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Microcitrus is one of five genera that are partially sexually cross-compatible with the genus Citrus. The genus Microcitrus contains seven species with characteristics that may be valuable for breeding citrus scions and rootstocks, including zygotic embryony, short juvenile period, short fruit maturation time, and resistance to nematodes and Phytophthora. However, relatively few F1 hybrids between Microcitrus and Citrus have been reported, and most of these have been pollen- and ovule-sterile. Some of these intergeneric hybrids have also been highly susceptible to cold damage. To create a genetic bridge for recombination of useful traits from Microcitrus into Citrus, two selections of Citrus ichangensis (an exceptionally cold hardy species with zygotic embryony and short juvenile period) were hybridized with Microcitrus warburgiana and two selections of Microcitrus inodora. Seed were collected from these crosses and germinated in a warm greenhouse. A total of 94 M. inodora xC. ichangensis hybrids and 34 M. warburgiana xC. ichangensis hybrids) were obtained and transplanted to 4-gallon pots in a screenhouse. At 33 months after planting the seed, 42% of the M. inodora × C. ichangensis hybrids and 67% of the M. warburgiana × C. ichangensis hybrids had flowered. Pollen germination tests on agar plates indicated that several hybrids produced large quantities of viable pollen. Numerous crosses were completed using some of these F1 hybrids as pollen and seed parents. Several F1 hybrids were confirmed to be highly fertile by recovery of healthy F2 and backcross hybrids with Microcitrus sp., Citrus sp., Poncirus trifoliata, and other Microcitrus (C. ichangensis) selections.
Citrus tree size and growth form are important traits that can be influenced by the genotype of both scion and rootstock cultivars. However, there have been very few reports of size or growth habit traits within Citrus or sexually compatible genera that might be transmitted genetically in breeding programs. A procumbent growth habit has been described for `Cipo' (Citrus sinensis [L.] Osbeck), a unique sweet orange cultivar maintained in the USDA germplasm repository. Sexual hybrids were produced between this selection and four related species, and these progenies were evaluated for two distinct traits associated with the unusual growth habit of `Cipo'. Inheritance of both drooping petiole and horizontal shoot growth were observed among the `Cipo' hybrids. Investigations are continuing on these four populations to verify segregation patterns and identify individuals possessing favorable combinations of growth habit with other desirable tree characteristics.
`Cipo' sweet orange [Citrus sinensis (L.) Osbeck] is distinctive among citrus selections because of reduced tree height and procumbent growth habit. Open-pollinated seeds were collected from `Cipo' orange and `Pineapple' sweet orange (C. sinensis) at Riverside, California, and grown under cool greenhouse conditions. Seedlings of `Cipo' were relatively uniform in morphology (including drooping shoot habit) and were presumed to be apomicts derived from nucellar embryos. `Cipo' seedlings were distinctly different from `Pineapple' in several characteristics, including smaller shoot altitude/extension ratios (a measure of uprightness) and broader stem-petiole angles (`Cipo' 1.33 radians; `Pineapple' 0.84 radians). The procumbent habit of `Cipo' appeared to be related to a preference for horizontal shoot orientation rather than a weakness of stem structure. Some increased sensitivity to ethylene was observed in the `Cipo' seedlings. `Cipo' is proposed as a resource for hormone research and a potential parent in breeding for unique tree morphology and reduced tree size.
`Cipo' sweet orange [Citrus sinensis (L.) Osbeck] combines typical midseason fruit characteristics with a unique procumbent growth habit. This distinctive habit may be of value in breeding smaller and more procumbent scion cultivars if the growth habit is transmitted to hybrid seedlings. Two hybrid populations were created using `Clementine' mandarin (Citrus reticulata Blanco) as the female parent and either `Cipo' sweet orange or `Pineapple' (another midseason sweet orange with a more typical upright growth habit) as the male parent. The `Clementine' × `Cipo' cross yielded many hybrids with the procumbent habit, many with the upright habit, and some that appeared intermediate. Both hybrid populations were compared with nucellar seedling populations from `Cipo' and `Pineapple' using two morphological characteristics that differentiate between the procumbent habit of `Cipo' and the upright habit of `Pineapple'. All the `Clementine' × `Pineapple' hybrids were of upright growth habit, while the `Clementine' × `Cipo' progeny segregated into two groups based on growth habit (upright and procumbent). The two measured characteristics were tightly correlated in the segregating population and are probably pleiotropic effects of the same genetic mutation. The observed population distributions were as expected if the procumbent habit in `Cipo' is controlled by a single dominant allele in the heterozygous condition.
