Abstract
Freeze-injured citrus fruit produced above-normal amounts of ethylene 1 to 4 days after injury. Elevated ethylene levels were often found in fruit 3 weeks after injury. Cellulase activity in the abscission zone increased 4 to 8 days after injury and preceded abscission. Some severely injured fruit that did not abscise were responsive to abscission-inducing chemicals. High internal ethylene content did not correlate as well with abscission as did high rates of abscission-zone cellulase activity.
Abstract
Peel injury in 3 citrus fruit cultivars fumigated with ethylene dibromide (EDB) was due to the persistence of resudue of the fumigant in the fruit peel. Initial resudues after fumigation were proportional to the concn and time of exposure. The desorption rate during aeration increased with temp. Incidence of peel injuries was highest in fruit stored at low temp or wrapped in polyethylene bags, probably due to prolonged action of EDB residues on the peel.
Storage of fumigated fruit in an atmosphere containing an increased concn of CO2 delayed the appearance of damage. Susceptibility was greatest in ‘Marsh’ grapefruits, followed by ‘Shamouti’ and ‘Valencia’ oranges.
A cDNA library was constructed from satsuma mandarin (Citrus unshiu Marc.) fruit tissues during the rapid cell enlargement phase. A total of 950 individual cDNA clones was partially sequenced and compared with GenBank databases for characterizing the gene repertoire expressed during this developmental phase. Among these, 426 cDNA clones (44.8%) showed sequence identity with previously characterized genes with optimized (OPT) scores of ≥200, while 524 clones (55.2%) resulted in low OPT scores (<200) and did not show any significant sequence identity with previously published genes. Based on nucleotide sequence, most clones with OPT scores of ≥200 were assumed to be transcription-, translation-, cell-wall-metabolism-, and stress-response-related genes. Other clones showed homology with published sequences related to housekeeping and stress-response-related genes, including metallothionein-like proteins, late-embryogenesis-abundant (LEA) proteins, and heat-shock proteins. These results suggested that Citrus fruit during rapid cell enlargement were metabolically active and expanding in response to biotic and abiotic stress. For clones with low nucleotide sequence homology, the recurrence was evaluated by aligning nucleotide sequences of each clone and generating contig maps. Expressed sequence tags (ESTs) of 162 clones with OPT scores <200 have not been reported for any other organism. Collectively, randomly sequenced cDNA clones described in this study provided information on types of genes expressed during the rapid cell enlargement phase in Citrus fruit. These genes should be used as candidates for Citrus genome mapping projects.
Valencia oranges [Citrus sinensis (L.) Osbeck cv. Valencia] and Marsh grapefruit [Citrus paradisi Macf.] were treated with single or double layers of coating. In cases where two coatings were applied, the first coating was a moisture-barrier wax; the second was either polyethylene wax or a mixture of shellac and resin ester. The inner coating reduced weight loss, and the outer coating imparted gloss. Fruit gloss, as measured by reflectometer, decreased more rapidly during 1 week at 20C with a single glossy coating than with the same coating applied as a second layer over a wax-based first coating. For citrus fruit, using resin ester or shellac as a high-gloss second coating tended to overly restrict the exchange of O2 and CO2; however, two layers of wax did not.
Abstract
Various weak acids produce citrus fruit abscission in Florida. Erythorbic (ascorbic) acid (1-2%) or hexamic acid (1-2%), alone or combined with citric acid (total concn 1-2%), produced acceptable abscission but only of early and midseason oranges for cannery use. Phosphoric acid (1/2%) and ferric chloride (0.5-2.5%) produced erratic loosening, phytotoxicity, severe peel injury, and sometimes damaged spray equipment. The type of peel injury resulting from weak acid sprays may cause extensive rotting (40% or more) if extended periods of wet weather occur prior to picking. As weather forecasting is not sufficiently accurate to predict conditions more than 48 hr in advance, and fruit abscission normally occurs from 3-7 days following spraying, the periodic economic losses that could be expected under these conditions preclude their use in Florida.
