Oxygen uptake and glycosidase activities were examined in normal and granulated juice vesicles of several citrus fruit. Oxygen uptake was low in normal juice vesicles isolated from freshly harvested `Lee' tangelos [Citrus reticulate Blanco cv. Clementine × (Citrus paradisi Macf. cv. Duncan × Citrus reticulate Blanco cv. Dancy)] and stored `Dancy' tangerine (C. reticulate Blanco) and `Marsh' grapefruit (Citrus paradisi Macf.) (35.7, 17.9, and 11.6 μl O2/hr per g fresh weight, respectively), but was 2- to 3-fold higher in granulated juice vesicles. As severity of granulation increased in grape. fruit, O2 uptake increased. Oxygen uptake in normal and disordered juice vesicles of all citrus fruit examined was reduced to nondetectable levels with 0.1 mM KCN and was insensitive to salicylhydroxamic acid. α - and β -galactosidase and α- and β -glucosidase activities were present in extracts of normal grapefruit juice vesicles (123, 214, 51, and 25 nmol·hr-1·g-1 fresh weight, respectively) and was 2- to 3-fold higher in extracts of granulated tissue. α- and β -mannosidase activities, nondetectable in normal juice vesicle extracts, were present in extracts from granulated tissue. The results suggest that increased metabolic activity occurs in granulated juice vesicles and the energy produced may be used to support cell wall synthesis and modification. Increases in O2 uptake and glycosidase activities correlate well with observed symptoms of section-drying in citrus.
Jacqueline K. Burns
Jacqueline K. Burns
1-Methylcyclopropene (1-MCP) is a gaseous ethylene-binding inhibitor used to control or delay ethylene-related postharvest effects in a range of horticultural commodities. The potential for preharvest applications of 1-MCP to prevent unwanted defoliation using ethephon to loosen mature citrus fruit is presented. Although there was no difference in mature fruit loosening by ethephon + 1-MCP treatments, 1-MCP reduced defoliation caused by ethephon. The gaseous nature of 1-MCP is an impediment to uniform application and consistent efficacy. A sprayable 1-MCP formulation would be of great value for preharvest use in many horticultural crops.
Jacqueline K. Burns
Luis Pozo and Jacqueline K. Burns
To successfully use abscission agents for ‘Valencia’ sweet orange mechanical harvesting throughout the harvest season, unwanted flower, fruitlet, and leaf drop must be assessed and minimized. Ethephon (400 mg·L−1), 1-methylcyclopropene (1-MCP; 5 mm), ethephon + 1-MCP, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP; 200 mg·L−1), and a kinetic adjuvant control [0.15% (v/v)] were applied to ‘Valencia’ branches at various times from full bloom in Mar. 2006 to the end of full bloom in Mar. 2008. Effects of these treatments on fruit detachment force (FDF) and abscission of developing and mature fruit, flowers, and leaves were recorded. Three separate response periods to abscission agent applications were observed: the first spanned the first 100 days after bloom (DAB) and was characterized by high initial response followed by decreasing sensitivity; the second spanned between 100 and 225 DAB and was characterized by little to no response; and the third spanned from 225 DAB to harvest and was characterized by a gain in sensitivity. Young fruitlets in the first response period were highly sensitive to ethephon but were less sensitive to CMNP or ethephon + 1-MCP. Mature fruit in the third response period were highly sensitive to CMNP and less sensitive to ethephon or ethephon + 1-MCP. The application of ethephon resulted in high leaf abscission and showed no clear sensitivity pattern throughout both cropping years. CMNP or ethephon + 1-MCP application caused minimal leaf abscission. The same abscission agent treatments were applied on whole tree canopies 6 and 28 DAB in Mar. 2007. Application date had no significant effect on the measured parameters. Although ethephon application induced high initial leaf drop, leaf area indices determined 7 months after any compound application were not significantly different. However, subsequent 2008 yield in trees sprayed with ethephon in 2007 was significantly less, whereas 2008 flower number was higher. The results indicate a complex interaction of fruitlet abscission and leaf loss during the first response period contributed to yield reduction and increased flower number in ethephon-treated trees.
