1-MCP is a gaseous ethylene binding inhibitor that controls or delays ethylene-related postharvest problems in a range of horticultural commodities. Our previous work demonstrated that exposure of calamondin to 1-MCP 16 hours before canopy sprays of ethephon greatly reduced unwanted leaf drop while only partially inhibiting the ability of ethephon to cause fruit loosening. The objective of this work was to determine whether formulated 1-MCP (SmartFresh) could be used in the field to stop defoliation caused by abscission agent applications without significantly altering abscission agent-induced fruit loosening. Spray solutions containing 400 mg·L-1 ethephon with 0, 1, 2.5, and 5 mm 1-MCP were applied to canopies of `Hamlin' and `Valencia' (Citrus sinensis). Timing of 1-MCP applications was a) 24 hours before, b) in combination with, or c) 24 hours after ethephon. Ethephon at 400 mg·L-1 significantly reduced fruit detachment force (FDF) but caused >70% leaf drop within 15 days after application in both cultivars. Applications of 1-MCP reduced ethephon-associated leaf abscission but had little effect on the ability of ethephon to reduce FDF. Timing of 1-MCP applications did not affect the ability of ethephon to cause fruit loosening; however, the best consistent treatment for control of leaf drop was achieved with the combined application of 5 mm 1-MCP and 400 mg·L-1 ethephon. 1-MCP was used in combination with the abscission agents coronatine, methyl jasmonate (MeJa) and 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) to determine its effect on leaf drop and fruit loosening. Leaf drop in trees treated with ethephon, coronatine, and MeJa was reduced by addition of 1-MCP. However, fruit loosening was largely prevented when 1-MCP was used in combination with coronatine or MeJa. Like ethephon, CMNP-induced fruit loosening was not affected by 1-MCP. The results demonstrate the ability to control ethephon-induced leaf abscission without affecting mature fruit loosening by targeting ethylene binding in citrus.
Luis Pozo, Rongcai Yuan, Igor Kostenyuk, Fernando Alférez, Guang Yan Zhong, and Jacqueline K. Burns
Frederick S. Davies, Glenn R. Zalman, Ed Stover, and Scott Ciliento
EcoLyst, a formulation of N-N-diethyl-2-(4-methylbenzyloxy) ethylamine hydrochloride containing 1 g/floz [4.5 oz/gal (33.8 g·L-1)] a.i., is a plant growth regulator that has been reported to increase soluble solids concentration (SSC) in juice oranges by 0.6% to 1.2%. Our objectives were to determine the effectiveness of EcoLyst application for increasing SSC in Florida oranges (Citrus sinensis) and grapefruit (C. paradisi), and to identify the optimum rate and time of application. Experiments were conducted for three seasons using `Hamlin,' `Pineapple,' and `Valencia' sweet oranges; and for two seasons using `Flame,' `Marsh,' and `Ray Ruby' grapefruit, all in commercial groves. EcoLyst was applied at 6 and 12 floz/acre (0.44 and 0.88 L·ha-1) for oranges and 16 and 32 ppm (mg·L-1) [effectively 9 and 18 floz/acre (0.66 and 1.32 L·ha-1) in most sprays] for grapefruit, and included Silwet L-77 adjuvant at 0.05%. Applications were made at several stages of development from prebloom to initial fruit set. In all cases, SSC was determined as juice corrected SSC, by adjusting refractometer readings based on titratable acidity. In 13 trials with sweet orange only five displayed significant increases in SSC (P ≤ 0.05) resulting from EcoLyst application. Two additional trials produced SSC increases significant at P < 0.10. Even where significant increases in SSC occurred they were typically observed in only one harvest and at one time of application and were always relatively low in magnitude (highest increase over controls was 0.38%). No rate or timing of EcoLyst application was consistently associated with best response, although eight of nine SSC increases observed in orange occurred with applications ranging from prebloom to 25% open flowers. Only one significant increase in SSC was observed in five trials with grapefruit. In these studies, increases in SSC resulting from EcoLyst application were neither sufficiently consistent nor large enough to justify a recommendation for commercial use in Florida citrus.
