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Robert C. Ebel

Apple leaves were shown to increase 6 volatile compounds in response to drought stress severe enough to promote senescence. Apple trees were allowed to dry to -2.0 MPa and -2.7 MPa, levels that were previously shown to reduce fruit growth by 50% and 70%, respectively. The 6 volatile compounds measured included hexanal, (E)-2-hexenal, 1-hexanol, (E)-2-hexen-l-ol, hexyl acetate, and (Z)-3-hexenyl acetate. Hexanal, (E)-2-hexenal, and 1-hexanol have been previously shown to be byproducts of lipoxygenase (LOX) activity. There is considerable information in the literature implicating LOX as a key enzyme involved in senescence, whether induced by pathogenic infection, insect feeding, or in ripening climacteric fruit and vegetables. It is reasonable to propose that LOX is also involved in promotion of senescence induced by drought stress.

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Naveen Kumar and Robert C. Ebel

5-Chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) is an abscission agent, standardized for the mechanical harvesting of late season ‘Valencia’ sweet oranges in Florida. This work was conducted to investigate the role of CMNP to induce oxidative stress in the abscission zone (AZ) of ‘Valencia’ sweet orange. Fully mature ‘Valencia’ sweet orange trees in a commercial grove were sprayed with 2.0 mm of CMNP. The experiment was repeated three times during the Apr.–May 2013 harvest season. Fruit were harvested at 0, 1, 2, and 3 days after CMNP application. Hydrogen peroxide (H2O2) concentration and malonic dialdehyde (MDA) concentration, as well as superoxide dismutase (SOD), ascorbate peroxidase (APOD), glutathione reductase (GR), peroxidase (POD), and lipoxygenase (LOX) specific activities were measured 0, 1, 2, and 3 days after CMNP treatment (DAT). Rate of lipid peroxidation remains unchanged throughout the abscission period. However, LOX activity increased 1 DAT in AZ of treated fruit, which might produce jasmonic acid (JA), known to promote abscission in citrus. Levels of H2O2 were similar in the AZ of control and treated fruit except at 3 DAT. The specific activity of SOD declined at 2 DAT, which showed compromised SOD defense against superoxide radicals (O·−). APOD activity declined sharply at 3 DAT. Interestingly, GR activity was 1.9-fold higher in CMNP-treated fruit at 3 DAT. Higher GR and low APOD activity reflects limited functioning of the APOD/GR cycle (e.g., APOD and GR) in scavenging of H2O2 at 3 DAT. Guaiacol POD activity transiently increased at 1 DAT and then declined. POD plays an important role in cell wall lignification and indole acetic acid (IAA) oxidation. The decline in POD activity may cause a decrease in lignification while higher activity made the AZ sensitive to ethylene and thus promote abscission in citrus fruit. This work also showed that CMNP-induced abscission is a collaborative effort of oxidative metabolism in flavedo tissue (FT) and AZ.

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Robert C. Ebel, Byron Wallace and Charles Elkins

Imidacloprid is a long-term systemic insecticide that is currently labeled under the trade name Marathon (imidacloprid, 1-[(6-chloro-3-pyridyl)methyl-4,5-dihydro-N-nitro-1-H-imidazol-2-amine, 1% granular on fritted clay, Bayer Corp., Kansas City, Mo.) for ornamental crops grown in greenhouses. The company that markets Marathon is seeking to expand its label to greenhouse-grown vegetable crops, although the rates they plan to label have not yet been divulged. Marathon was applied to cucumber (Cucumis sativus L. `Turbo') and tomato (Lycopersicon esculentum Mill. `Rutgers') at 0, 1/8, 1/4, 1/2, 3/4, and 1 tsp (0, 5, 10, 20, 30, and 40 mg a.i.) per 4.5-inch (550-mL) pot. Both species developed phytotoxicity symptoms of leaf chlorosis of the oldest leaves and distorted growth and marginal necrosis of newer leaves within 1 week after application. By the end of the experiment, even the lowest rate caused phytotoxicity symptoms. The symptoms were similar in appearance to Ca deficiency but cucumber foliar analysis revealed no difference in Ca, Zn, Fe, or Co across imidacloprid rates, however, Mg and B decreased whereas K and Mn increased linearly across imidacloprid rates. P, Cu, and Mo varied quadratically with 1/2 tsp (20 mg a.i.) per pot having the lowest P and Mo, and Cu increasing at the higher rates. These data indicate that imidacloprid can alter plant nutrition. The rates of imidacloprid applied here are not recommended for use on greenhouse-grown cucumber and tomato under similar growing conditions as in this study.

