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Rongcai Yuan, Ulrich Hartmond, and Walter J. Kender

The seasonal abscission response of mature `Valencia' oranges [Citrus sinensis (L.)Osb.] to 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-Pyrazole) was examined in relation to young fruit, shoot, and root growth. CMN-Pyrazole dramatically increased ethylene production in fruit and effectively reduced the fruit detachment force (FDF), except in a period of reduced response to CMN-Pyrazole in early May. Root growth was inhibited by trunk girdling, in combination with removal of spring vegetative flushes and flowers, but not by their removal alone. During the responsive period, there was no difference in both ethylene production and FDF of CMN-Pyrazole-treated mature oranges between 1) the unmanipulated trees and those manipulated by either 2) girdling, removal of spring flushes and flowers, or 3) removal of flushes and flowers alone. However, during the less-responsive period, ethylene production in CMN-Pyrazole-treated mature oranges was significantly lower while the FDF was higher from non-manipulated trees than from trees treated by either girdling and removal of flush, or only removal of flush. There was no difference in either ethylene production or FDF of CMN-Pyrazole-treated mature oranges between trees manipulated by girdling and removal of flush, and those by removal of flush alone. Flush growth terminated at least 2 weeks before the onset of the less responsive period. This suggests that the hormones from rapidly growing young fruit may be responsible for the less responsive period.

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Rongcai Yuan, Ulrich Hartmond, and Walter J. Kender

Endogenous concentrations of IAA and ABA in the peel, pulp, seed, and abscission zone of mature `Valencia' oranges [Citrus sinesis (L.) Osbeck] were determined by high-performance liquid chromatography and enzyme-linked immunosorbent assay from early November 1998 to mid-June 1999. Ethylene production of mature `Valencia' oranges during the same period was determined by gas chromatography. IAA concentrations in the pulp and seed were three to five times lower than those in the peel over the 7-month observation period. IAA concentration in the abscission zone and peel was high from late April to mid-May, the period of less responsiveness to abscission chemicals. ABA concentration in the pulp was low over the entire observation period. ABA concentration in the abscission zone and peel was low during the less responsive period. Ethylene production was always low except for a slight increase during late December and early February. The IAA to ABA ratio was high in the fruit abscission zone during the less responsive period. Fruit detachment force of CMN-pyrazole-treated fruit was positively correlated with the ratio of endogenous IAA to ABA or endogenous IAA, but negatively to endogenous ABA in the fruit abscission zone. These data suggest the balance between IAA and ABA in the fruit abscission zone may be an important factor in determining sensitivity and thereby the response of mature `Valencia' orange fruit to abscission chemicals. Chemical names used: abscisic acid (ABA); indole-3-acetic acid (IAA); 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole).

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S. V. Kossuth and R. H. Biggs

Abstract

‘Hamlin’ and ‘Valencia’ oranges [Citrus sinensis (L.) Osbeck] were treated by stem uptake or dipping in 5-chloro-3-methyl-4nitro-1H, pyrazole (Release) and (2-chloro-ethyl) phosphonic acid (ethephon) of varying concentrations. Depending upon chemical concentration and fruit maturity, fruit removal force (FRF) was reduced within 24 or 48 hours. More buffer and salt extractable cellulase activity were produced after ethephon than after Release treatment, both exceeding the control. Buffer soluble cellulase activity was treatment concentration dependent while salt released cellulase activity was chemical (Release, ethephon, or control) dependent, increasing by the same amount regardless of treatment concentration. Two ethylene peaks, resulting in an M-shaped curve, occurred after dipping oranges in Release. The peak due to chemical treatment (chemical peak) appeared first and was concentration dependent, being of greater magnitude and earlier in time with the higher concentrations. Measurable ethylene production after stem uptake of the chemicals was erratic and represents an inadequate and erroneous procedure for determining effectiveness of a potential abscission chemical.

