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P.H. Brown and H. Hu

We have demonstrated that boron (B) is freely phloem mobile in a number of crop species and we predict that B will be mobile in all species that transport polyols (mannitol, sorbitol, dulcitol). This finding directly contradicts accepted dogma and profoundly influences the diagnosis and management of B in almond, apple, apricot, cherry, pear, peach, plum, prune, celery, and other species. In the majority plants, B moves in the xylem with the transpiration stream. Once B enters the leaf, it remains there with little or no redistribution. As a result, there is always a decreasing concentration gradient of B from old to young leaves and B toxicity symptoms always occurs in the old leaves first, typically exhibiting tip and margin burn. In species in which B is mobile, these symptoms do not occur. When almond, peach, and plum were exposed to high B in the growth medium, the predominant site of B accumulation was fruit, young stems and apical meristems. As a consequence, the earliest symptoms of B toxicity in species in which B is phloem mobile are observed in the young shoot meristems and fruits. Foliar application of 10B isotope demonstrates that B is readily transported to neighboring fruits and buds of almond, apple, and nectarine. In apple seedlings, plant B requirements can be fully satisfied solely by foliar application to a few mature leaves. This strongly suggest that foliar B applications can be used as an efficient means for B fertilization in Malus, Prunus, and Pyrus species. Details of the studies and their implications for B management will be discussed.

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Scot H. Hulbert, Gongshe Hu, and Jeff A. Drake

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Patrick H. Brown, Hening Hu, and Rawia El-Mntaium

Excessive soil and water boron (B) occur widely in California, often in conjunction with high soil salinity. Descriptions of the symptoms of B toxicity and quantification of its impact on Prunus species are not available. In these experiments we describe the impact of high B and saline conditions on uptake, distribution and growth depression in almond, peach, plum and peach/almond hybrid rootstocks

A series of experiments are described that indicate an important additive effect of B on sensitivity of Prunus species to salinity. Boron concentrations in excess of lppm in the irrigation solution, significantly impair plant growth under moderate (non-limiting) salinity conditions and lead to plant death at higher salinity levels. Symptoms of B toxicity in Prunus include stem necrosis and vascular occlusion. Unlike most other species, B does not appear to accumulate in the leaf margins and leaf symptoms are generally not observed. Differences in sensitivity of a range of Prunus species to B toxicity are described.

Initial results suggest that differences in rootstock sensitivity to B and salinity are the result of differential uptkake and partitioning of B, Na and Cl within the plant.

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Patrick H. Brown, Hening Hu, and Warren G. Roberts

The phloem mobility of boron (B) in plants varies dramatically among species. Variations in phloem B mobility occur as a consequence of the presence of sugar alcohols (polyols) in some species but not in others, and these differences in phloem B mobility profoundly affect the expression of B toxicity symptoms. Twenty-four species including common ornamental species varying in sugar alcohol content, were selected to test their response to B toxicity. Species that do not produce sugar alcohols exhibited previously described B toxicity symptoms that include accumulation of high concentrations of B in, and burning of, the tip and margin of old leaves. In the sugar-alcohol-producing species these symptoms were absent, and B toxicity was expressed as meristematic dieback and an accumulation of B in apical tissues. These symptoms have not previously been associated with B toxicity in these species and hence may have been frequently misdiagnosed.

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Clyde W. Fraisse, Zhengjun Hu, and Eric H. Simonne

