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Kathryn M. Santos, Paul R. Fisher, and William R. Argo

2) supply nutrients for direct foliar uptake. Once severed from the stock plant, hormones such as ethylene, jasmonates, and auxins increase at the stem base and subsequently play diverse roles in initiating adventitious root development ( Ahkami et

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Qinglong Zhang and Patrick H. Brown

The distribution and transport of foliar applied Zn were determined for pistachio (Pistachio vera L.) seedlings and mature trees using stable 68Zn isotope. In seedlings, ≈5.4% of Zn adsorbed by the leaf was transported out of the treated leaves and this Zn was detected in all other plant parts to varying extent. In mature trees, the transport of Zn occurred both acropetally and basipetally within the leaflets with more basipetal movement; however, no significant amount of Zn was transported out of the treated leaflets during the first 10 days after application. The total percentage of Zn transported to other plant parts 20 days after application was significantly greater when Zn was applied to immature leaflets (6.5%) than to mature leaflets (2.1%), though the majority of the absorbed Zn remained within the treated leaflets. The limited mobility of foliar-absorbed Zn in pistachio may partially be attributed to the high binding capacity of leaf tissue for Zn.

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Qinglong Zhang and Patrick H. Brown

The characteristics and mechanisms of foliar Zn uptake and translocation in pistachio (Pistachio vera L.) and walnut (Juglans regia L.) were investigated using 68Zn labelling in both intact and detached leaves. Following washing, mature walnut and pistachio leaves retained 8% and 12% of the total Zn applied, respectively. About half of retained Zn (3.5% and 6.5% of total Zn respectively) was absorbed into the leaf and translocated outside the treated area. Leaf age affected the Zn absorption capacity of pistachio but not walnut. Immature pistachio leaves absorbed more Zn than mature leaves. The absorption of Zn by walnut leaves at high concentrations (7.5 to 15 mm Zn) was not significantly affected by the pH of the solution. In pistachio Zn absorption was greatest at pH 3.5 and declined as pH increased to 8.5. The uptake process was not affected by light or addition of metabolic inhibitors. Foliar leaf absorption was only slightly affected by changes in temperature with an average Q10 of 1.2 to 1.4. This study suggests that foliar Zn uptake is dominated by an ion exchange and/or diffusion process rather than an active one. This study also demonstrates the usefulness of stable isotope labelling in studies of foliar Zn absorption.

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Geno A. Picchioni, Steven A. Weinbaum, and Patrick H. Brown

Factors affecting the phloem mobility of foliar-applied B have received little study. The purpose of this experiment was to evaluate foliar retention of B solutions, foliar uptake kinetics, and phloem mobility of foliar-applied B among four tree fruit species. Leaves on current-year shoots of nonbearing 'Red Delicious' apple, 'Bartlett' pear, 'French' prune, and 'Bing' cherry were immersed in 1000 mg/liter B solutions (supplied as 10B-enriched boric acid) in midsummer. Export of the applied label from leaves was monitored between 0 and 24 h, and throughout the following 20 days by ICP-mass spectrometry. Uptake by leaves increased steadily in all species between 0 and 24 h, and reached 80% to 95% of the applied quantity within 24 h. By 24 h, 62% to 75% of the applied label, depending on species, had been exported from the treated leaves. Apple leaves retained, absorbed, and exported over twice the amount of labelled B as prune and pear leaves, and nearly four times the amount of cherry leaves. Foliar retention largely controlled the capacity for uptake and export.