Interploid hybridization was conducted using `Key' lime [Citrus aurantifolia (Cristm.) Swing.], `Lakeland' limequat hybrid [C. aurantifolia × Fortunella japonica (Thumb.) Swing.], Palestine sweet lime (C. limettioides Tan.), `Etrog' citron (C. medica L.), and seven lemon [C. limon (L.) Burm. F.] varieties as female progenitors and five allotetraploid somatic hybrids {`Hamlin' sweet orange [C. sinensis (L.) Osbeck] × `Femminello' lemon (C. limon)]; `Key' lime × `Valencia' sweet orange (C. sinensis); `Valencia' sweet orange × rough lemon (C. jambhiri Lush); Milam lemon (purported C. jambhiri hybrid) × `Femminello' lemon (C. limon); and `Valencia' sweet orange × `Femminello' lemon} and two autotetraploids [`Giant Key' lime (C. aurantifolia) and `Femminello' lemon] as pollen progenitors. A few tetraploid × diploid crosses were also performed. Thirty-five parental cross combinations were accomplished in 2000, 2001, and 2002. The breeding targets were seedlessness, cold-tolerance, and disease resistance. Triploid hybrids were recovered through embryo culture. Generation of triploid citrus hybrids was affected by several factors including sexual compatibility, cross direction, embryo developmental stage, pollen viability, as well as horticultural practices and climatic conditions. Efficiency of triploid hybrid production was higher in diploid × tetraploid crosses than the reciprocal. Many more triploid hybrids were generated from lemon seed progenitors compared to the other acid citrus fruit progenitors. `Todo el Año', `Lisbon', and `Limonero Fino 49' showed the highest sexual compatibility. Embryo germination rate and normal plant recovery were also higher in lemons as compared to the other seed progenitors. Low winter temperatures might have affected the hybrid production efficiency from tropical acid fruit progenitors. A total of 650 hybrids (mostly triploid) were transferred to soil. The novel genetic combinations of these progenies should be valuable for the genetic improvement of acid citrus fruit (lemons and limes).
The Citrus Guide, Teaching Healthy Living Through Horticulture (Citrus Guide) is an activity guide designed to help teachers integrate nutrition education into their classrooms. The objectives of this research project were to: 1) help teachers integrate nutrition education, specifically as it relates to citrus fruit, into their curricula by using the Citrus Guide; and 2) evaluate whether students developed more positive attitudes towards citrus fruit by participating in activities from the Citrus Guide. The nutritional attitudes of 157 second through fifth grade students were measured with a citrus fruit preference questionnaire divided into two sections: one targeting citrus fruit and the other targeting citrus snacks. After participating in the activities, no differences were detected in attitudes towards citrus fruit. However, students did have more positive attitudes towards citrus snacks after participating in the activities, with female students and younger students having the greatest improvement in citrus snack attitude scores. Also, there was a direct positive correlation between more grapefruit and oranges consumed daily and students' attitudes towards citrus fruit.
To compare to two types of Citrus fruit rind [i.e., soft type (satsuma mandarin, Citrus unshiu Marc.) and firm type (Hassaku, C. Hassaku Hort. Tanaka)], rind firmness and contents of cell wall polysaccharides were measured from August to January. In August, firmness was measured by a puncture test and found to be ≈3000g in both species. Firmness of satsuma mandarin decreased drastically with time from August to September and decreased slightly thereafter. In contrast, Hassaku firmness increased slightly from August to September, decreased from September to November, and fluctuated. Hassaku firmness, therefore, was significantly higher than satsuma mandarin firmness after September. We measured sugar content in each fraction after fractionalizing cell wall polysaccharides. In flavedo tissue, sugar content in cellulose fraction was the highest, followed by hot-water and EDTA fraction; hemicellulose fraction was the lowest. Although both species were almost the same in sugar content in cellulose and EDTA fraction in August, satsuma mandarin was significantly higher than Hassaku in January. These data showed that changing of rind firmness in citrus was related to the sugar content of cellulose and EDTA fraction in flavedo tissue. In albedo tissue, sugar content in the cellulose fraction was the highest, followed by hemicellulose and hot-water fraction, and EDTA fraction was the lowest. However the extent of seasonal fluctuation in albedo tissue was smaller than that of flavedo tissue, not having any relation to the changing of the firmness.
Citrus fruit with sector chimeras were collected in commercial packinghouses and from the field. Chimeric fruit from eight cultivars of sweet oranges [Citrus sinensis (L.) Osbeck], grapefruit (C. paradisi Macf.), tangelo (C. paradisi × C. reticulate Blanco), and tangors (C. reticulate × c. sinensis) were found at a frequency of 0.009% to 0.271%. Tetraploid plants obtained from one type of sector mutant (termed gigas) and albino plants obtained from another type of sector mutant confirmed that some genetic mutations observed in fruit rind can be recovered in nucellar seedlings. The gigas chimeras were identified as a source of citrus tetraploids. Several types of potentially useful sector mutants with altered rind color were observed, and plants were produced from some mutant sectors by developed seed or culture of aborted ovules. HPLC analysis of rind tissues from sectors of one chimeric fruit revealed substantial quantitative and qualitative differences in pigment composition. Propagation of plants from mutant sectors may yield cultivars with improved fruit color, altered maturation date, and reduced disease or mite susceptibility and may eventually lead to breeding of seedless triploid hybrids.
Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of Citrus L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of Citrus species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [C. sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose. Sucrose concentration was not affected, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.