Most of the winter vegetable production in the southeastern United States is located in Florida. High-value vegetable crops are grown under intensive fertilization and irrigation management practices using drip, overhead, or seepage irrigation systems. Rainfall events may raise the water table in fields irrigated by seepage irrigation resulting in leaching of nutrients when the level is lowered to remove excess water. The objective of this study was to assess the effect of El Niño–Southern Oscillation (ENSO) phases on rainfall distribution and leaching rain occurrences during the fall, winter, and spring tomato (Solanum lycopersicum) growing seasons using long-term weather records available for main producing areas. Differences in fall growing season mean precipitation during El Niño, La Niña, and neutral years were found to be nonsignificant. Winter and spring mean precipitations during El Niño, La Niña, and neutral years were found to be significantly different. Winter and spring average rainfall amounts during La Niña and neutral years were lower than during El Niño years. During El Niño years, at least one leaching rainfall event of 1.0 inch or more in 1 day occurred at all locations and all planting seasons and two of these events occurred in more than 9 of 10 years except during the winter and spring planting seasons at the Tamiami Trail station located in Miami–Dade County. During the fall growing season of El Niño years, three to four 1.0 inch or more in 1-day leaching rainfalls may be expected at least 4 of 5 years at all locations. In the case of larger leaching rainfall events (3.0 inches or more recorded in 3 days or 4.0 inches or more recorded in 7 days), the probability of having at least one event was mostly less than 0.80. Based on these results, nitrogen fertilizer supplemental applications of 30 to 120 lb/acre could be applied during the fall growing season of all ENSO phases and during all planting seasons of El Niño years. Using current fertilizer prices, one supplemental fertilizer application of 30 lb/acre nitrogen and 16.6 lb/acre potassium costs $55/acre. Assuming a median wholesale price of $12 per 25-lb box, this additional cost may be offset by a modest yield increase of 4.6 boxes/acre (compared with a typical 2500 25-lb box/acre marketable yield). These results suggest that ENSO phases could be used to predict supplemental fertilizer needs for tomato, but adjustments to local weather conditions may be needed.

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Rawia El-Motaium, Hening Hu, and Patrick H. Brown

The influence of B and salinity [3 Na2SO4 : 1 CaCl2, (molar ratio)] on B toxicity and the accumulation of B, sodium, and SO4 in six Prunus rootstocks was evaluated. High salinity reduced B uptake, stem B concentrations, and the severity of toxicity symptoms in five of the six rootstocks. Forward and backward stepwise regression analyses suggested that stem death (the major symptom observed) was related solely to the accumulation of B in the stem tissue in all rootstocks. The accumulation of B and the expression of toxicity symptoms increased with time and affected rootstock survival. No symptoms of B toxicity were observed in leaf tissue. The Prunus rootstocks studied differed greatly in stem B accumulation and sensitivity to B. The plum rootstock `Myrobalan' and the peach-almond hybrid `Bright's Hybrid' were the most tolerant of high B and salinity, whereas the peach rootstock `Nemared' was very sensitive to high B and salinity. In all rootstocks, adding B to the growth medium greatly depressed stem SO4 concentrations. In every rootstock except `Nemared' peach, adding salt significantly depressed tissue B concentrations. A strong negative correlation between tissue SO4 and B was observed. Grafting experiments, in which almond was grafted onto `Nemared' peach or `Bright's Hybrid', demonstrated the ability of rootstocks to influence B accumulation and scion survival.

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Xunzhong Zhang, Erik H. Ervin, Yiming Liu, Guofu Hu, Chao Shang, Takeshi Fukao, and Jasper Alpuerto

