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Susan C. Miyasaka, Jeff B. Million, Nguyen V. Hue, and Charles E. McCulloch

Possible boron (B) deficiency symptoms were observed on avocado (Persea americana Mill. `Sharwil') grown in Kona, Hawaii. To determine the B requirement of young, `Sharwil' avocado trees, two greenhouse experiments were conducted. In a soil study, seven B treatments (0, 3.7, 11, 22, 44, 89, and 178 mg·kg–1 soil fines) were applied to 1-year-old grafted `Sharwil' avocado trees grown for 13 weeks in a Tropofolist soil. Due to the low and variable fractions of soil fines in this rocky soil, extractable soil B concentration did not appear to be a good predictor of B requirements by avocados. Adequate foliar B concentrations in `Sharwil' avocado trees based on dry weight and area of new leaves ranged from 37 (±3) to 65 (±4) and from 31 (±10) to 78 (±13) mg·kg–1 (dry-weight basis), respectively. (Means are followed by standard errors of the mean in parentheses.) In a hydroponics study, 6-month-old grafted `Sharwil' avocado trees were supplied with four levels of B (0, 1, 10, and 100 μm). At 11 months after B treatment initiation, leaves with deformed margins and a “shot-hole” appearance were first observed at a solution level of 0 μm B. At 14 months after B treatment initiation, foliar B concentrations that were associated with 12% to 14% incidence of deformed leaves ranged from 9.8 to 13.5 mg·kg–1 (dry-weight basis). Although `Sharwil' avocados are reportedly susceptible to B deficiency, foliar B concentrations required for adequate growth and those associated with B deficiency symptoms are similar to those for other cultivars.

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Roberto Núñez-Elisea, Bruce Schaffer, Mongi Zekri, Stephen K. O'Hair, and Jonathan H. Crane

Most tropical fruit trees in southern Florida are grown in calcareous gravelly soil that is mechanically trenched to a depth of about 50 cm (about 20 inches). Fruit trees are often planted at the intersections of perpendicular trenches to provide space for root development. Tree root systems are concentrated in the top 10 to 20 cm (about 4 to 8 inches) of soil. Extreme soil rockiness has made it difficult to obtain consistent and reliable measurements of soil water status and to collect soil samples for constructing soil-water characteristic curves in the laboratory. Multisensor capacitance probes andlow-tension [0 to 40 kPa (centibars) (0 to 5.8 lb/inch2)] tensiometers were installed adjacent to star fruit (Averrhoa carambola L.) and avocado (Persea americana Mill.) trees in trenches to simultaneously measure volumetric soil water content and soil matric potential in situ. Capacitance probes consisted of four sensors centered at depths of 10, 20, 30, and 50 cm (3.9, 7.9, 11.8, and 19.7 inches). Tensiometers were installed at 10- and 30-cm depths, adjacent to the 10- and 30-cm deep capacitance sensors. Measurements obtained with both instruments were used to generate in situ soil-water characteristic curves. Rock fragments were more abundant at 30 cm than at 10 cm (71% to 73% versus 26% to 38% of bulk soil volume, respectively) soil depth, which limited the precision of tensiometers at the greater depth. In situ soil water characteristic curves for the 10-cm soil depth can be used to determine parameters needed for irrigation scheduling by techniques such as the water budget method.

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Nydia Celis, Donald L. Suarez, Laosheng Wu, Rui Li, Mary Lu Arpaia, and Peggy Mauk

Avocado (Persea americana Mill.) is one of the most salt-sensitive crops and one of the highest value crops per acre. In the United States, avocados are grown primarily in California, in regions experiencing both scarcity of freshwater and salinization of available water supplies. Thus, our objectives were to evaluate avocado rootstocks for salt tolerance and evaluate the relationship between leaf ion concentrations, trunk diameter, leaf burn, and fruit yield. Our field experiment evaluated the salt tolerance of the Hass scion grafted onto 13 different avocado rootstocks using the Brokaw clonal rootstock technique. The experiment consisted of 156 trees arranged in a randomized complete block design with six replications of each saline [electrical conductivity (EC) = 1.5 dS·m–1, Cl = 4.94 mmol·L–1] and nonsaline (EC= 0.65 dS·m–1, Cl = 0.73 mmol·L–1) irrigation water treatment. We collected soil samples and leaves, then analyzed them for major ions. The rootstocks R0.06, R0.07, PP14, and R0.17, which had high concentrations of Cl and Na in the leaves, were the least salt tolerant, with 100% mortality in the rows irrigated with saline water for 23 months. The rootstocks R0.05, PP40, R0.18, and Dusa, which had low concentrations of Cl ions in the fully expanded leaves, were least affected by salinity, and these rootstocks exhibited the greatest yields, largest trunk diameters, and greatest survival percentages in the saline treatment. Yield and growth parameters correlated well with leaf Cl concentration, but not Na, indicating that salt damage in avocado is primarily a result of Cl ion toxicity. Under arid inland environments, no variety performed satisfactorily when irrigated with an EC = 1.5 dS·m–1 water (Cl = 4.94 mmol·L–1). However, the more tolerant varieties survived at soil salinity levels that would apparently be fatal to varieties reported earlier in the literature.

