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J.K. Burns, U. Hartmond, R. Yuan and W.J. Kender

Methyl jasmonate (Me-Ja) is a naturally occurring ubiquitous compound in plants. Me-Ja is considered to be a putative plant hormone because of its effect on plant processes such as senescence, germination, tuber formation, signal transduction, ethylene production, and abscission at low exogenous concentrations. We applied Me-Ja to fruit or whole trees of `Hamlin' or `Valencia' orange to determine the potential of this compound as a mature fruit abscission agent. Me-Ja (0, 1, 5, 10, or 20 mM in 0.1% Kinetic adjuvant) was applied to whole trees with a handgun or boom sprayer rates of 4850 and 1790 L·ha–1, respectively. Alternatively, tree fruit were dipped in Me-Ja solutions. Fruit drop, leaf drop and ethylene production in both fruit and leaves and fruit detachment force in fruit were monitored at various times up to 2 weeks after application. Me-Ja treatment resulted in increased ethylene production in fruit and leaves 1 to 2 days after application. Fruit detachment force significantly declined 6 to 10 days after application followed by significant fruit drop. Applications of Me-Ja >10 mM resulted in an unacceptable amount of canopy defoliation. The results suggest that Me-Ja has potential as an abscission agent for citrus. Future work will focus on improving uniformity of application and response.

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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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Walter J. Kender, Ulrich Hartmond and Jacqueline K. Burns

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.

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Jacqueline K. Burns, Ulrich Hartmond and Walter J. Kender

The abscission action of two sulfonylureas and one imidazolinone was evaluated in laboratory studies with harvested orange (Citrus sinensis L. cv. Valencia) fruit and greenhouse studies with orange (cv. Hamlin) and grapefruit (Citrus paradisi Macf. cv. Marsh) trees. Dipping harvested fruit in 90 mg·L–1 imazameth, 2 mg·L–1 metsulfuronmethyl, or 30 mg·L–1 prosulfuron solutions increased levels of internal ethylene. Internal ethylene concentration was higher when fruit were dipped in 2 mg·L–1 metsulfuron-methyl solutions at low pH. Fruit retained on trees and dipped in 2 mg·L–1 metsulfuron-methyl solutions produced more ethylene than control fruit. Drop of treated fruit began when ethylene production was at a maximum. High temperatures (average 33 °C) suppressed ethylene production and fruit drop of metsulfuron-methyl–treated fruit. The results indicate the importance of environmental conditions in evaluating the potential of sulfonylureas and imidazolinones as abscission agents for citrus. Chemical names used: ±-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid (imazameth); methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl); 1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl) phenylsulfonyl] urea (prosulfuron); N-(phosphonomethyl) glycine (glyphosate); 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H-imidazol-2-yl]-3-quinolinecarboxylic acid (imazaquin).

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E. Echeverria, J.K. Burns and W.M. Miller

The effect of fruit temperature and fruit maturity on the development of blossom end clearing (BEC) in Florida grapefruit (Citrus paradisi Macf. vars. Ruby Red and Marsh) was investigated. Field and storage temperature studies indicated that development of BEC was directly associated with temperature; BEC increased when fruit temperature rose above 21 °C. Cooling fruit prior to packingline operations reduced BEC significantly. Older fruit were more susceptible to BEC than were younger fruit.

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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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W.J. Kender, U. Hartmond, M. Salyani, J.K. Burns and J.D. Whitney

A field experiment was conducted to determine effects of concentration and spray volume of metsulfuron-methyl as an abscission aid for mechanical harvesting of citrus. Concentrations of 1, 2, and 4 mg·L–1 metsulfuron-methyl were applied to `Hamlin' orange [Citrus sinensis (L.) Osbeck] trees at 470, 1900, and 4700 L·ha–1 (0.5 to 19 g·ha–1 a.i.). Effective fruit loosening was achieved with all applications >1.9 g·ha–1 (4 mg·L–1 at all volumes, 2 mg·L–1 at 1900 and 4700 L·ha–1, and 1 mg·L–1 at 4700 L·ha–1). Heavy defoliation and twig dieback were observed on trees receiving 2 and 4 mg·L–1 at all volumes. Defoliation and dieback became more severe and flower development and fruit set were inhibited as fruit loosening increased. The use of metsulfuron-methyl as an abscission agent for `Hamlin' oranges is not recommended until conditions for its safe application can be determined. Chemical names used: methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl).