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Jacqueline K. Burns*, Richard S. Buker, and Fritz M. Roka

A study was initiated in `Hamlin' orange to determine if a selective citrus abscission material, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), could improve mature fruit removal and recovery when used with mechanical harvesters. A trunk shaker and continuous-moving canopy shaker equipped with catch-frames were used at the flatwoods and ridge growing regions in Florida. Both trials were conducted during the first 2 weeks of Dec. 2003. Plots were constructed as randomized complete blocks containing four replicates for each treatment. Each replicate contained five trees at the ridge site or four trees at the flatwoods site. CMNP was applied using a commercial airblast sprayer at 0, 125, 250 and 500 mg·L-1 at a rate of 2,800 L·ha-1 4 days before scheduled harvest. The trunk shaker was operated for either 7 seconds or 2 seconds/tree, whereas the canopy shaker was operated at either 260 cpm or 140 cpm/tree. The data show that the correct time was selected for harvest. Over a 50% reduction in FDF was achieved with the 250 and 500 mg·L-1 treatments, while post-application fruit drop was less than 1.5%. The greatest benefit of abscission agent use was seen when the mechanical harvesters were operated at the least aggressive setting (2 seconds or 140 cpm), where increases in % fruit removal and recovery were over 20% higher than the controls. At the most aggressive settings, numeric increases in % fruit removal and recoveries were measured, but these changes were not statistically significant. The results demonstrated that statistically similar % fruit removal and recoveries could be achieved using less aggressive harvester settings with abscission agent use when compared with most aggressive settings using no abscission agent.

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Rongcai Yuan, Walter J. Kender, and Jacqueline K. Burns

The effects of removal of young fruit and application of auxin transport inhibitors on endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations were examined in relation to the response of mature `Valencia' orange [Citrus sinensis (L.) Osb.] fruit to abscission materials. ABA concentrations were increased in the fruit abscission zone and pulp but not in the pedicel, peel, or seed of mature fruit by removal of young fruit during the period of reduced response of mature fruit to abscission materials in early May. However, removal of young fruit slightly decreased IAA concentrations in leaves and the abscission zone and pedicel of mature fruit but had no effect on the IAA concentrations in the peel, pulp, or seed of mature fruit. Young fruit had higher IAA concentrations in the abscission zone and pedicel than mature fruit. Application of 2,3,5-triiodobenzoic acid (TIBA), an IAA transport inhibitor, reduced IAA concentrations in the abscission zone of mature fruit but did not influence the IAA concentrations in the pedicel and peel when applied directly to an absorbent collar tied around the pedicel 2 cm above the fruit abscission zone during the less responsive period in early May. ABA concentrations were increased drastically in the fruit abscission zone and pedicel but not in peel by TIBA application. Applications of ABA, or IAA transport inhibitors such as naringenin, quercetin, or TIBA comparably increased the response of mature fruit to the abscission material 5-chloro-3-methyl-4-nitro-1 H-pyrazole (CMN-pyrazole) in early May. These data suggest that young fruit reduce the response of mature `Valencia' oranges to abscission materials through increasing IAA concentrations and decreasing ABA concentrations in the abscission zone of mature `Valencia' orangees.

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Rongcai Yuan, Ulrich Hartmond, Angela Grant, and Walter J. Kender

Influence of young fruit, shoot, and root growth on response of mature `Valencia' oranges [Citrus sinensis (L.) Osbeck] to the abscission chemical CMN-pyrazole was examined in 1999 and 2000. CMN-pyrazole dramatically increased ethylene production in mature fruit and reduced the fruit detachment force (FDF), except during a period of reduced response to CMN-pyrazole in early May when spring vegetative growth, young fruit of the following year's crop, and mature fruit were all on the trees. Removal of spring flushes, which included spring vegetative shoots and leafy and leafless inflorescences, prevented any young fruit and shoot growth, but did not inhibit root growth. However, trunk girdling in combination with removal of spring flushes not only prevented growth of young fruit and shoots but also inhibited root growth. During the responsive period, there were no differences in either ethylene production or FDF of CMN-pyrazole-treated mature oranges between 1) the nonmanipulated trees and those manipulated by either 2) removal of spring flushes alone, or 3) in combination with trunk girdling. However, during the less responsive period, ethylene production in CMN-pyrazole-treated mature oranges was significantly lower while the FDF was higher in nonmanipulated trees than in trees treated by either removal of spring flushes alone, or in combination with trunk girdling. There was no difference in either fruit ethylene production or FDF between trees manipulated by (2) removal of spring flushes alone, and (3) removal of spring flushes in combination with trunk girdling regardless of CMN-pyrazole application. Shoot growth terminated at least 2 weeks before the onset of the less responsive period. Removal of young fruit increased response of mature fruit to CMN-pyrazole during the less responsive period. This suggests that hormones from rapidly growing young fruit may be responsible for the occurrence of the less responsive period. Chemical name used: 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole).