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Robert C. Ebel and Jonathan P. Lynch

Kinematic analysis allows accurate description of physiological changes along root axes by additionally taking into account changes due to dilution as cells expand. In previous studies using kinematic analysis, roots have been marked with ink by fine-tipped pens or single hair brushes. These methods have occasionally reduced root growth and limited resolution to the width of the marks, usually 1 mm. We describe a new method of marking roots with the fluorescent dye calcofluor which does not reduce root growth. The terminal 7 mm of bean root tips were grown vertically in a glass chamber into which a constant flow of aerated nutrient solution was passed. A 0.001% calcofluor solution was pulsed through the chamber for 1 min. Excess calcofluor was removed rapidly by a high rate of nutrient flow (200 ml·h–1) for 3 min. after which flow was reduced to 20 ml/hr. Roots were magnified 11.5× under a microscope mounted horizontally and five digitized images captured every 5 min. Imaging software allowed determination of fluorescence of individual pixels along the length of the root. Fluorescence decreased in the zone of cell elongation due to dilution as cells expanded. This method may improve resolution of kinematic analysis to the length of individual pixels, which was 18 microns at 11.5× magnification.

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David G. Himelrick and Robert C. Ebel

`Chandler' strawberry plants were established in a recirculating nutrient flow hydroponic system under six nutrient solution N levels (35, 70, 140, 210, 280, and 350 ppm). Various morphological and fruiting responses were measured. Average berry weight was greatest in the 280 ppm range and lowest in the 350 ppm solution N treatments. Percent soluble solids were greatest in the 35 ppm and lowest in the 140 ppm N treatments. Titratable acidity was greatest in the 75 and 210 ppm treatments and lowest in the 140, 280, and 350 ppm N treatments. Nitrate N was greatest in the juice of the 280 and 350 and lowest in the 35 ppm N treatment. Interior and exterior fruit firmness followed a general trend of the greatest firmness being found at 35 ppm and the least firm berries being from the 350 ppm treatment.

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Robert T. Boozer, Robert C. Ebel and James A. Pitts

A Phil Brown Corporation, hydraulic operated rope thinner was evaluated in 1995 and 1997 to determine performance for bloom thinning under Alabama peach growing conditions. Using detailed pruned trees in 1995, the rope thinner removed 55% and 57% of the blooms from two double pass treatments and 42% from single pass. Thinning was 9% to 31% higher in the upper one-half of the fruiting zone. In 1997, nondetail pruned trees were used and ground speed was evaluated. Percent blooms removed by single pass were 28, 23, and 22 for 1.6, 3.2, and 4.8 km·h-1, respectively. Double pass clockwise removed 38% of the blooms at 3.2 km·h-1. Greatest time saving for follow-up hand thinning was 15 minutes per tree with double pass over hand-thinned only.

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Robert C. Ebel, Edward L. Proebsting and Robert G. Evans