Rongcai Yuan and Jacqueline K. Burns
The effect of temperature on the ability of 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) and ethephon to induce ethylene evolution and abscission of mature fruit and leaves was determined using 3-year-old potted `Hamlin' orange [Citrus sinensis (L.) Osb.] trees in environment-controlled growth rooms in seasons 2001-02 and 2002-03. Ethylene evolution and abscission of CMNP or ethephon-treated fruit and ethephon-treated leaves were highly temperature dependent. Fruit detachment force (FDF) and fruit ethylene evolution were not affected by application of ethephon at 200 mg·L-1 or CMNP at 200 mg·L-1 when air temperature was 10 °C for ethephon treatment or ≤15.6 °C for CMNP treatment. However, ethylene evolution of CMNP or ethephon-treated fruit increased sharply, and FDF decreased drastically as temperature increased from 10 to 26.7 °C for ethephon treatment or from 15.6 to 26.7 °C for CMNP treatment. Several 10 hour day/14 hour night temperature regimes were explored to determine the effect of varying daily and nightly temperatures on efficacy and ethylene evolution. At least 3 days of exposure to 21/10 °C were required for CMNP to effectively loosen fruit, whereas only one day of exposure to 26.7/15.6 °C was enough to induce similar changes. At 21/10 °C, CMNP significantly reduced FDF to<25 N and markedly enhanced fruit ethylene evolution, regardless of interruption by 1 day of low temperature at 10/10 °C in the first 5 d after application. Ethephon had no significant effect on leaf ethylene evolution and leaf abscission when temperature was 10 °C, but caused a marked increase in both leaf ethylene evolution and leaf abscission as temperature increased from 10 to 26.7 °C. CMNP did not stimulate leaf ethylene evolution and leaf abscission regardless of temperature. Chemical names used: 5-chloro-3-methyl-4-nitro-1 H-Pyrazole (CMNP); 2-chloroethylphosphonic acid (ethephon).
Anish Malladi and Jacqueline K. Burns
Plant growth regulators (PGRs) play important roles in the way plants grow and develop. Myriad processes important to horticultural crops are regulated by PGRs. Changes in the presence, balance, and distribution of PGRs communicate developmental, stress-related, or environmental cues that alter growth. Short-distance communication involves changes in biosynthesis or metabolic conversion, whereas longer-distance communication may also require export and translocation of PGRs, their precursors or metabolites. Examples are presented that demonstrate PGR communication between roots and shoots in horticultural commodities. For example, increased duration and intensity of flooding stress can result in synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC), precursor of the PGR ethylene, in roots. ACC transported to the shoot through the transpiration stream is converted to ethylene and causes leaf epinasty. Roots sense the onset of water stress and can communicate the need to close leaf stomata by altering abscisic acid (ABA) levels in the shoot. Daylength and temperature regulate synthesis and transport of gibberellins, which promote stem elongation and stolon formation and inhibit tuberization in potato. Outgrowth of axillary buds following the decapitation of the apical meristem is dependent on synthesis and transport of cytokinin from root to the axillary buds as well as the balance of indole-3-acetic acid (IAA) cytokinin, and additional messengers. Current research in the field of long-distance communication within plants is uncovering novel messengers and altering our view of the central roles for PGRs in such signaling.
Madhulika Sagaram and Jacqueline K. Burns
Chlorophyll fluorescence and photochemical and nonphotochemical quenching parameters were measured in 20 genotypes of Citrus spp. or relatives grown in the greenhouse and commercial ‘Valencia’ sweet orange (Citrus sinensis) trees at two Florida locations. The purpose was to determine the utility of measurements for early huanglongbing [HLB (Candidatus Liberibacter asiaticus)] detection in asymptomatic trees and to examine the leaf response to HLB infection. Polymerase chain reaction (PCR)-negative healthy and PCR-positive symptomatic, asymptomatic, and distant asymptomatic leaves were used for fluorescence analysis using a portable chlorophyll fluorometer. Greenhouse-grown genotypes were separated into mild, moderate, and severe symptom groups based on leaf mottling, color, and size. In general, mild symptom genotypes were characterized by increased photosystem II (PSII) excitation pressure and unregulated heat dissipation and decreased regulated heat dissipation, whereas moderate and severe symptom genotypes increased loss of photosynthetic efficiency and increased unregulated and regulated heat dissipation. Distant asymptomatic leaves could be distinguished from healthy ones in moderate and severe symptom genotypes by increased total and regulated heat dissipation measurements. In the field, overall photosynthetic efficiency and total regulated heat dissipation measurements could distinguish between healthy and asymptomatic ‘Valencia’ sweet orange leaves at the location with slow or more recent infection, but not at the location where infection appeared to progress faster or was of longer duration. Starch content followed a similar pattern. The results indicate that no single measurement uniquely described the relationship between HLB and the host in asymptomatic and healthy leaves, but accuracy of field-based detection could be strengthened by a combination of total nonphotochemical quenching, overall photosynthetic efficiency, starch content, and PCR analyses. Chlorophyll fluorescence and quenching measurements suggest a PSII-based explanation for, and temperature dependency of, leaf symptom development.