Guy D'hallewin, Mario Schirra, Enzo Manueddu, Antonio Piga, and Shimshon Ben-Yehoshua
`Washington Navel', `Biondo Comune', `Tarocco', and `Valencia Late' orange [Citrus sinensis (L.) Obsek] fruit, harvested at various periods of time, were subjected to ultraviolet-C (UV-C) irradiation at 0.5, 1.5, or 3.0 kJ·m-2 doses and then stored at 7 °C and 90% to 95% relative humidity (RH) for 4 weeks plus one additional week at 20 °C and 80% RH. Following UV-C treatment, there was varying amounts of rind browning and necrotic peel damage, depending on cultivar, treatment dose, and harvest date. `Tarocco' fruit were damaged more easily by UV-C treatment than the other cultivars. `Valencia L.' were the most resistant to UV-C irradiation, showing no adverse effects at the lowest dosage and having the lowest percentages of damaged fruit at higher dosages. `Washington Navel' and `Biondo Comune' oranges showed an intermediate susceptibility to UV-C treatment, with negligible differences between these cultivars. The percentage of damaged fruit after irradiation at the higher UV-C dosages decreased in most fruit samples as the season progressed. UV-C irradiation at 0.5 kJ·m-2 effectively reduced decay development compared with nontreated fruit. Irradiation with 1.5 kJ·m-2 was more effective compared with 0.5 kJ·m-2 only in early harvested fruit. In `Washington Navel' and `Biondo Comune' oranges in the later harvests, treatment with 3.0 kJ·m-2 improved decay control further, compared with 0.5 kJ·m-2. Following UV-C treatments the phytoalexins, scoparone and scopoletin, accumulated in flavedo tissue depending on the cultivar, fruit age, and UV-C treatment. Both phytoalexins displayed a similar accumulation pattern, however, the levels of scopoletin were very low compared with scoparone. Concentrations of phytoalexins rose as the irradiation dose increased. No scoparone and scopoletin could be detected in nontreated fruit. The highest concentration of phytoalexins among cultivars was recorded in `Valencia Late' oranges, the lowest in `Tarocco', with similar intermediate accumulations in `Washington Navel' and `Biondo Comune'. In `Washington Navel', `Biondo Comune', and `Tarocco' oranges, the rate of scoparone accumulation was significantly higher in fruit harvested earlier in the season while `Valencia late' oranges exhibited an opposite trend.
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).
H.K. Wutscher and K.D. Bowman
Twenty-one selections consisting of 13 numbered hybrids, one ornamental, and seven named cultivars were tested as rootstocks for `Valencia' orange, Citrus sinensis L. Osbeck. The test included six, four-tree replications in randomized complete blocks on sandy soil typical of the center of the Florida peninsula. Trees propagated on Vangasay lemon, HRS 812 (Sunki × Benecke trifoliate orange), and HRS 942 (Sunki × Flying Dragon trifoliate orange) produced more fruit than trees on the other 18 rootstocks in the test. Trees on 10 rootstocks, including the widely used commercial rootstocks, Swingle citrumelo and Carrizo citrange, were intermediate in cumulative fruit production. Trees on five rootstocks, including Sun Chu Sha, Gou Tou #1, and Tachibana, had low yields and trees on HRS 939 (Flying Dragon trifoliate orange × Nakorn pummelo) and sour orange #2 were extremely dwarfed and were minimally productive because of tristeza virus disease. Fouryear cumulative fruit production ranged from 52 to 317 kg per tree. Fruit from trees on HRS 954 and HRS 952 (Pearl tangelo × Flying Dragon trifoliate orange) had the highest, and fruit from trees on Vangasay and Gou Tou #1 had the lowest total soluble solids concentration.
M.L. Marin and N. Duran-Vila
A study was conducted to evaluate the potential of in vitro techniques for genetic conservation of citrus. A tissue culture system was developed using explants of juvenile `Pineapple' sweet orange. It consisted of: a) establishment of primary cultures from nodal stem segments followed by the recovery of plants in vitro; and b) successive cycles of secondary cultures consisting of the culture of nodal stem segments from in vitro-grown plants, rooting of shoots obtained from nodal stem segments, and recovery of whole plantlets. Two parameters, K and K', based on the multiplication factors of the different stages of primary and secondary cultures are proposed to monitor the system as a potential tool for genetic conservation of citrus. The system also can be successfully used for the conservation of juvenile tissues of two sweet orange varieties [Citrus sinensis (L.) Osb.], trifoliate orange [Poncirus trifoliata (L.) Raf.], Mexican lime [C. aurantifolia (Christm.) Swing.], and `Eureka' lemon [C. limon (L.) Burro. f.]. Chemical names used: 6-benzylaminopurine (BA); α- naphtbaleneacetic acid (NAA).
Dangyang Ke and Adel A. Kader
`Valencia' oranges [Citrus sinensis (L.) Osbeck] tolerated up to 20 days of exposure to 0.5%, 0.25%, or 0.02% O2, at 5 or 10C followed by holding in air at 5C for 7 days without any detrimental effects on external and internal appearance. Oranges stored in 0.5%, 0.25%, or 0.02% O2 had lower respiration rates, but higher resistance to CO, diffusion and higher ethanol evolution rates than those stored in air at 10C. Similar, but less pronounced, effects of the low O2 atmospheres were observed at O and SC. Respiration rates, internal CO2 concentrations, and ethanol evolution rates were generally higher at 10C than at 0C, while resistance to CO2 diffusion was lower at the higher temperature. `Valencia' oranges kept in 60% CO2 at 5C for 5 to 14 days followed by holding in air at 5C for 7 days developed slight to severe injury that was characterized by skin browning and lowered external appearance scores. Juice color, soluble solids content, pH, titratable acidity, and ascorbic acid content were not significantly influenced by either the low O2 or the high CO2 treatments. However, these treatments increased ethanol and acetaldehyde contents, which correlated with the decrease in flavor score of the fruits. Ethanol content of the oranges transferred to air following low 02 treatment correlated with CO2 production rate of the fruits at the transfer temperature and was related to ethanol evolution and probably production rates after the transfer.
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.
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).
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).