Long-term identification of individual plants in the field is an important part of many types of research. In a previous report, we described methods for using implanted radiofrequency identification device (RFID) microchips to tag citrus trees for field research. This report provides an update on the RFID technology for use in plants, the effect of implanted chips on long-term plant growth, and survival of the microchips over time. The microchips were found to have no significant effect on plant health and growth, and most microchips continued to work well through the first 6 years after implantation. Implanted RFID microchips appear useful for long-term tagging of citrus and other woody plant species.
Secure identification of individual plants by some kind of labels in the field is an important part of many types of horticultural, plant science, and ecological research. This report describes implanted microchips as one method of plant tagging that is reliable, durable, and secure. This technology may be especially useful in long-term experiments involving perennial woody plants. Two methods are described for implanting microchips in citrus trees that would also be applicable to other woody plant species. One method of implanting microchips is demonstrated to have no deleterious effect on citrus tree growth through the first 18 months after implantation into the tree. Since microchips implanted beneath the bark will become more deeply embedded in wood as the plants grow, signal penetration through wood was evaluated and determined to be sufficient for long-term field utility. Implanted microchips are potentially useful for secure tagging of valuable or endangered plant species to deter theft by providing secure and conclusive identification.
Bending was compared to cutting off for effectiveness in forcing growth of sweet orange and mandarin scions budded on `Carrizo' citrange, `Swingle' citrumelo, and 17 new hybrid citrumelo rootstocks. For both scion types, more than twice as many plants from the bending treatment than the cut treatment had growing scion buds at 12 weeks. This advantage of the bending treatment was similar for most scion/rootstock combinations except with sweet orange scion on `Carrizo', which produced outstanding bud growth from both forcing methods. Length of growing shoots at 12 weeks was >14 times longer from the bending than the cut treatment for both scions and with all rootstocks. Tree survival and yield of usable trees at 35 weeks old were also significantly better for the bending treatment than for the cut treatment. There was an overwhelming advantage to using the bending treatment instead of cutting off in forcing scion bud growth for propagating citrus trees on citrumelo rootstocks.
Phytophthora parasitica Dast. causes several root and trunk diseases of citrus, including damping-off, root rot, foot rot, and gummosis. Phytophthora resistance is needed in Citrus rootstocks and is available in Poncirus trifoliata (L.) Raf. and some hybrids between Citrus and P. trifoliata. Field or greenhouse tests of rootstocks require large amounts of space and time. To provide a preliminary indication of rootstock resistance to P. parasitica, nucellar seedlings of P. trifoliata selections, and Citrus × P. trifoliata hybrids were tested for response to P. parasitica by in vitro inoculation. Seeds of individual selections were germinated in sterile culture and 3-week-old shoots were excised and inoculated with a cultured isolate of Phytophthora. After 1 week of incubation, response to the disease organism was measured by length of stem discoloration. Progression of Phytophthora in the stem also was measured by plating sequential 5-mm segments of the shoot and determining presence or absence of Phytophthora in individual segments. Stem discoloration length corresponded with location of Phytophthora in the stem. Relative resistance, as measured by this technique, approximated field resistance for several common rootstock cultivars. Resistant, intermediate, and susceptible selections were found in populations of Citrus × P. trifoliata rootstock hybrids using in vitro inoculation.
Modern citrus nursery production makes use of potted-tree propagation in greenhouses. Supplemental lighting is one method by which nursery tree growth and profitability may be significantly improved, but limited specific information is available. Five replicated experiments were conducted to determine the utility and effects of increasing daylength during the winter months by supplemental illumination from light-emitting diode (LED) or high-pressure sodium (HPS) lights in citrus nursery propagation. Studies used ‘Valencia’ sweet orange scion, the most common citrus cultivar grown in Florida, and the commercially important rootstocks sour orange, ‘Cleopatra’ mandarin, ‘US-812’, ‘US-897’, ‘US-942’, and ‘US-1516’. Comparisons used the three common types of citrus rootstock propagation: seed, stem cuttings, and micropropagation. Six responses were measured in the lighting experiments, including vegetative growth before budding, scion bud survival, and scion bud growth after budding. Supplemental HPS or LED light to extend daylength to 16 h in the citrus nursery during short-day winter months was observed to be effective in increasing unbudded rootstock liner growth and ‘Valencia’ scion growth on all rootstocks and propagation types. Generally, the positive effect on vegetative growth from an increased daylength was stronger with the HPS light than with LED light, while increasing daylength with LED light, but not HPS light, provided some increased bud growth initiation. Use of HPS or LED supplemental lighting to extend daylength offers significant growth advantage for the citrus nursery industry in winter.