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Brian J. Boman, Mongi Zekri, and Ed Stover

Although citrus (Citrus spp.) is sensitive to salinity, acceptable production can be achieved with moderate salinity levels, depending on the climate, scion cultivar, rootstock, and irrigation-fertilizer management. Irrigation scheduling is a key factor in managing salinity in areas with salinity problems. Increasing irrigation frequency and applying water in excess of the crop water requirement are recommended to leach the salts and minimize the salt concentration in the root zone. Overhead sprinkler irrigation should be avoided when using water containing high levels of salts because salt residues can accumulate on the foliage and cause serious injury. Much of the leaf and trunk damage associated with direct foliar uptake of salts can be reduced by using microirrigation systems. Frequent fertilization using low rates is recommended through fertigation or broadcast application of dry fertilizers. Nutrient sources should have a relatively low salt index and not contain chloride (Cl) or sodium (Na). In areas where Na accumulates in soils, application of calcium (Ca) sources (e.g., gypsum) has been found to reduce the deleterious effect of Na and improve plant growth under saline conditions. Adapting plants to saline environments and increasing salt tolerance through breeding and genetic manipulation is another important method for managing salinity.

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Anil P. Ranwala, Garry Legnani, and William B. Miller

Several experiments were conducted to find effective ways of utilizing gibberellin4+7 (GA4+7) and benzyladenine (BA) to prevent leaf chlorosis during greenhouse production of Easter lilies (Lilium longiflorum Thunb.) while minimizing the undesirable side effects on stem elongation. On an absolute concentration basis, GA4+7 was much more effective than BA in preventing leaf chlorosis. Excessive levels of GA4+7, however, tended to cause stem elongation. When applied at around the visible bud stage, if the foliage was well covered with the spray solution, 25 mg·L-1 of GA4+7 was adequate for maximum protection against leaf chlorosis. Increasing the GA4+7 concentration above 25 mg·L-1 gave no additional benefit on leaf chlorosis. Two possible modes of GA4+7 uptake during a foliar spray application (absorption through leaves and stems, and root uptake of the extra run-off) were studied in terms of their relative contribution to leaf chlorosis and stem elongation. Although both modes of uptake prevented leaf chlorosis, foliar uptake was much more effective than root uptake. However, GA4+7 taken up by the roots contributed mainly to stem elongation. When sprayed to leaves on only the lower half of the plant, a 10-mL spray of either 25 or 50 mg·L-1 of each GA4+7 and BA was enough for complete protection against leaf chlorosis. Increasing volumes had no additional benefit on leaf chlorosis, but increased the chances of unwanted stem elongation.

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G.A. Picchioni, S.A. Weinbaum, and P.H. Brown

Leaf retention, uptake kinetics, total uptake (per unit leaf area), export kinetics, and the total export of foliage-applied, labeled B (]0B-enriched boric acid) were determined for apple (Malus domestics Borkh.), pear (Pyrus communis L.), prune (Prunus domestics L.), and sweet cherry (P. avium L.). Foliar uptake of labeled B by shoot leaves was 88% to 96% complete within 24 hours of application. More than 50% of the B retained on shoot leaf surfaces following application was absorbed and exported within 6 hours of application. Genotypic differences in shoot leaf surface characteristics among the species tested greatly influenced the amount of solution retained per unit leaf area. Leaf retention capacity was the primary determinant of the quantity of B absorbed by and exported from shoot leaves following foliar application. On average, apple shoot leaves retained, absorbed, and exported at least twice as much labeled B per unit leaf area as prune and pear shoot leaves and three to four times as much as sweet cherry shoot leaves. The sink demand of nearby, mature apples did not affect the export of labeled B when applied to adjacent spur leaves, but the fruit imported 16% of their total B from the applied solution during a 10-day period. Despite extensive documentation for the immobility of B accumulated by leaves naturally (e.g., from the soil), the B accumulated by leaves following foliage application was highly mobile in all four species tested.