Water deficit is a major limiting factor for grass culture in many regions with physiological mechanisms of tolerance not yet well understood. Antioxidant isozymes and hormones may play important roles in plant tolerance to water deficit. This study was designed to investigate antioxidant enzymes, isozymes, abscisic acid (ABA), and indole-3-acetic acid (IAA) responses to deficit irrigation in two perennial ryegrass (Lolium perenne L.) cultivars contrasting in drought tolerance. The plants were subjected to well-watered {100% container capacity, 34.4% ± 0.21% volumetric moisture content (VWC), or deficit irrigation [30% evapotranspiration (ET) replacement; 28.6% ± 0.15% to 7.5% ± 0.12% VWC]} conditions for up to 8 days and rewatering for 4 days for recovery in growth chambers. Deficit irrigation increased leaf malondialdehyde (MDA) content in both cultivars, but drought-tolerant Manhattan-5 exhibited lower levels relative to drought-sensitive Silver Dollar. Superoxide dismutase (SOD) activity declined and then increased during water-deficit treatment. ‘Manhattan-5’ had higher SOD activity and greater abundance of SOD1 isozyme than ‘Silver Dollar’ under water deficit. Deficit irrigation increased catalase (CAT) and ascorbate peroxidase (APX) activity in ‘Manhattan-5’, but not in ‘Silver Dollar’. ‘Manhattan-5’ had higher CAT, APX, and peroxidase (POD) activity than ‘Silver Dollar’ during water limitation. Deficit irrigation increased mRNA accumulation of cytosolic cupper/zinc SOD (Cyt Cu/Zn SOD), whereas gene expression of manganese SOD (Mn SOD) and peroxisome APX (pAPX) were not significantly altered in response to deficit irrigation. No differences in Cyt Cu/Zn SOD, Mn SOD, and pAPX gene expression were found between the two cultivars under deficit irrigation. Water limitation increased leaf ABA and IAA contents in both cultivars, with Silver Dollar having a higher ABA content than Manhattan-5. Change in ABA level may regulate stomatal opening and oxidative stress, which may trigger antioxidant defense responses. These results indicate that accumulation of antioxidant enzymes and ABA are associated with perennial ryegrass drought tolerance. Activity and isozyme assays of key antioxidant enzymes under soil moisture limitation can be a practical screening approach to improve perennial ryegrass drought tolerance and quality.

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Arnold H. Hara, Trent Y. Hata, Victoria L. Tenbrink, Benjamin K.S. Hu, and Mike A. Nagao

Postharvest treatments significantly reduced or eradicated pests on various tropical cut flowers and foliage. Immersion in water at 49° C for 10 minutes killed armored scales on bird of paradise leaves, Strelitzia reginae Banks, as well as aphids and mealybugs on red ginger, Alpinia purpurata (Vieill.) K. Schum. Vapor heat treatment for 2 hours at 45.2° C provided quarantine security against armored scales on bird of paradise leaves. A 5 minute dip in fluvalinate combined with insecticidal soap eliminated aphids and significantly reduced mealybugs on red ginger. A 3 minute dip in fluvalinate, a 3 minute dip in chlorpyrifos, or a 3 hour fog with avermectin-B significantly reduced thrips on orchids, Dendrobium spp., without injury to the flowers. No postharvest treatment was both effective and nonphytotoxic on all commodities.

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Slavko Perica, Nacer Bellaloui, Carl Greve, Hening Hu, and Patrick H. Brown

Boron (B) remobilization, mannitol and glucose concentrations, and the effect of B application on changes in soluble carbohydrates were investigated in various organs of bearing `Manzanillo' olive trees (Olea europaea L. `Manzanillo'). Following foliar 10B application to leaves of various ages, there was significant 10B export out of the treated leaves, and significant 10B enrichment in nontreated adjacent organs, including inflorescences and fruit. Results demonstrated that B can be remobilized from leaves of various ages, and that foliar-applied B is phloem mobile in olive. Soluble carbohydrate analysis determined that mannitol and glucose are the predominant sugars in all organs analyzed and that the mannitol concentration in the leaves is adequate to account for all B transport. This is consistent with observations in other species, where the presence of mannitol is known to facilitate phloem B transport through formation of a mannitol-B complex. Previous reports have indicated that B application can alter carbohydrate metabolism. In the present study, foliar B application significantly suppressed glucose concentration in the leaf petioles of all ages and increased mannitol in petioles of the current-year-developed leaves.

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John S. Hu, Diane M. Sether, H. Michael Harrington, and Diane E. Ullman

Thermotolerance of pineapple crowns (`Champaka 153') to 50C and above was increased with a 30-min first treatment at 30, 35, or 40C. Pineapple crowns receiving a 30-min heat treatment, before a second heat treatment at 50 or 55C, exhibited significantly less leaf damage than controls receiving no first treatments (P ≤ 0.05). The degree of thermotolerance was dependent upon the season in which crowns were harvested; greater thermotolerance occurred in crowns harvested in April than those harvested in October. Maximum thermotolerance occurred after an interval of at least 8 h between the first treatment and the higher temperature heat treatment. Thermotolerance was stable for at least 24 h.