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Claudia Fassio, Ricardo Cautin, Alonso Pérez-Donoso, Claudia Bonomelli, and Mónica Castro

Root morphological traits and biomass allocation were studied in 2-year-old ‘Duke 7’ avocado (Persea americana) trees propagated using seedling and clonal techniques. The plants either were or were not grafted with the scion ‘Hass’. Whole tree excavation 1 year after planting revealed that the propagation technique affected the root growth angle of the main roots (third order roots), the root length density (defined as the total length of roots per volume of soil), and the number of first and second order roots present. The root system of clonal trees showed a typical morphology of rooted cuttings, with a crown of roots originating from a relatively short stem, resulting in a shallow root system. Clonal trees, compared with seedlings, produced main framework roots with shallower angles and more fine roots (first and second order roots) that increased the root length density (defined as the total length of roots per volume of soil). Nongrafted seedlings exhibited a main taproot and lateral roots with narrow angles that penetrated deeper into the soil and increased the aboveground biomass but had a lower root-to-shoot ratio than nongrafted clonal trees. The grafting of both clonal and seedling trees resulted in similar root architecture and revealed that grafting significantly decreased the soil volume explored and the shoot and root biomass. Although both root systems were shallow, grafted clonal trees had a higher root-to-shoot ratio than grafted seedlings. In this study, a distinct class of roots with large diameter and unbranched growth was more abundant in the root systems of clonal trees. These types of roots (previously undescribed in avocado trees), called pioneer roots, may enhance soil exploration in clonal trees.

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Samuel Salazar-García and Carol J. Lovatt

The objectives of the present research were to quantify 1) the contribution that vegetative shoots produced in the summer vs. fall and indeterminate vs. determinate inflorescences make to yield and 2) the effects of GA3 on flowering expression and inflorescence phenology of summer and fall shoots of `Hass' avocado (Persea americana Mill.) under field conditions. Anthesis started earlier on fall than summer shoots of 10-year-old `Hass' avocado trees; however, no difference in the date of full bloom was observed. Indeterminate inflorescences that underwent early anthesis set more fruit than those with delayed anthesis, conversely, determinate inflorescences with delayed anthesis set more fruit. Indeterminate inflorescences comprised 90% of total inflorescences and contributed 73% of total fruit yield, but individual determinate inflorescences were at least three times more productive than the indeterminate ones. Summer and fall shoots were sprayed with 0, 50, 100, or 1000 mg·L-1 GA3 in November, December or January. GA3 stimulated apical growth of all shoots. If secondary axes of an inflorescence bud were differentiated at the time of GA3 application, the inflorescence developed in advance of inflorescences on branches not treated with GA3. In addition, GA3 caused precocious development of the vegetative shoot of indeterminate inflorescences relative to the flowers in the same inflorescence and relative to the vegetative shoot of indeterminate inflorescences from untreated branches. Stimulation of vegetative growth at the inflorescence apex by GA3 inhibited growth of axillary buds. GA3 at 50 mg·L-1 had no effect on the number of determinate or indeterminate inflorescences produced by either summer or fall shoots. Higher concentrations of GA3 increased the number of vegetative shoots and inactive buds produced by both shoot types.

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Robert K. Prange, John M. DeLong, Peter A. Harrison, Jerry C. Leyte, and Scott D. McLean

A new chlorophyll fluorescence (F) sensor system called FIRM (fluorescence interactive response monitor) was developed that measures F at low irradiance. This system can produce a theoretical estimate of Fo at zero irradiance for which we have coined a new fluorescence term, Fα. The ability of Fα to detect fruit and vegetable low-O2 stress was tested in short-term (4-day) studies on chlorophyll-containing fruit [apple (Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.), pear (Pyrus communis L.), banana (Musa ×paradisiaca L.), kiwifruit (Actinidia deliciosa C.S. Liang & A.R. Ferguson), mango (Mangifera indica L.), and avocado (Persea americana Mill.)] and vegetables (cabbage (Brassica oleracea L. Capitata Group), green pepper (Capsicum annuum L. Grossum Group), iceberg and romaine lettuce (Lactuca sativa L.)). In all of these fruit and vegetables, Fα was able to indicate the presence of low-O2 stress. As the O2 concentration dropped below threshold values of 0 to 1.4 kPa, depending on the product, the Fα value immediately and dramatically increased. At the end of the short-term study, O2 was increased above the threshold level, whereupon Fα returned to approximately prestressed values. A 9-month study was undertaken with `Summerland McIntosh' apple fruit to determine if storing the fruit at 0.9 kPa O2, the estimated low O2 threshold value determined from Fα, would benefit or damage fruit quality, compared with threshold + 0.3 kPa (1.2 kPa O2) and the lowest recommended CA (1.5 kPa O2). After 9 months, the threshold treatment (0.9 kPa) had the highest firmness, lowest concentration of fermentation volatiles (ethanol, acetaldehyde, ethyl acetate) and lowest total disorders. Sensory rating for off-flavor, flavor and preference indicated no discernible differences among the three treatments.

