Lemon trees [Citrus limon (L.) Burm.[grown outdoors were moved into three greenhouses in Sept. 1993 before hot summer temperatures waned. Greenhouse (grh.) temperatures were 22 to 28C min./35 to 40C max. to maintain the hot growing condition effect. In late October, one grh. was set to 5 to 10C min./20C max. (cool), one to 20C min./33C max. (warm), and one to 25 to 30C min./35 to 40C max. (hot). Fruit was harvested after 1-, 2-, and 4-week exposures. Postharvest treatments applied 1 day after pick were hot water immersion (HWI) (55C/5 min.), cold (CT) (2C/3 weeks), or methyl bromide (MB) fumigation (60 g/m3/2 h/21C). Fruit was stored at 10C for 4 weeks after treatment and evaluated weekly. HWI caused more peel injury to fruit from cool (82%) and warm (60%) grhs. than from the hot grh. (32%). The incidence and severity of injuries increased with time fruit were held in the cooler grhs. CT caused more peel injury in fruit from the hot (94%) grh. than from the warm (48%) or cool (16%) grh. Severity of injury decreased with longer exposure in the cooler grhs. MB injury was least in fruit from the cool and warm grhs. (2% to 3%) and most from the hot grh. (14%). MB injury increased slightly with fruit age. In HWI, CT and MB tests `Eureka' fruit were injured more than `Lisbon'.
In accordance with the currently approved Australian citrus disinfestation protocol for export to Japan, degreened `Eureka' lemons [Citrus limon (L.) Burm.] were cold-stored for 2 weeks at 1C. Following cold treatment, fruit were stored at 5C for 3 weeks, then transferred to 20C for an additional week to simulate transportation and handling. Fruit harvested early in the season were more susceptible to chilling injury than fruit harvested later, with 62% having lesions >1 cm2 after 2 weeks at 1C. Most of the chilling injury occurred after subsequent storage (at 5C) rather than immediately after the 1C treatment. Injury was different from surface pitting or oleocellosis, manifesting as large uniform surface lesions 2 to 3 cm in diameter that rapidly discolored following storage at 20C. Although the oil glands were flattened, the collenchyma layer immediately above the oil gland remained intact. Cellular discoloration was localized around the oil gland, possibly indicating a lateral release of oil gland contents. Nondegreened late-season fruit developed substantially lower levels of chilling injury.
Commercially packed lemons (Citrus limon (L.) Burm.), grapefruit (C. paradisi Macfayden) and oranges (C. sinensis (L.) Osbeck) from CA and AZ were fumigated in corrugated fiberboard shipping boxes with methyl bromide (MB) at doses efficacious for controlling various postharvest insect pests. Fruit developed no rind injury when fumigated at 24 or 32 g MB/m3 for 2 hr at 21C. At 40 g MB fruit developed slight to moderate peel injury, and sometimes there were more decayed fruit. More rind injury developed at 48 gm MB, the injury was more severe, and there were more decays. Curing fruit for 3-4 days at 15-20C before fumigation, and extending the aeration period after fumigation from a few hours to 1 or 3 days reduced fruit injury. Early-season fruit were not injured as severely as late-season fruit. Lemons picked with green-colored peel but fumigated after they turned yellow (by holding at 13C for 4-10 weeks to degreen) were not injured as much as silver or yellow lemons.
Green lemons (Citrus limon (L.) Burm.) were imaged for chlorophyll fluorescence (CF) 30 minutes after immersion of the fruit into 55 °C water for 5 minutes to determine if CF could be used to identify areas of hot water-induced rind injury before the appearance of visible symptoms. Fluorescence was variable in intensity over the surface of the rind with defined areas of enhanced fluorescence being present that corresponded in shape and location with visible injury that later developed during 24 hours of storage. Images showing minimum fluorescence (F0) and maximal fluorescence (Fm) provided the best image contrast between injured and noninjured areas of the rind. Total F0 present in the image was closely correlated (r 2 = 0.87) with the area of rind injury present following storage. Holding the fruit under conditions of low humidity for 24 h before hot water treatment prevented both the formation of areas of enhanced fluorescence and the corresponding rind injury. Imaging of CF has potential as a means to identify areas of incipient rind injury in citrus to facilitate study of the causal mechanisms of postharvest rind disorders.
Eureka lemon (Citrus limon L. `Eureka') trees were inoculated with ecotypes of VAM fungi isolated from either a subtropical desert (HVAM) or a temperate grassland (LVAM), and grown for five months at 40.5C/32.2C (high) or 29.4C/21.1C (low) day/night, respectively. Diurnal measurements of leaf carbon assimilation (A), transpiration (E) and stomatal conductance (gs) were then made with a portable photosynthesis system. At high temperatures, afternoon A, E and gs were highest for trees inoculated with LVAM and lowest for trees inoculated with HVAM. At low temperatures, afternoon A, E and gs were highest for trees inoculated with HVAM and lowest for trees inoculated with LVAM. Compared to controls, trees inoculated with HVAM and LVAM displayed rapid mid-day fluctuations in stomatal conductance. At low temperatures, water use efficiency (WUE) during the morning was lowest for trees inoculated with LVAM; whereas, afternoon WUE was not affected by HVAM or LVAM. HVAM and LVAM did not affect WUE at high temperatures. Results indicate that long-term physiological adaptations of lemon trees to temperature are uniquely affected by different VAM fungal ecotypes.
