Grapefruit ( Citrus paradisi ), like all citrus fruits, produce very low levels of ethylene throughout development and do not exhibit an ethylene climacteric during ripening ( Aharoni, 1968 ; Eaks, 1970 ). Citrus fruit do exhibit elevated levels
Stylar-end russetting (SER) is a cosmetic defect of Florida citrus fruit most frequently associated with navel orange. SER is evident as spots or streaks of corky tissue that often form a network of intersecting lines. Occurrence of SER is reported to vary widely from year to year, but some orchards have a history of severe SER, with fruit culled annually for this defect. Growers report that SER is typically first evident in August. The cause of SER has not been determined. Reports of yeast-like fungi inducing russet in pome fruit suggest that similar organisms may be implicated in SER. Yeast-like fungi were isolated on acid PDA from navel oranges in an orchard with frequent severe SER. Strains were selected with a wide range of colony morphology, but were not identified taxonomically. These strains, and strains of Aureobasidium pullulans and Rhodotorula glutinis that caused russetting in pome fruit, were grown in liquid suspension and sprayed on navel orange trees with three repeated applications during July and Aug. 1998. No increase in SER was observed on strain-inoculated trees compared to controls. Two broad-spectrum fungicides were sprayed on other navel orange trees to further explore the possibility that fungi may be involved in SER. GA (gibberellic acid) was also applied in this experiment because it can reduce russetting in apples. All applications were made five times at 3-week intervals in June through Sept 1998. SER was assessed in fruit harvested late Sept. 1998. The proportion of fruit with less than 10% of the surface exhibiting SER was 51% for controls, increased to 69% where myclobutanil was applied at 74 mg a.i./L and increased further to 93% where manganese ethylenebisdithiocarbamate was applied at 1775 mg a.i./L. GA did not significantly influence SER.
The synthetic auxins NAA and 3,5,6-TPA were investigated for reducing abscission of mature citrus fruit in California (CA). NAA was investigated on navel orange trees in San Joaquin Valley and southern CA locations. Of the seven NAA experiments presented, five had substantial fruit drop. In these five experiments, a treatment of NAA reduced drop by 31% to 88% compared to the untreated control. Although NAA treatments as low as 25 mg·L-1 (acid equivalent) reduced drop, the greatest reductions in drop were obtained by spray concentrations in the 100 to 400 mg·L-1 range. 3,5,6-TPA was investigated for fruit drop control properties on navel orange and grapefruit grown in various CA locations. The untreated control in seven of the ten 3,5,6-TPA experiments had substantial fruit drop. In each of these cases, a treatment of 10, 15 or 20 mg·L-1 (acid equivalent) of 3,5,6-TPA reduced drop 69% to 96% compared to the untreated control. A strong linear response from 3,5,6-TPA in these seven experiments indicates maximum fruit drop reduction from the highest rate investigated. On an acid equivalent basis 3,5,6-TPA seems to be comparable to 2,4-D. Both NAA and 3,5,6-TPA were effective in controlling preharvest fruit drop in citrus under CA conditions. Both materials provided fruit holding late into the harvest season. NAA, and in particular 3,5,6-TPA, offer the potential to provide a substitute for 2,4-D which is commonly used for controlling fruit drop in many countries. Chemical names used: naphthaleneacetic acid (NAA); 3,5,6-trichloro-2-pyridinyloxyacetic acid (3,5,6-TPA, triclopyr); 2,4-dichlorophenoxyacetic acid (2,4-D).
Citrus is a major fruit crop in the acid red soils (Ultisol) of hilly areas in Southeast China. These soils are normally deficient in P, K, and other elements. Integrated nutrient management is important for sustainable production of citrus in these areas. In this study, a systematic approach was used to identify the limiting factors for plant growth, using sorghum as a test species. Long-term field experiments were conducted with seven different P and K supply levels to determine optimal application rates for citrus (cv. Ponkan), following alleviation of other limiting factors. The primary nutritional limitations to plant growth in red acid soils included: severe deficiencies in N, P, and K, and moderate deficiencies in Ca, Zn, and B. With increasing application of P and K to field soil, N concentrations in citrus leaves decreased up to 60% due to dilution from increased growth, whereas P and K concentrations increased 2-3 fold. After 2 years of fertilizer application, the N: P: K ratio in leaves reached 1:0.5:1 for the optimal P and K treatment. The available P and K in the soils, measured after harvest each year, increased with increasing P and K application rates. However, within each treatment, increase in P and K with additional years of fertilization was modest. Citrus fruit yields generally increased with increasing P and K and reached a maximum at P and K rates of 125 kg P2O5/ha and 500 kg K2O/ha. In 3 years of successive field experiments, the highest net income was obtained by a balanced NPK fertilization practice using N: P2O5: K2O input of 450: 125: 500 kg/ha per year.
