CITRUS AREAS: SURFACES AND PRODUCTION
Most of the citrus fruit are grown in two regions of Spain: a) the Levante area, which extends along the East coast and includes the provinces of Castellón, Valencia, Alicante, and Murcia, with about 80% of the total plantations; and b) the Andalusian area, with plantings both along the Guadalquivir river in the provinces of Cordoba and Sevilla, and near the coast, mainly in the provinces of Huelva, Málaga, and Almería, the area representing about 15% of the country's total (4). The rest of citrus area is scattered in many other provinces of Spain (4), mostly near the coast (Fig. 1). Plantings of citrus by varietal groups are listed in Table 1 (5).
peel disorders are key factors affecting external fruit quality for fresh consumption. Different postharvest peel disorders have been described in citrus fruit. However, the responsible causes for many of them are not well understood, their incidence
Hot water immersion of citrus fruit is a potential postharvest quarantine treatment for insect disinfestation. Little is known about fruit injury in the temp. ranges/exposure times required to control surface insects. We immersed lemons in water at 25, 50, 52.5 or 55C for 5, 7.5 or 10 min. Fruits were held overnight at 20, 25 or 30C before hot water immersion. Fruits were stored at 10C for 4 wk after treatment. We compared (1) fresh-picked late-season (July-Aug.) coastal “silver” maturity lemons with (2) fresh-picked ripe but green-colored early/mid-season (Oct.) desert lemons and (3) similar desert lemons commercially degreened 7 days with ethylene to attain desirable yellow color prior to heat treatment. Heat injury symptoms were small-large light-dark brown necrotic lesions or discoloration which developed on peel surface within 2-3 wk after treatment. Order of sensitivity to heat was: most sensitive = coastal silver (≥ 90% of fruit injured at 55C/10 min) > degreened desert > green (≥ 34% of fruit injured at 55C/10 min) desert lemons. Up to 50C/5 min could be used on coastal and 52.5C/5 min on desert lemons without appreciable injury. There were no differences between fruit cured overnight at 20, 25 or 30C before heat treatments.
Randomly amplified polymorphic DNA (RAPD) analysis was used to investigate the histogenic structure of leaf and fruit tissues in four graft chimeras, two intentional chimeras that were produced in combination with `Hamlin' orange [Citrus sinensis (L.) Osbeck] and `Satsuma' mandarin (C. unshiu Marc.), and two naturally occurring periclinal chimera cultivars, Kobayashi Mikan (a graft chimera of C. unshiu and C. natsudaidai Hayata), and Kinkoji Unshu (a graft chimera of C. unshiu and C. obovoidea hort. ex Takahashi). RAPD profiles of the lamina epidermis and the mesophyll cells of specific individuals indicated that the four graft chimeras were interspecific monekto chimeras, whose outermost layer (histogenic layer L-1) of the shoot apical meristem consisted of a species that was different from that in the inner layers (histogenic layers L-2 and L-3). Moreover, juice vesicles, which develop from the inside cells of the pericarp and become the main edible parts of Citrus fruit, were a mixture of the cells from both parental source cultivars. Therefore, the vesicles were at least composed of L-1 and subepidermal inner L-2 cells. This determination of interspecific chimeral construction (which was made possible by molecular techniques) is a valuable finding, in terms of improving Citrus through intentional use of periclinal chimerism.
High-pressure washing (>689 to 3446 kPa or 100 to 500 psi at the spray nozzle) has been used recently in citrus packinghouses to improve the action of surfactant solution and brushing on the removal of dirt and superficial molds. Although high-pressure washing has no obvious detrimental effect on citrus fruit (e.g., no cellular breakage), its effects on physiology have not been fully examined. In this study gas samples were taken from the fruit core of `Orlando' tangelos, `Hamlin' oranges, and `Ruby Red' and white `Marsh' grapefruit prior to and following washing. An apparent wound ethylene response was measured for all varieties and was a function of prolonged exposure (>20 s) and excessive pressure (>2067 kPa). For the responding fruit, internal ethylene was initially detected about 3 h after washing, reached a maximum around 24 h (range: 0.1 to 0.6 ppm), and diminished to near background levels (0.0 ppm) after 48 h. No wound ethylene was observed when fruit were washed for the recommended exposure time (10 s) and pressure (1379 kPa). Concurrent decreases in internal O2 and increases in CO2 were observed for white and red grapefruit. High-pressure washing (1379 or 2757 kPa) did not affect water loss and water, O2, and CO2 exchange. The effects of subsequent waxing of the fruit (increased internal ethylene and CO2 levels and reduced of internal O2 levels) were amplified by washing at the higher pressure (2757 kPa).
