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Carol J. Lovatt

Foliar fertilization efficiently meets the nutrient demand of tree fruit crops during periods when soil conditions (low or high temperatures, low or excess soil moisture, pH, salinity) render soil-applied fertilizers ineffective, when nutrients (e.g., phosphate, potassium, and trace elements) become fixed in the soil, and when tree nutrient demand is high. Applying nutrients directly to leaves ensures that the metabolic machinery of the tree is not compromised by low availability of an essential nutrient. It should be noted that phloem mobile nutrients applied to the foliage are translocated to all tree parts, even feeder roots. Because foliar application of fertilizers can reduce nutrient accumulation in soil, runoff water, surface water (streams, lakes, ocean), and groundwater (drinking water supply), where they contribute to salinity, eutrophication, or nitrate contamination, with negative consequences to the environment and humans, it is highly desirable to replace soil-applied fertilizers at least in part with foliar-applied fertilizers. However, not all nutrients are taken up through leaves and, even if taken up, some nutrients are not phloem mobile. In addition, although foliar fertilizer rates are typically lower than soil fertilizer rates, application can be more costly. The goal has been to time the application of foliar fertilizers to key stages of citrus (Citrus sp.) and avocado (Persea americana) tree phenology when demand for the nutrient is likely to be high and especially when soil conditions are likely to compromise nutrient uptake by roots. This approach has proven successful for increasing yield, fruit size, and grower income even when the tree is not nutrient deficient by standard leaf analysis. Winter prebloom foliar-applied low-biuret urea was previously documented to increase total yield of both navel and ‘Valencia’ oranges (Citrus sinensis). Adaptation of this treatment to ‘Nules’ Clementine mandarin (Citrus reticulata) increased the yield of commercially valuable size fruit in two experiments. Foliar application of boron or low-biuret urea to ‘Hass’ avocado trees at the “cauliflower” stage of inflorescence development increased total yield; potassium phosphite applied at this stage of tree phenology increased the yield of commercially valuable size fruit.

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Carol J. Lovatt

The goal of this research was to identify the role essential nutrients play in the physiology of tree crops, and then to apply the nutrient as a foliar fertilizer to stimulate a specific metabolic process at phenological stages when nutrient demand is high. This approach has proven successful. A single winter prebloom foliar application of nitrogen as low-biuret urea [0.16 kg N/tree (0.35 lb N/tree)] to 30-year-old `Washington' navel orange (Citrus sinensis L. Osbeck) trees during flower initiation significantly increased yield and fruit number per tree for each of 3 consecutive years (P ≤ 0.05). The number of commercially valuable largesize fruit also increased significantly with yield increases (r 2 = 0.88). Sodium tetraborate applied foliarly to `Hass' avocado (Persea americana Mill.) trees at the cauliflower stage of inflorescence development (elongation of inflorescence secondary axes, pollen and ovule development) increased the number of pollen tubes reaching the ovule, ovule viability and cumulative yield (P ≤ 0.05). Additional examples are presented.