Green mold, a predominant disease of citrus fruit, develops when spores of Penicillium digitatum infect extant wounds in fruit epidermal tissue. Development of green mold during shipping limits the distance grapefruit can be surface transported. The objective of this research was to evaluate whether altering the atmosphere during refrigerated storage could suppress development of green mold. In the first two experiments, growth of green mold was evaluated after fruit were stored in ultra-low oxygen (0.05 or 1 kPa) at 14, 16, or 18 °C for up to 21 days. In the last two experiments, grapefruit were stored for 14 or 21 d at 12, 13, or 14 °C in atmospheres containing 2, 5, or 10 kPa oxygen with or without 2, 5, 10, or 20 kPa carbon dioxide. In all experiments, grapefruit were inoculated with 10 or 20 μL of a spore suspension of P. digitatum. Decay progression after storage was monitored by measuring the diameter of the lesion in cm at the demarcated site of inoculation or by subjectively rating percent decayed fruit surface area. Grapefruit not inoculated with P. digitatum had no visible symptoms of green mold. Grapefruit stored under controlled atmosphere had less fruit surface covered with mycelium (5% to 64%) than grapefruit stored in air. Inoculated grapefruit stored in 0.05 kPa oxygen for up to 14 d at 14 or 18 °C had no visible symptoms of green mold upon removal from cold storage, but developed a characteristic green mold lesion after 5 additional days of storage in air at ambient temperature. Results suggest that refrigerated controlled-atmosphere storage combined with wax and a fungicide can enhance control of green mold during shipping.
There are two ways salinity can damage citrus: direct injury due to specific ions, and osmotic effects. Specific ion toxicities are due to accumulation of sodium, chloride, and/or boron in the tissue to damaging levels. The damage is visible as foliar chlorosis and necrosis and, if severe enough, will affect orchard productivity. These ion accumulations occur in two ways. The first, more controllable and less frequent method, is direct foliar uptake. Avoiding irrigation methods that wet the foliage can easily eliminate this form of specific ion damage. The second way specific ion toxicity can occur is via root uptake. Certain varieties or rootstocks are better able to exclude the uptake and translocation of these potentially damaging ions to the shoot and are more tolerant of salinity. The effect of specific ions, singly and in combination, on plant nutrient status can also be considered a specific ion effect. The second way salinity damages citrus is osmotic effects. Osmotic effects are caused not by specific ions but by the total concentration of salt in the soil solution produced by the combination of soil salinity, irrigation water quality, and fertilization. Most plants have a threshold concentration value above which yields decline. The arid climates that produce high quality fresh citrus fruit are also the climates that exacerbate the salt concentration in soil solution that produces the osmotic effects. Osmotic effects can be slow, subtle, and often indistinguishable from water stress. With the exception of periodic leaching, it is difficult to control osmotic effects and the cumulative effects on woody plants are not easily mitigated. This review summarizes recent research for both forms of salinity damage: specific ion toxicity and osmotic effects.
In cybridization, new combinations of nuclear and cytoplasmic genes result in a unique genotype that may bring cellular, physical, physiological, and biochemical changes to the plant. This has been demonstrated in the unexpected cybrids generated from the fusion of citrus (Citrus sp.) protoplasts in two independent experiments. The first experiment was conducted to generate potentially seedless triploids by fusing diploid protoplasts of embryogenic ‘Dancy’ mandarin (Citrus reticulata) suspension culture cells with haploid ‘Ruby Red’ grapefruit (C. paradisi) protoplasts derived from tetrad-stage microspores. After multiple attempts, only one triploid was recovered, but several diploid plants with typical grapefruit morphology were also regenerated. In the second experiment, protoplasts derived from embryogenic ‘Dancy’ mandarin suspension culture were fused with nonembryogenic protoplasts from ‘Duncan’ grapefruit leaves in an effort to produce an allotetraploid somatic hybrid. The fruit from the resulting trees resembled grapefruit in morphology and type, and maintained excellent quality throughout the summer, when commercial grapefruit rapidly loses quality. Fruit on these trees remained firm with exceptional sweetness and good flavor into August, and without seed germination. The regenerants obtained in the protoplast fusion experiments were confirmed as cybrids by genetic marker analyses. The test grapefruit were identical to commercial ‘Ruby Red’ grapefruit at six nuclear simple sequence repeat (SSR) marker loci, but identical to ‘Dancy’ with respect to a mitochondrial intron marker. The plastid genomes of individual trees originated from either fusion partner. In the first experiment, haploid ‘Ruby Red’ protoplast preparations must have also contained contaminant diploid protoplasts. Apart from the value of altered fruit quality attributes in the marketplace, these plants provide an opportunity to understand the contributions of cytoplasmic organelle genetics to important citrus fruit-breeding objectives.
regular orchard management systems. Fig. 2. Major plant growth regulators (PGRs) treatments during stages of citrus fruiting processes. The Productivity Complex It is not easy to distinguish among the physiological problems of flowering, fruit set, and
natural phenomenon following activation of the abscission process and is significantly exacerbated by HLB, the disease associated with the bacterium Candidatus Liberibacter asiaticus ( C Las) that has drastically reduced citrus fruit production in the
The citrus fruit is botanically classified as a hesperidium berry and has a leathery rind that is important from a horticultural perspective in that it is essentially this structure that determines the fruit’s commercial value in the fresh fruit