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Samuel Salazar-García and Carol J. Lovatt

Avocado trees (Persea americana Mill.) bearing a heavy crop produce a light “off” bloom the next spring. This results in a light crop and a subsequent intense “on” bloom the year after. The objective of the study was to quantify the effects of GA3 canopy sprays applied to `Hass' avocado trees during the months preceding an “off” or “on” bloom on inflorescence and vegetative shoot number and yield. The experiment was initiated approximately seven months before an anticipated “off” bloom in an attempt to increase flowering intensity and yield. GA3 (25 or 100 mg·L-1) was applied to separate sets of trees in September (early stage of inflorescence initiation), November (early stage of inflorescence development), January (initial development of the perianth of terminal flowers), March (cauliflower stage of inflorescence development; only 25 mg·L-1), or monthly from September through January (only 25 mg·L-1). Control trees did not receive any treatment. GA3 (100 mg·L-1) applied in September reduced inflorescence number in both years, but not yield. GA3 (25 or 100 mg·L-1) applied in November before the “on” bloom reduced inflorescence number with a concomitant increase in vegetative shoot number and 47% yield reduction compared to control trees. This treatment might provide avocado growers with a tool to break the alternate bearing cycle by reducing yield in an expected “on” crop year to achieve a higher yield the following year. GA3 (25 mg·L-1) applied in November or January stimulated early development of the vegetative shoot of indeterminate inflorescences. January and March applications did not affect the number of flowering or vegetative shoots produced either year. GA3 (25 mg·L-1) applied in March at the start of an “off” bloom increased 2-fold the production of commercially valuable fruit (213 to 269 g per fruit) compared to the control.

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Allan B. Woolf, Asya Wexler, Dov Prusky, Elana Kobiler, and Susan Lurie

Effect of direct sunlight on the postharvest behavior of five avocado (Persea americana Mill.) cultivars (Ettinger, Fuerte, Hass, Horshim and Pinkerton) was examined. Probes placed 6 to 7 mm under the peel showed that the temperature an the side exposed to the sun could be as much as 15 to 20 °C higher than the temperature of shade fruit, while the nonexposed side of the fruit was ≈5 °C higher than the shade fruit. With the exception of `Ettinger', sun fruit, and especially the exposed side, were found to be most tolerant to postharvest 50 and 55 °C hot water treatments. Similarly, storage of fruit at 0 °C for between 3 to 6 weeks caused severe chilling injury to shade fruit, with less effect on sun fruit. Furthermore, there was little or no damage on the exposed side of the sun fruit. During postharvest ripening at 20 °C, sun fruit showed a delay of between 2 to 5 days in their ethylene peak compared with shade fruit. The exposed side of the sun fruit was generally firmer than the nonexposed side, and the average firmness was higher than that of shade fruit. Activities of polygalacturonase and cellulase were similar in shade and sun fruit of similar firmness. After inoculation with Colletotrichum gloeosporioides (Penz.) Penz@sacc., the appearance of lesions on sun fruit occurred 2 to 3 days after shade fruit. Levels of heat-shock proteins were examined using western blotting with antibodies for Class I and II cytoplasmic heat-shock proteins. Class I reacted with proteins from the exposed side of sun fruit and Class II with proteins from both sides of sun fruit. Thus, it is clear that preharvest exposure of fruit to the sun can result in a wide range of postharvest responses.

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Michael V. Mickelbart and Mary Lu Arpaia

Effect of salinity (1.5, 3.0, 4.5, or 6.0 dS·m-1) on growth and physiology of 1-year-old `Hass' avocado (Persea americana Mill.) trees on one of three rootstocks, `Thomas', `Toro Canyon', or `Duke 7', was investigated to determine the relative salinity tolerance of these rootstocks and to determine possible reasons for any observed differences in tolerance. Leaves of trees on `Thomas' rootstock had the highest leaf Na+, Cl-, and necrosis compared to trees on the other two rootstocks. Exposure to salinity resulted in decreased growth of shoots on all rootstocks, but was greatest on `Thomas' and least on `Duke 7'. The oldest leaves on all rootstocks had the highest proportion of leaf necrosis, whereas younger leaves exhibited almost no necrosis. Salinity reduced net CO2 assimilation (A) and chlorophyll concentrations of scion leaves on all rootstocks, but more in older leaves than in younger leaves. Although the effects of salinity on A were greater for trees on `Thomas' on one measurement date, overall, rootstock differences in A were not significant for any leaf age. Differences in response to salinity among rootstocks were noted primarily in morphological traits such as growth and leaf necrosis, rather than physiological traits such as gas-exchange and water relations. Based on overall growth and physiological response to salinity, trees on `Thomas' performed poorest, whereas trees on `Duke 7' exhibited the greatest salt tolerance. The relative tolerance of the various rootstocks appeared to be due primarily to their ability to exclude Na+ and Cl- from the scion.

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Xuan Liu, James Sievert, Mary Lu Arpaia, and Monica A. Madore

Avocado (Persea americana Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO2 fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated d-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g-1 fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.