Release, localization, and concentration of essential oils in chilling-injured and noninjured lemon [Citrus limon (L.) Burm.] fruit were investigated to enhance understanding of how chilling injury (CI) occurs in lemon. CI in the form of moderate to severe pitting of the flavedo was initially apparent after 3 weeks at 1 °C, followed by a gradual increase in severity until termination of the experiment after 7 weeks at 1 °C. Curing the fruit at 15 °C for 1 week before cold treatment greatly reduced the severity of CI. Release from the fruit of d-limonene, a major component of essential oil in lemon, increased with increasing amounts of CI. The enhancement of d-limonene release, however, lagged behind the development of CI. Studies of the internal anatomy of the flavedo using confocal microscopy indicated that essential oils were abundantly present inside the oil gland and in oil bodies outside the gland. Chilling-injured flavedo exhibited no obvious disruption of either the oil glands or the oil bodies. Extraction and quantification of d-limonene from chilling-injured and noninjured flavedo indicated that similar amounts of oil were present in the tissue, regardless of injury. Damage to the flavedo after 3 weeks at 1 °C was noted in the form of flattened or collapsed cells between the top of the gland and the epidermis, whereas collapse of the oil gland only was observed in later stages of injury development.
Protoplasm culture following the chemical fusion of `Valencia' sweet orange [Citrus sinensis (L.) Osb.] protoplasts, isolated from an embryogenic suspension culture, with `Femminello' lemon [Citrus limon (L.) Burro. f.] leaf protoplasts resulted in the regeneration of an interspecific allotetraploid somatic hybrid plant, two autotetraploid lemon plants, and diploid plants from both parents. The regeneration of plants from lemon leaf protoplasts is an example of protoplast-to-plant regeneration from non-nucellus-derived tissue for Citrus. Regenerated plants were classified according to leaf morphology, chromosome number, and analyses of phosphohexose isomerase (PHI), peroxidase (PER), and 6-phosphoglucose dehydrogenase (PGD) zymograms. The somatic hybrid plant was vigorous, with leaves morphologically intermediate to the parents. The tetraploid lemon plants were similar to diploids, although less vigorous and with thicker leaves. The tetraploid lemon and somatic hybrid plants, if fertile, could be used in interploid sexual crosses to breed triploid seedless lemon cultivars with tolerance of mal secco disease from sweet orange. Further investigation of plant regeneration from leaf protoplasts could increase the number of totipotent Citrus clones amenable to somatic hybridization and genetic transformation experiments.
Fruit production of ‘Owari’ satsuma mandarin (Citrus reticulata Blanco) on 16 rootstocks was best on Argentina, Christiansen, Benecke, and English Small trifoliate oranges (Poncirus trifoliata (L.) Raf.) and Carrizo citrange (P. trifoliata (L.) Raf. × C. sinensis (L.) Osbeck). The lowest yielding trees were on Rusk citrange and English Large trifoliate. Navel orange on Rubidoux trifoliate, Troyer, Carrizo and Uvalde citranges were more productive than trees on Rusk or Citremon C46216 (P. trifoliata (L.) Raf. × Citrus limon (L.)). Satsuma and navel on Troyer and Carrizo citranges produced the heaviest fruit. Fruit weight and production were not always correlated. In general, the larger fruit had thicker rinds. Satsuma on Rubidoux, Christiansen, Norton and Carrizo had darker orange colored rinds; whereas those on Troyer were much greener in color. Navel orange on Rubidoux had the better rind color. Rootstocks had no influence on juice color of navels or satsumas. Fruit from navel orange trees on Rusk had the highest solids/acid ratio of the navel oranges. Satsuma on Rubidoux had the highest juice percentage, those on English Large had the highest soluble solids and titratable acidity. Satsuma and navel orange trees on citrange rootstocks, with the exception of Rusk, were larger than those on trifoliate oranges. However, trees on citranges had less cold hardiness than on trifoliates.
Comparisons were made of some physical and chemical characteristics of lemons (Citrus limon L.) imported into western Europe from 11 countries of origin. Statistically significant differences in quality characteristics were found. Fruit from Chile, Cyprus, Israel, and the United States was the most uniform in yellow coloring. Lemons from Argentina, Israel, South Africa, and Uruguay tended to be rounder than more elongated fruit from Spain and Turkey. Thickest peels were found in Chilean, Greek, Italian, and Spanish fruit, and thinnest peels in South African and Uruguayan fruit. Regardless of origin, the largest fruit had the thickest peel and contained a lower percentage of juice. Israeli lemons had the highest percentage of juice and Argentine, Chilean, and Spanish fruit, the lowest. Total soluble solids (TSS) concentration was highest in juice of fruit from Cyprus and Turkey and lowest in fruit from South Africa and Spain. Total acid (TA) concentration was highest in Turkish fruit and lowest in Italian, South African, and Spanish fruit. Smaller fruit from most countries had higher concentrations of ascorbic acid than larger fruit. Chilean lemons had the highest, and Turkish and U.S. lemons the lowest, ascorbic acid levels.
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.