Fruit of 11 citrus cultivars were evaluated for their response to the experimental abscission material metsulfuron-methyl at 2 mg·L-1 (ppm) active ingredient as an aid to mechanical or hand harvest. Cultivars evaluated included `Ambersweet', `Glen Navel', `Hamlin', and `Valencia' oranges [Citrus sinensis (L.) Osb.], `Robinson' tangerine (Clementine × Orlando, C. reticulata Blanco), `Sunburst' tangerine [`Robinson' × `Osceola', C. reticulata × (C. paradisi Macf. × C. reticulata)], `Murcott' and `Temple' tangor (C. reticulata × C. sinensis), `Orlando' tangelo (C. reticulata × C. paradisi), `Ray Ruby', and `Marsh' grapefruit (C. paradisi). Six of the 11 cultivars were effectively loosened by sprays of metsulfuron-methyl (`Hamlin', `Valencia', `Orlando', `Murcott', `Temple', and `Ray Ruby'). Addition of an adjuvant (Kinetic, 0.125%) was necessary for abscission activity in fruit and leaves. Trees sprayed with metsulfuron-methyl in combination with an adjuvant had higher percent cumulative fruit drop, higher internal ethylene, and lower fruit detachment forces (FDF) than trees sprayed with metsulfuron-methyl alone. `Sunburst' tangerine responded poorly to the abscission material in the presence or absence of Kinetic. Leaf loss was greatest in trees sprayed with metsulfuron-methyl and adjuvant, intermediate in trees sprayed with metsulfuron-methyl alone, and least in control trees. Twig dieback was observed in trees of `Valencia' orange and `Marsh' grapefruit sprayed with metsulfuron-methyl. The peel of some cultivars had irregular coloration and developed pitted areas after harvest. Although metsulfuron-methyl is an effective abscission agent for mature citrus fruit, further work is needed to more accurately define conditions for its safe and dependable use.
Although no longer as glamorous as it was a few decades past, the routine application of embryo rescue techniques, leading to plant recovery, is a valuable tool for citrus cultivar improvement. Embryo rescue approaches can be used to generate useful variation or to capture various kinds of spontaneous genetic variation. Embryo rescue, by in vitro culture of undeveloped, and presumably unfertilized, ovules in colchicine-supplemented media is a practical method of producing tetraploid clones, which are used then in crosses with diploids to produce seedless triploid hybrids. This same approach, i.e., in vitro culture of undeveloped ovules, is also used to recover plants from chimeric sectored fruit exhibiting economically important mutations for fruit characteristics, and for producing potentially variant somaclones. Seedlessness is an important objective for fresh citrus fruit cultivar improvement, and triploidy following 2x × 4x hybridizations is one approach being exploited for this objective. When monoembryonic diploid seed parents are crossed with tetraploid pollen parents, however, normal seed development is not usually possible. Embryos must be excised from abortive seeds fairly early in development and cultured appropriately to ensure the recovery of sufficient numbers of 3x offspring from these crosses, to increase the likelihood of identifying superior seedless hybrids. These applications will be described in some detail, and progress toward breeding objectives are highlighted.
An approved quarantine treatment for Tephritid fruit fly control of citrus fruit requires fruit be held at 0.0-2.2C for 10-22 days, depending on fruit fly species involved and actual temperature attained. However, this treatment causes chilling injury (CI) in California-Arizona desert lemons harvested in late summer or early autumn. We found that temperatures at which lemons are held before cold treatment affects the susceptibility of lemon fruit to CI. Commercially packed lemons obtained from Yuma, AZ packinghouses in Sept.-Nov. 1987 and 1988 were held at 1C for 3 or 6 weeks, or cured for one week at 5, 15 or 30C, or at 15C for one week, followed by 30C for one week, before receiving the 1C cold treatment. Lemons cured one week at 5 or 15C before the cold treatment developed at least 25-30% less CI during 4 weeks peat treatment storage at 10C than noncured fruit. The other curing treatments were not as effective for reducing CI.
Postharvest pitting of citrus fruit is a recently defined peel disorder that is caused by high-temperature storage (>10°C) of waxed fruit. We examined the anatomy of pitted white grapefruit peel to improve our understanding of this disorder and assist in its diagnosis. Scanning, light, and transmission micrographs showed that postharvest pitting is characterized by the collapse of oil glands. Cells enveloping the oil glands are the cells of primary damage. Oil gland rupture may occur anywhere around the oil gland, but often occurs in regions farthest from the epidermal cells. Adjacent parenchyma cells are damaged as the oil spreads. Epidermal and hypodermal cells are often damaged during severe oil gland collapse. In contrast, chilling injury is characterized by the collapse of epidermal and hypodermal cells. Oil glands are affected only in severe cases of chilling injury. Oleocellosis (oil spotting) is often characterized by the collapse of epidermal and hypodermal cells, but cells enveloping the oil gland are typically not damaged. Physical damage is characterized by damage of epidermal cells, a wound periderm, and presence of secondary pathogens.
An enzymatic peeling process is currently used to produce peeled citrus fruit that are convenient for consumption. By this process, fruit are scored and infused with pectinase or pectinase and cellulase solution and are incubated at 20 to 45C for 0.5 to 2 h. While enzyme solution apparently weakens of the albedo and thus improves separation of the fruit from its peel, we expect that enzyme infused into the flesh reduces storage quality. In these studies, fruit were vacuum- or pressure-infused with or without pectinase in water. The time required to peel white `Marsh' and `Ruby Red' grapefruit infused with solution containing enzyme were only 10% to 20% less than for fruit infused with water alone. `Hamlin' orange and `Orlando' tangelo peeling times were not improved by enzyme treatment. This suggests that water is the primary operative component of the enzyme solution and that the enzyme is an active, but nonessential, supplement. For white grapefruit and oranges stored at 5, 10, 15, or 25C, nonenzyme-treated fruit had significantly less juice leakage than enzyme-treated fruit. For example, 0.2% and 5.0% of the peeled fruit weight was lost by non-enzymatically and enzymatically peeled fruit, respectively, for vacuum-infused oranges stored at 5C for 7 days. Moreover, the enzyme treatment significantly reduced firmness, as determined by a sensory panel. Microbial levels and rates of respiration and ethylene emanation during storage were not significantly affected by enzyme treatment. Similar results were found for vacuum- and pressure-infused fruit.