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Anwar G. Ali and Carol J. Lovatt

This study investigated the effect of irrigation treatments and time of foliar applications of low-biuret urea on yield of 30-yr-old navel orange trees grown under optimal N fertilization. The experiment was carried out at the Agricultural Experiment Station of the University of California, Riverside, using split plot design with 12 replications. The two irrigation treatments were the mainplots and the urea applications were the subplots. Irrigation was withheld from one set of trees from October to March, the other set was irrigated according to commercial practice. Foliar-urea (0.17 kg/tree) was applied in November, December, January, or February.

The results of the first year showed no significant differences between irrigation treatments with regard to total fruit weight or total number of fruit per tree, All trees receiving urea had significantly higher fruit weight and fruit number per tree than the control trees. The specific time of urea application had no significant effect.

With respect to fruit size distribution, the irrigated treatment resulted in a significantly higher number of fruit of size 7.0-8.0 cm (box sizes 88-72). Generally, the non-irrigatd trees had more fruit of size 6.0 cm or less. No interaction between irrigation treatments and foliar urea sprays was observed.

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Izulme R. Santos and Cecil Stushnoff

Economically, citrus is the second most-important fruit crop grown worldwide; thus germplasm conservation of commercial cultivars, as well as of wild relatives, is essential. Presently, citrus germplasm has been conserved mainly in field genebanks. This approach is helpful; however, it is costly, exposes germplasm to climatic and biological hazards, and is not a long-term conservation system. Cryopreservation (conservation in liquid nitrogen, at –150°C to –196°C) is a technique that can ensure long-term storage of plant material. Attempts to cryopreserve citrus are restricted to a few reports, but the results obtained are encouraging. The basic purpose of this study is to define cryopreservation protocols for embryo axes and axillary buds of `Pineapple' sweet orange using the encapsulation-dehydration method. Embryo axes encapsualted in Na-alginate beads, precultured with high levels of sucrose and dehydrated over silica gel before freezing in liquid nitrogen had 60% survival. No survival was obtained for buds treated the same way, however buds isolated from plants acclimated at 0°C over a 30-day period survived exposure to –20°C when slow cooled at 2°C/hour. Additional experiments will combine cold acclimation, slow cooling and pre-treatment with sugars and other chemical compounds as an attempt to enhance cold hardiness of axillary buds and obtain survival after freezing in liquid nitrogen. Different approaches will be used to increase embryo axes survival rates.

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Charles A. Powell and Robert R. Pelosi

Sixty-eight percent of the `Pineapple', 52% of the `Navel', 46% of the `Valencia', 38% of the `Hamlin', and 0% of the `Ambersweet' orange [Citrus sinensis (L.) Osh.] trees in five Florida citrus nurseries were infected with severe strains of citrus tristeza virus (CTV), as demonstrated by reaction with a monoclinal antibody specific for severe strains of the virus. Severe strains of CTV infected 4%, 46%, 76%, 30%, and 48% of the trees at each of the five nurseries, respectively, indicating a considerable difference in severe strain prevalence among the nurseries. Thirty-five percent of the trees in the scion blocks (budwood source) of the nurseries also contained severe strains of CTV.

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Brandon R. Hockema and Ed Etxeberria

The nature of sink strength in orange fruit and changes occurring during drought stress were investigated. Potted trees of `Hamlin' orange [Citrus sinensis (L.) Osbeck] grafted on Troyer citrange [Citrus sinensis × Poncirus trifoliata (L.) Raf.] were irrigated using a microsprinkler system creating either well-watered or water-stressed conditions, as determined by stem water potentials. Fruit were harvested every other week from trees of both well-watered and drought-stressed treatments during the final stage of fruit development when sugars accumulate rapidly. Fruit quality indices and activities of sucrose synthase (SuSy), invertase, sucrose-P synthase, sucrose-P phosphatase, V-ATPase, and V-PPase were measured. Acids and soluble sugar concentrations were elevated in drought-stressed fruit, whereas juice pH decreased in those same fruit. Results indicate that increased sink strength in fruit from stressed trees was accompanied by an increase in SuSy activity and lowered juice pH. The remaining enzymes examined in this experiment showed no changes in activity between control and treated fruit, as was the case for plasmalemma and tonoplast sucrose carriers. Based on the present data, we conclude that SuSy and vacuolar pH are the predominant factors controlling photoassimilate accumulation in orange fruit under enhanced sink conditions brought about by imposition of a mild drought stress.