William C. Kazokas and Jacqueline K. Burns
Mature and immature `Valencia' orange [Citrus sinensis (L.) Osbeck] and immature `Valencia' orange and `Tahiti' lime (Citrus latifolia Tan.) fruit with attached pedicels were treated with 8 μL·L-1 ethylene for periods up to 24 hours. Endo-β-1,4-glucanase (cellulase) activity and gene expression were determined in fruit abscission zones during and after ethylene exposure. Cellulase activities were not detected in mature `Valencia' orange and immature `Tahiti' lime fruit abscission zones immediately following harvest and after 6 hours of ethylene treatment. After 12 hours of ethylene treatment, cellulase activity increased and was highest after 24 hours. Cellulase gene expression preceded the rise in cellulase activity and was detectable after 6 hours of ethylene treatment, but then declined after 12 hours. Following transfer to air storage, abscission zone cellulase activity in mature `Valencia' fruit remained high, whereas activity in immature `Tahiti' fruit declined. After 168 hours air storage, activity in abscission zones of mature `Valencia' fruit decreased slightly, but activity in abscission zones of immature `Tahiti' lime fruit increased to the highest level. Expression of abscission zone cellulase gene Cel-a1 in abscission zones of mature `Valencia' fruit markedly increased after transfer to air and was highest after 48 hours air storage. Cel-a1 expression returned to low levels after 168 hours of air storage, but expression of cellulase gene Cel-b1 remained at low levels throughout the air storage period. Expression of Cel-a1 and Cel-b1 declined in fruit abscission zones of immature `Valencia' and `Tahiti' lime fruit upon transfer to air. After 168 hours of air storage, expression of Cel-a1 again rose to high levels but Cel-b1 remained low. The results suggest that differences in cellulase activity and gene expression measured in mature and immature fruit abscission zones during ethylene treatment and subsequent air storage may, in part, explain the differential response of mature and immature fruit to abscission agents.
Karthik-Joseph John-Karuppiah and Jacqueline K. Burns
When applying abscission agents to tree fruit to facilitate harvest, it is desirable to loosen fruit and not leaves or other organs, but mechanisms controlling leaf and fruit drop are not fully understood. The effect of 450 μL·L−1 ethephon (ethylene-releasing agent) alone or in combination with 1-methylcyclopropene [1-MCP (ethylene perception inhibitor)] on leaf and mature fruit abscission of ‘Valencia’ sweet orange (Citrus sinensis) was studied. Leaf abscission increased and fruit detachment force (FDF) decreased significantly especially 4 days after ethephon treatment. Leaf drop rose to over 80% 7 days after application, whereas FDF was only 30% less than untreated control fruit. When 1-MCP was combined with ethephon and applied to ‘Valencia’ sweet orange canopies, leaf abscission was greatly reduced, but reduction in FDF proceeded unabated. We hypothesized that differential response of ‘Valencia’ sweet orange fruit and leaves to 1-MCP was correlated with expression of ethylene biosynthetic and signaling genes and their downstream action. Partial or full-length nucleotide sequences were obtained for ‘Valencia’ sweet orange homologs of 1-amino-cyclopropane-1-carboxylate synthase-1 (CsACS1), 2 (CsACS2), 1-amino-cyclopropane-1-carboxylate oxidase (CsACO), ethylene response sensor-1 (CsERS1), ethylene response-1 (CsETR1), 2 (CsETR2), 3 (CsETR3), constitutive triple response-1 (CsCTR1), ethylene insensitive-2 (CsEIN2), and ethylene insensitive 3-like-1 (CsEIL1) and 2 (CsEIL2). Ethephon application increased expression of biosynthesis genes CsACS1 and CsACO and receptors CsERS1 and CsETR2 in the abscission zones of leaves and mature fruit. Ethephon-induced increase in gene expression was completely suppressed by 1-MCP application in all but CsACS1 and CsACO in fruit abscission zones. Although gene expression was suppressed initially, CsACS1 and CsACO expression in fruit abscission zones treated with 1-MCP in the presence or absence of ethephon increased over the 7-day measurement period, suggesting that CsACS1 and CsACO expression were negatively regulated by basal ethylene production in this tissue. However, 1-MCP treatment alone did not loosen fruit, indicating that CsACS1 and CsACO played minor roles in fruit abscission. To determine if the difference in ethylene sensitivity was the basis of differential response to ethylene within the same organ, potted ‘Valencia’ sweet orange plants were treated with ethylene, and rates of blade and petiole drop and detachment forces at the laminar and petiolar abscission zones were studied. Although leaf blades abscised earlier than petioles, the force of detachment was similar, indicating no differences in ethylene sensitivity. Overall, the most significant difference between fruit and leaf abscission zones was seen in the expression of CsACS1 and CsACO genes, but the expression pattern was poorly correlated with abscission.