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U. Hartmond, J.D. Whitney, J.K. Burns and W.J. Kender

Two field studies were conducted to evaluate the effect of metsulfuron-methyl and 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole) on abscission of `Valencia' orange [Citrus sinensis (L.) Osbeck] during the 3-month harvest season. Solutions of metsulfuron-methyl at 0.5, 1, and 2 mg·L-1 active ingredient (a.i.) were applied at 10-day intervals beginning on 13 Feb. and ending 18 May 1998. Early in the harvest season, 1 or 2 mg·L-1 metsulfuron-methyl significantly reduced fruit detachment force (FDF) 14 days after application. Metsulfuron-methyl was less effective during a 4- to 6-week period following bloom (“less-responsive period”). After this period, metsulfuron-methyl regained the ability to loosen fruit. Applications of 2 mg·L-1 a.i. were more effective than 1 mg·L-1 in reducing FDF and causing leaf drop, but 0.5 mg·L-1 a.i. had little or no effect on FDF. Flowers and leaflets on developing shoots and young fruit completely abscised with 1 and 2 mg·L-1 a.i. Defoliation and twig dieback was extensive at all concentrations and spray dates, eliminating metsulfuron-methyl as a commercially viable abscission agent for citrus. In a separate experiment CMN-pyrazole at 50 and 100 mg·L-1 a.i. and metsulfuronmethyl at 0.5 mg·L-1 a.i. were applied to `Valencia' trees to determine fruit removal with a trunk shake and catch harvesting system. Application of both abscission materials before and after the “less-responsive period” resulted in a 10% to 12% increase in fruit removal when compared to control trees. Less than a 35% reduction in FDF was sufficient to significantly increase fruit removal. Only 100 mg·L-1 a.i. CMN-pyrazole significantly increased fruit removal when applied during the “less-responsive period.” Chemical names used: Methyl-2-(((((4-Methoxy-6-Methyl-1,3,5-Triazin-2-yl)-Amino)Carbonyl) Amino)Sulfonyl)Benzene (Metsulfuron-methyl); 5-Chloro-3-methyl-4-nitro-1-H-pyrazole (CMN-pyrazole).

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Luis Pozo, Ana Redondo, Ulrich Hartmond, Walter J. Kender and Jacqueline K. Burns

Two formulations of the plant growth regulator dikegulac (2,3:4,6-di-O-isopro-pylidene-α-L-xylo-2-hexulofuranosoic acid), consisting of dikegulac-sodium (Atrimmec) or dikegulac:ascorbic acid (1:1) (DAA), as well as 5-chloro-3-methyl-4-nitro-pyrazole at 200 mg·L-1, were applied as foliar sprays to `Hamlin' and `Valencia' orange trees (Citrus sinensis L. Osbeck) at two dates during the harvest season for each cultivar (11 Nov. and 10 Jan. for `Hamlin', 22 Mar. and 25 May for `Valencia'). Fruit detachment force was evaluated 10 days after application, whereas cumulative leaf abscission was monitored up to 60 days after application. In both cultivars, Atrimmec and DAA at 3,000 mg·L-1 induced moderate fruit loosening when applied at the earlier application date, but fruit loosening improved when applied at the later application date. In `Hamlin', both formulations caused higher leaf abscission when applied at the later date. DAA applications resulted in low leaf loss in `Valencia' regardless of application time, whereas Atrimmec caused unacceptably high leaf loss at either application date. No differences in internal fruit quality were found as a result of any abscission material treatment. The results indicate that DAA could be a promising option to induce fruit loosening in late harvested `Valencia' orange trees with minimal undesirable side effects.

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R. Potjewijd, M.O. Nisperos, J.K. Burns, M. Parish and E.A. Baldwin

Varying the cellulose component of coating formulations affected the survival of two yeast biocontrol agents, Candida guillermondii (Castelani) Langeron and Guerra strain US7 and Debaryomyces sp. strain 230, when these yeasts were incorporated into the coating. Using methylcellulose as the main film-former gave the most recovery of the yeasts after an incubation period for both strains. Significant control of decay on naturally infected `Pineapple' and `Valencia' oranges [Citrus sinensis (L.) Osb.] was demonstrated for US7 in a methylcellulose-based coating for the first 2 to 4 weeks of storage at 16C and 90% relative humidity. During this time, US7 in methylcellulose formulations was similar in decay control to a commercial shellac coating with imazalil at 2000 mg·liter–1. A US7 concentration of at least 105 colony-forming units/cm was maintained on the coated fruit surface of `Valencia' oranges for 3 weeks of storage. Suppression of decay by US7 was improved by the addition of glucose and calcium chloride to the coating formulation. Although nearly equal in concentration recovered, Debaryomyces strain 230 was not as effective as US7 in disease suppression of `Pineapple' oranges. The addition of US7 to Nature Seal, a coating material made with methylcellulose, had neither a quantitative nor a qualitative effect on the pathogen population compared to the same formulation without the antagonist. Chemical name used: 1-[2-(2,4-dichlorophenyl)-2-(2-propenyloxy)ethyl]-1H-imidazole (imazalil).