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Bhaskar Bondada, Peter D. Petracek, and Jim Syvertsen

Recent interest in reducing nitrate levels in ground water has stimulated the re-examination of foliar application of urea on citrus trees. Because the cuticle is the primary barrier to foliar uptake, we examined the diffusion of 14C-urea through isolated citrus leaf cuticles. Cuticles were enzymatically isolated from leaves of the four youngest nodes (1 month to 1 year old) of pesticide-free grapefruit trees. The diffusion system consisted of a cuticle mounted on a receiver cell containing stirred buffer solution. Urea (1 μL) was pipetted onto the cuticular surface, and buffer solution was sampled periodically through the side portal of the receiver cell. The time course of urea diffusion was characterized by lag (time to initial penetration), quasi-linear (maximum penetration rate), and plateau (total penetration) phases. Apparent drying time was less than 30 min. Average lag time was about 10 min. The maximum penetration rate occurred about 40 min after droplet application and was about 2% of the amount applied per hour. Rewetting stimulated further penetration. The total penetration averaged about 35% and tended to decrease with leaf age. Dewaxing the second node cuticles by solvent extraction significantly increased maximum penetration rates (30% of the amount applied per hour) and total penetration (64%).

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Bruce W. Wood, Charles C. Reilly, and Andrew P. Nyczepir

The discovery of nickel (Ni) deficiency in field plantings of pecan [Caryaillinoinensis (Wangenh.) K. Koch] (Wood et al., 2004) has led to efforts to identify appropriate management approaches to correct tree deficiency and to identify the causes for Ni deficiency. Evaluation of several inorganic and organic forms of Ni have indicated that solutions from all sources function well to correct deficiencies when timely applied as a foliar spray to affected trees at Ni concentrations >10 mg·L-1. Addition of urea, ammonium nitrate, or nicotinic acid to Ni spray solutions increased apparent foliar uptake from Ni sprays. The lower critical level of Ni, based on foliar analysis, appears to be in the 3-5 mg·L-1 dw range, with the upper critical level appearing to be >50 mg·L-1 dw. The cause of Ni deficiency in soils possessing plenty of Ni is associated with excessive amounts of one or more metals (e.g., Ca, Mg, Fr, Mn, Cu, and Zn) that inhibit Ni uptake and/or utilization. Root damage by nematode feeding and cool/dry soils during early spring also contributes to Ni deficiency. Foliar application of Ni to foliage in the autumn and subsequent appearance of Ni in dormant season shoot tissues indicates that Ni can be mobilized from senescing foliage to dormant season shoots and is therefore available for early spring growth. Evidence indicates that pecan has a higher Ni requirement than most other crop species because it transports nitrogenous substances as ureides. Thus, there is evidence that Ni-metalloenzymes are playing either a direct or indirect role in ureide and nitrogen metabolism. It is postulated that crop species that are most likely to exhibit field level Ni deficiencies are those that transport N as ureides. Candidate crops will be discussed.

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Louise Ferguson and Steven R. Grattan

There are two ways salinity can damage citrus: direct injury due to specific ions, and osmotic effects. Specific ion toxicities are due to accumulation of sodium, chloride, and/or boron in the tissue to damaging levels. The damage is visible as foliar chlorosis and necrosis and, if severe enough, will affect orchard productivity. These ion accumulations occur in two ways. The first, more controllable and less frequent method, is direct foliar uptake. Avoiding irrigation methods that wet the foliage can easily eliminate this form of specific ion damage. The second way specific ion toxicity can occur is via root uptake. Certain varieties or rootstocks are better able to exclude the uptake and translocation of these potentially damaging ions to the shoot and are more tolerant of salinity. The effect of specific ions, singly and in combination, on plant nutrient status can also be considered a specific ion effect. The second way salinity damages citrus is osmotic effects. Osmotic effects are caused not by specific ions but by the total concentration of salt in the soil solution produced by the combination of soil salinity, irrigation water quality, and fertilization. Most plants have a threshold concentration value above which yields decline. The arid climates that produce high quality fresh citrus fruit are also the climates that exacerbate the salt concentration in soil solution that produces the osmotic effects. Osmotic effects can be slow, subtle, and often indistinguishable from water stress. With the exception of periodic leaching, it is difficult to control osmotic effects and the cumulative effects on woody plants are not easily mitigated. This review summarizes recent research for both forms of salinity damage: specific ion toxicity and osmotic effects.