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J.G. Williamson, W.S. Castle, and K.E. Koch

Three commercially employed treatments to force scion bud growth were compared using greenhouse-grown `Carrizo' citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings budded with `Hamlin' orange [Citrus sinensis (L.) Osb.]. Scion buds were forced either by 1) cutting off (removing the rootstock shoot above the bud union); 2) lopping (cutting half-way through the rootstock shoot above the bud union and breaking over the rootstock stem); or 3) bending (bending the rootstock shoot to its base and tying it in position). Plants were harvested, dried, and weighed at various times after scion shoot emergence. Plants on which rootstock shoots remained attached (lopping or bending) had the largest scion leaf area and gained the most scion and whole-plant dry weight. Bending rootstock shoots also resulted in a longer scion, more leaves, and higher root dry weight than did removal of rootstock shoots by cutting off. Few differences in overall growth were observed among trees retaining their rootstock shoots after two scion growth flushes. Removal of rootstock shoots after the first scion growth flush reduced leaf area and dry weight gain of the second scion growth flush; however, it did not affect total scion leaf area or dry weight. Analysis of 14C-photosynthate transfer from labeled rootstock leaves showed that bending allowed the greatest movement of labeled assimilates to other plant parts during the 24 hours after 14CO2 fixation. Radiolabeled photosynthates from rootstock leaves were partitioned primarily to shoots during scion growth flushes and to roots during periods between growth flushes.

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Charles A. Powell, Robert R. Pelosi, and Phyllis A. Rundell

None of 4190 sweet orange [Citrus sinensis (L.) Osb.] nursery trees of `Hamlin', `Midsweet', `Navel', and `Valencia' sampled from five Florida citrus nurseries were infected with a decline-inducing isolate of citrus tristeza virus (CTV) as judged by enzyme-linked immunosorbent assay (ELISA) using isolate-specific monoclonal antibodies. Two of the nurseries had a relatively high level of infection (37% to 100% of composite samples containing tissue from 10 trees) with nondecline-inducing (mild) isolates of CTV, depending on the cultivar. Three of the nurseries had a lower incidence of mild CTV (0% to 22% of 10 tree composite samples). No nursery was CTV-free. ELISA of individual trees used as budwood sources by the nurseries revealed that one tree out of 260 tested contained decline-inducing CTV, and 83 contained mild CTV. These results suggest that the budwood certification program adopted in 1997 has virtually eliminated decline-inducing CTV from commercial budwood supplies.

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Mosbah M. Kushad, Andrea R. Orvos, and George Yelenosky

Several stages of citrus (Citrus sinensis L. Osbeck. cv. Valencia) flowers, from very small bud (stage 1) to anthesis (stage 6), were evaluated for free and conjugated polyamines. The concentration of putrescine and spermidine synthesis increased markedly during the early stages, and then declined as the flower buds grew. At anthesis, putrescine and spermidine concentrations had increased significantly. Spermine concentration was very low and showed no significant change during the first three floral developmental stages; however, by stages 5 and 6, spermine concentration showed a slight but significant increase. Eighty percent of the total polyamine content in fully developed flowers is localized in the reproductive organs and only 20% is localized in the petals and the calyx. This study relates changes in conjugated and free polyamines to citrus flower growth.

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Thomas E. Marler and Frederick S. Davies

Growth responses of young `Hamlin' orange [Citrus sinensis (L.) Osbeck] on sour orange (C. aurantium L.) trees to microsprinkler irrigation were studied under field conditions from 1985 to 1987 to determine the most-efficient irrigation rates and duration. Trees were irrigated when available soil water depletion (SWD) reached 20% (high frequency), 45% (moderate frequency), and 65% (low frequency). Trees at the moderate and low levels received 49% and 13%, respectively, as much irrigation water as the high treatment. Canopy volume, trunk cross-sectional area, dry weight, shoot length, leaf area, total root dry weight and volume, and new root dry weight were similar for the high and moderate levels in 2 of 3 years, but were significantly reduced at the low level. Summer and fall growth flushes were delayed or did not occur at the moderate and low levels. More than 90% of root dry weight was within 80 cm of the trunk at the end of the first growing season.

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Amir Heiman and Eliezer E. Goldschmidt

The share of brand-name fruit and vegetables is much lower than that for general foods or processed industrial foods. The paper analyzes consumers' choice between generic and brand-name products and shows that consumers' preferences for horticultural brands increase with their appreciation of quality and with a low quality of the generic products, and that brand preferences are affected by socio-economic variables. The theoretical findings are supported by an orange (Citrus sinensis) consumer survey held in the UK and Israel. We found that in both countries consumers assign a relatively low value to orange brands. Consumers who are willing to pay for better quality are those who perceive brands as more important.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

Juice quality of `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks was determined for fruit harvested by canopy quadrant and separated into size categories to ascertain the direct role of rootstock selection on juice soluble solids concentration (SSC) and soluble solids (SS) production per tree of citrus fruit. SS production per fruit and per tree for each size category was calculated. Juice quality was dependent on rootstock selection and fruit size, but independent of canopy quadrant. Fruit from trees on Carrizo citrange had >20% higher SSCs than fruit from trees on rough lemon, even for fruit of the same size. Large fruit accumulated more SS per fruit than smaller fruit, despite lower juice content and SSC. Within rootstocks, SS content per fruit decreased with decreasing fruit size, even though SSC increased. Rootstock effect on juice quality was a direct rather than an indirect one mediated through differences in fruit size. The conventional interpretation of juice quality data that differences in SSC among treatments, e.g., rootstocks or irrigation levels, or fruit size, are due to “dilution” of SS as a result of differences in fruit size and, hence, juice volume, is only partly supported by these data. Rather, accumulation of SS was greater for fruit from trees on Carrizo citrange than rough lemon by 25% to 30%.