performance evaluation.

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Jude W. Grosser

Citrus protoplast technology has advanced to where several practical applications in variety improvement and plant pathology are routine. We will report on progress in the following areas: somaclonal variation—`Valencia` and `Hamlin' sweet orange protoclones have been selected for improved juice color, higher soluble solids, seedlessness, and altered maturity dates; somatic hybridization for scion improvement—allotetraploid breeding parents have been created from numerous combinations of elite parental material, and are now being used as pollen parents in interploid crosses to produce seedless triploid varieties; somatic hybridization for rootstock improvement—numerous somatic hybrids combining complementary rootstock germplasm are under commercial evaluation and several look promising for wide adaptation, improved disease resistance, and tree size control; transformation—an alternative protoplast-based transformation that utilizes EGFP for selection has been developed; virus resistance assays—a protoplast-based assay is being used to screen varieties and candidate sequences for resistance to citrus tristeza virus at the cell level, saving time and greenhouse space.

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Lisa McFadyen, David Robertson, Margaret Sedgley, Paul Kristiansen and Trevor Olesen

Yields of macadamia (Macadamia integrifolia, M. tetraphylla, and hybrids) orchards tend to increase with increasing tree size up to ≈94% light interception. Beyond this, there is some indication that crowding leads to yield decline, but the evidence is limited to one site. Increasing tree size and orchard crowding also present numerous management problems, including soil erosion, harvest delays, and increased pest and disease pressure. The aim of this study was to better characterize long-term yield trends in mature orchards and to assess the effects of manual and mechanical pruning strategies on yield, nut characteristics, tree size, and economics. We monitored yield at four sites in mature ‘344’ and ‘246’ orchards for up to seven years and confirmed a decline in yield with crowding for three of the sites. There was a small increase in yield over time at the fourth site, which may reflect the lower initial level of crowding and shorter monitoring period compared with the other sites, and highlights the need for long-term records to establish yield trends. Pruning to remove several large limbs from ‘246’ trees to improve light penetration into the canopy increased yield relative to control trees but the effect was short-lived and not cost-effective. Removal of a codominant leader from ‘344’ trees reduced yield by 21%. Annual side-hedging of ‘246’ trees reduced yield by 12% and mechanical topping of ‘344’ trees caused a substantial reduction in yield of up to 50%. Removal of limbs in the upper canopy to reduce the height of ‘344’ trees had less effect on yield than topping but re-pruning was not practical because of the extensive regrowth around the pruning cuts. Tree size control is necessary for efficient orchard management, but in this study, pruning strategies that controlled tree size also reduced yield. Research into the physiological response to pruning in macadamia is required to improve outcomes.

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François Mademba-Sy, Zacharie Lemerre-Desprez and Stéphane Lebegin

topography hinders mechanical harvesting techniques. Various procedures to tree size control based on the use of pathogenic agents ( Broadbent et al., 1986 ; Golomb, 1988 ), and specific horticultural or cultural techniques ( Golomb, 1988 ; Krezdorn, 1978

Open access

Haijun Zhu and Eric T. Stafne

biosynthesis, offers a superior method of tree size control of pecan compared with traditional and generally unsuccessful pruning or hedging methods ( Sparks, 1979 ). Previous research indicated the strong effect of PBZ on pecan seedlings grown in a greenhouse

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William S. Castle

usually selected for tree size control and their tolerance to various pests or diseases. Another distinction is the nature of the citrus fruit itself and its morphology and physiology. The citrus fruit consists of small sacs filled with juice. The

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Haishan An, Feixiong Luo, Ting Wu, Yi Wang, Xuefeng Xu, Xinzhong Zhang and Zhenhai Han

associated with dwarfing habit. A majority of standard or seedling rootstocks exhibited a deep root development ( Manschadi et al., 2008 ). Most East Malling apple dwarfing rootstocks or their descendants have shown excellent capacity for apple tree size