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J.K. Burns, C. Arias, I. Kostenyuk, and M. Obraztnova

The process of abscission results in shedding of plant parts such as leaves, fruit, flowers, and in citrus, shoot tips and entire shoots. Growers must successfully manage abscission in their operations to avoid unnecessary defoliation or loss of yield due to floral abscission or preharvest fruit drop. Conversely, abscission enhancement may be desired during harvest. Yet despite its importance to horticulture, little is known about mechanisms that control abscission. We know that abscission can be induced by ethylene and altered to some extent by auxin. Over the years, many physiological and anatomical events of abscission have been described. For example, cellulase, polygalacturonase and pectin methylesterase genes are induced during abscission, and they are thought to have a role in alteration and depolymerization of middle lamella polysaccharides located in the abscission zone area. Other genes, such as those associated with the process of pathogen resistance, are also induced during abscission. We are interested in using tools of molecular biology to examine abscission-related gene expression prior to organ separation in Florida field-grown Valencia orange (Citrus sinensis L. Osbeck) and greenhouse-grown calamondin (Citrus madurensis Loureiro) citrus trees. Subtractive cDNA library screening and differential display were used to examine gene expression in fruit, leaf and floral abscission zones 6, 24 and 48 h after induction of abscission with 5-chloro-3-methyl-4-nitro-1H-pyrazole or Ethrel® (Rhone-Poulenc, [2-chloroethyl] phosphoric acid). Some isolated cDNAs encoded polypeptides with no significant matches in the database or share significant similarities with unknown proteins isolated from Arabidopsis. Other cDNAs encoded polypeptides with similarity to cell wall modifying proteins such as polygalacturonases and expansin, PR proteins such as chitinase, proteins associated with secondary and xenobiotic metabolism such as amine oxidase, benzoquinone reductase, caffeic acid methyltransferase, phenylalanine ammonia lyase and squalene synthase, and proteins associated with signal transduction such as several serine/threonine kinases. Temporal and spatial expression of these genes and others will be presented. Use of this information to target potential points of abscission control will be discussed.

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Matthew W. Fidelibus, Karen E. Koch, and Frederick S. Davies

enhancement with 5-chloro-3-methyl-4-nitro-1H-pyrazole) ( Alferez et al., 2006 ). Gibberellic acid maintained sucrose gradients in the peel. An intriguing aspect of the GA 3 response observed here was that the consistent reduction in sucrose depletion

Open access

Biwek Gairhe, Peter Dittmar, Davie Kadyampakeni, Ozgur Batuman, Fernando Alferez, and Ramdas Kanissery

Alferez, F. Pozo, L. Burns, J.K. 2006 Physiological changes associated with senescence and abscission in mature citrus fruit induced by 5-chloro-3-methyl-4-nitro-1H-pyrazole and ethephon application Physiol. Plant. 127 66 73 https://doi.org/10.1111/j

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Muhammad Farooq, Masoud Salyani, and Jodie D. Whitney

Field experiments were conducted to investigate the effect of sprayer type, airflow rate, and nozzle output on deposition of active ingredient and mechanical harvesting of `Valencia' orange (Citrus sinensis). Fruit detachment force (FDF) and percentage of fruit removal (PFR) by trunk shaker were used as mechanical harvesting parameters. A PowerBlast sprayer discharging radially and a Titan sprayer discharging over the entire canopy were used. The spray mixture contained an abscission chemical (CMN-pyrazole), a surfactant (Kinetic) and a fluorescent tracer (Pyranine-10G). Deposition was determined at three different heights outside and inside of the canopy. With the PowerBlast, higher airflow and lower nozzle output reduced deposition of the active ingredient. The mean FDF of sprayed treatments was less than that of the non-sprayed control but the difference among the four spray treatments was not significant. The lower airflow rate with lower nozzle output had higher PFR than that of the control. With the Titan sprayer, the mean deposition at lower airflow was similar to or higher than the higher airflow. At higher airflow, the lower nozzle output gave higher mean deposition. The Titan sprayer treatments resulted in less FDF than the control. At both airflow rates, the FDF was less at lower nozzle output than at higher nozzle output. The PFR of these treatments were not different from that of control.