Drought stress was imposed in two `Delicious' apple (Malu×domestica Borkh.) orchards on a sandy loam soil of different soil depths (0.8 and 1.2 m) in the semi-arid environment of central Washington by withholding irrigation all season or from 3, 5, 7, 9, 11, 13, 15, or 17 weeks before harvest. Total pan evaporation was 1005 mm and precipitation was negligible from May through Sept. Soil of the control trees was near field capacity all season, and stem water potential (Ψstem) averaged -1.29 MPa. Total available soil water (TAW) declined after irrigation was terminated for each treatment. As TAW declined to 35%, the TAW that commercial growers are recommended to allow soil to dry to before irrigating, Ψstem was 93% of the controls, fruit growth rate was 97% of the controls, and leaf senescence did not exceed the control trees. As TAW decreased below 30%, leaves senesced acropetally starting with transition leaves near the bud-scale scar. Soil moisture of nonirrigated trees was depleted in July in the orchard on shallow soil and in late August in the orchard on deep soil. Normal June drop was reduced in the driest treatments, but crop load was not affected in the other treatments. There was no difference in drought response between the two rootstocks studied (M.7 and MM.111), but nonspur-type trees exhibited slightly greater symptoms of drought stress than the smaller spur-type trees. A Crop Water Deficit Index (CWDI) based on Ψstem measurements was linearly related to fruit weight at harvest (r 2 = 0.87). All trees were well-watered the following year and yield was reduced only for trees that were severely stressed the previous year.

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Kelly T. Morgan, Robert E. Rouse and Robert C. Ebel

Huanglongbing (HLB) causes citrus root systems to decline, which in turn contributes to deficiencies of essential nutrients followed by decline of the canopy and yield. This study was conducted on a 6-year-old ‘Valencia’ [Citrus sinensis (L.) Osb.] on Swingle rootstock (Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.) trees in a commercial grove near Immokalee, FL, to evaluate the effects of foliar applications of selected essential nutrients (N, K, Mn, Zn, B, and Mg) on growth and productivity of citrus trees infected with Candidatus Liberibacter asiaticus (CLas), the pathogen putatively associated with HLB in Florida. Mn, Zn, B, and Mg were applied in all experiments to drip at 0×, 0.5×, 1.0×, and 2.0×/spray of what has been traditionally recommended in Florida to correct deficiencies. Treatments were applied foliarly 3×/year with the sprays occurring during each growth flush for 5 years (2010–14). Thus, the 0×, 0.5×, 1.0×, and 2.0×/spray treatments resulted in 0×, 1.5×, 3.0×, and 6.0×/year to correct deficiencies. MnS04 and ZnSO4 were applied with or without KNO3 and in separate experiments were compared with Mn3(PO3)2 and Zn3(PO3)2, respectively. Disease incidence, foliar nutrient content, canopy volume, and yield were measured. At the beginning of the experiment, foliar N, P, Ca, Mg, Cu, and B were in the sufficient range and K, Mn, Zn, and Fe were slightly low. Disease incidence was very high with 83% and 98% of trees testing positive for CLas in 2010 and 2014, respectively. Nutrients that are not mobile or have limited mobility in plants, namely Mn, Zn, and B, demonstrated an increase in foliar concentration immediately after spray and in the annual averages. Foliar K increased from the deficient to the sufficient level by KNO3 sprays, but the mobile nutrients N and Mg did not show an increase in foliar levels, indicating that intraplant transport occurs in the presence of HLB. Foliar KNO3 application had a stronger effect on growth than yield. Yield was most strongly affected by application of MnSO4 where yield of the 3×/year treatment was 45% higher than that of the unsprayed control, but yield declined by 25% for the 6×/year treatment. Yield within 95% of the maximum occurred with foliar Mn concentrations of 70–100 µg·g−1 dry weight when Mn was applied as MnSO4, which is at the high end of the traditionally recommended 25–100 µg·g−1 dry weight range. The phosphite form of Mn [Mn3(PO3)2] depressed yield by an average of 25% across all application concentrations. Zn, B, and Mg did not significantly impact yield. Canopy volume demonstrated concave relationships across application concentrations for MnSO4 and ZnSO4 without KNO3 and Mn3(PO3)2, Zn3(PO3)2, Boron, and MgSO4 with KNO3, with the minimum occurring near the 3×/year application concentration. These data indicate a complex interaction in the amount of nutrients applied and their corresponding effects on foliar concentration, growth, and yield for HLB-affected trees. The results of this study at least partially explain the current confusion among scientists and the commercial industry in how to manage nutrition of HLB-affected citrus trees. The traditionally recommended approaches to correcting nutrient deficiencies need to be reconsidered for citrus with HLB.