Search Results
Water and carbon budgets of individual citrus fruit were determined throughout their growth to quantify the demand for sucrose and water relative to developmental changes. Fruit transpiration, water accumulation, photosynthesis, respiration, and C gain were measured during this period for grapefruit (Citrus paradisii Macf.) and calamondin (Citrus madurensis Lour.). On a whole-fruit basis, estimated rates of grapefruit transpiration and mean daily water inflow decreased after the first third of development, whereas water apparently was lost freely throughout growth of the smaller, thin-peeled calamondins. Estimates of daily fruit C import remained relatively similar during the majority of grapefruit growth, increasing rapidly only as fruit neared maturation. A similar trend was observed in calamondins, although rates were more variable. Overall, estimated mean daily water inflow into “developing grapefruit decreased relative to that of sucrose inflow, resulting in a progressively higher ratio of sucrose transport to net water inflow. Values for these ratios rose from ≈; 10 to >300 g sucrose/liter of water, reaching levels of net water and sngar transfer that could both be accommodated by citrus phloem alone. Any additional entry into grapefruit appears to have been offset by xylem back-flow, because no other net water influx was observed.
The effect of high-pressure washing (HPW) on the surface morphology and physiology of citrus fruit was examined. Mature white (Citrus paradisi Macf. `Marsh') and red (Citrus paradisi Macf. `Ruby Red') grapefruit, oranges (Citrus sinensis L. `Hamlin'), and tangelos (Citrus reticulata Blanco × Citrus paradisi Macf. `Orlando') were washed on a roller brush bed and under a water spraying system for which water pressure was varied. Washing white grapefruit and oranges for 10 seconds under conventional low water pressure (345 kPa at cone nozzle) had little effect on peel wax fine structure. Washing fruit for 10 seconds under high water pressure (1380 or 2760 kPa at veejet nozzle) removed most epicuticular wax platelets from the surface as well as other surface debris such as sand grains. Despite the removal of epicuticular wax, HPW did not affect whole fruit mass loss or exchange of water, O2, or CO2 at the midsection of the fruit. Analysis of the effect of nozzle pressure (345, 1380, or 2760 kPa), period of exposure (10 or 60 seconds), and wax application on internal gas concentrations 18 hours after washing showed that increasing nozzle pressure increased internal CO2 concentrations while waxing increased internal ethylene and CO2 concentrations and decreased O2 concentrations. An apparent wound ethylene response was often elicited from fruit washed under high pressures (≥2070 kPa) or for long exposure times (≥30 seconds).
Abstract
The effects of ethrel on the color, respiration, ethylene evolution and internal quality of citrus fruit were studied. Concentrations between 100 and 5000 ppm were tested and compared to 50 ppm ethylene treatment. Fruit treated with 1000 ppm ethrel reached a satisfactory color 7 days after treatment at 17°C. Respiration and ethylene evolution levels were higher in ethrel and ethylene treated lemons than in the controls. Five thousand ppm ethrel caused rind injuries to lemons and delayed the degreening of Clementine and Shamouti oranges. Ethrel efficiency was found to be temperature dependent. Waxing the fruit delays the degreening processes. Ethrel induced eifects similar to those of ethylene.
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.
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).
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.
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.
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.
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.
Arsenic compounds generate diverse effects in all living organisms. In citrus (Citrus L. sp.), they reduce acidity and improve fruit quality by unknown mechanisms. The major organic acid in citrus fruit is citric acid, which begins accumulating early in fruit development, reaches a peak in middle-sized fruit and then, in most species, declines as the fruit matures. In an attempt to understand the basis of the effect of arsenite, it was applied to `Minneola' tangelo (Citrus paradisi Macf. × C. reticulata Blanco) ≈6 weeks postanthesis, and a detailed analysis of total titratable acidity and citric acid concentration was performed throughout fruit growth. Within 35 days after arsenite application, total acid content and citrate concentration were slightly lower compared with the controls, and this difference persisted throughout fruit development. The concentrations of other organic acids were not reduced by the treatment. Sodium arsenite reduced the citrate concentration in `Eurieka' lemon callus [Citrus limon (L.) Burm.] also, without affecting tissue growth. Extractable activity of citrate synthase in treated fruit was inhibited within 1 day following arsenite spray, but recovered to a normal level a few days later. In contrast, gene expression was remarkably induced 1 day following treatment, which might explain the recovery in enzyme activity. Data suggest that reduction in acid accumulation may not be related to the initial inhibition of citrate synthase activity.