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Carol J. Lovatt

To protect groundwater from potential nitrate pollution, `Hass' avocado (Persea americana Mill.) growers in California divide the total annual soil-applied nitrogen (N) fertilizer (N at 56 to 168 kg·ha-1) into small applications made during the period from late January to early November. However, no research had been conducted to test the efficacy of this fertilization practice, and there was concern that the amount of N in the individual applications may be too little to meet the demand of the tree at some stages of its phenology. The research presented herein addressed the question of whether yield of `Hass' avocado could be increased by doubling the amount of N currently applied during specific stages of tree phenology. The control in this experiment was the practice of annually applying N as NH4NO3 at 168 kg·ha-1 (168 trees/ha) in six small doses of N at 28 kg·ha-1 in January, February, April, June, July, and November. From these six application times, five were selected on the basis of tree phenology and additional N as NH4NO3 at 28 kg·ha-1 was applied at each time for total annual N of 196 kg·ha-1. Two phenological stages were identified for which N application at 56 kg·ha-1 in a single application (double dose of N) significantly increased the 4-year cumulative yield (kilograms fruit per tree) 30% and 39%, respectively, compared to control trees (P ≤ 0.01). In each case, more than 70% of the net increase in yield was commercially valuable large size fruit (178 to 325 g/fruit). The two phenological stages were when shoot apical buds have four or more secondary axis inflorescence meristems present (mid-November); and during anthesis to early fruit set and initiation of the vegetative shoot flush at the apex of indeterminate floral shoots (about mid-April). When the double dose of N was applied at either of these two stages, the kilograms and number of large size fruit averaged across the 4 years of the study was significantly greater than the control trees (P ≤ 0.01). Averaged across the 4 years of the study, only the November treatment increased yield compared to the control trees (P ≤ 0.05). Application of the double dose of N at flower initiation (January), during early-stage gynoecium development (February), or during June drop had no significant effect on average or cumulative yield or fruit size compared to control trees. Application of the double dose of N in April significantly reduced the severity of alternate bearing (P ≤ 0.05). Yield was not significantly correlated with leaf N concentration. Time and rate of N application are factors that can be optimized to increase yield, fruit size, and annual cropping of `Hass' avocado. When the amounts of N applied were equal (196 kg·ha-1), time of application was the more important factor.

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Oded Sagee and Carol J. Lovatt

Maximum leaf NH3-NH4 + content and activity of the de novo arginine biosynthetic pathway occurred during the 1st week after transfer of 5-year-old rooted cuttings of the `Washington' navel orange (Citrus sinensis L. Osbeck) from 8 weeks of low-temperature stress [8-hour days (500 μmol·s-1·m-2) at 15 to 18C/16-hour nights at 10 to 13C]. Both aspects declined in parallel during the subsequent 4 weeks of 12-hour days (500 μmol·s-1·m-2) at 24 C/12-hour nights at 19C, which culminated in maximum bloom. Apical flowers of inflorescences initiated in response to 8 weeks of low-temperature stress exhibited maximum tissue concentrations of NH3-NH4 + and putrescine, and maximum activity of the de novo arginine biosynthetic pathway 1 week after transfer of the trees from the low-temperature induction to the higher temperature (flower buds were 7 × 5 mm, length/width). All three criteria decreased in parallel as flowers developed through Stage V (petal fall). In contrast, spermine concentration increased 7-fold during Stage IV of flower development (flower opening). By Stage V, ovaries contained about equal concentrations of putrescine, spermidine, and spermine. The activity of the de novo tyrosine biosynthetic pathway exhibited a pattern of change independent of flower NH3-NH4 + concentration. Observed changes were not due to increased organ weight or size and persisted when the data were expressed per milligram protein. The results of this study demonstrate that leaves and floral buds undergo parallel changes in N metabolism in response to low-temperature, stress-induced flowering and provide evidence that flower NH3-NH4 + content and putrescine synthesis via argine are metabolically correlated during flower development in C. sinensis.

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Isa Bertline and Carol J. Lovatt

Tryptophan is known to be a precursor of IAA in plants. The amount of IAA available for the development of avocado fruit might be a limiting factor for its growth. It is well known that IAA is not transported into developing fruit along its strictly basipetal transport route. Therefore, IAA present in fruit must be synthesized in situ. We investigated the possibility that tryptophan or its metabolites are transported from leaf to fruit.

An HPLC method was developed to quantitatively isolate and measure tryptophan and all well known intermediates in the synthesis of IAA. Avocado leaves were fed L-[side chain-3-14C] tryptophan and its transport and metabolism to IAA within the leaf and within the fruit were monitored over time. Significant movement of tryptophan or a metabolite from leaf to fruit occurs in 24 h.

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Yusheng Zheng and Carol J. Lovatt

Rough lemon seedlings [Citrus limon (L)] were hydroponically-cultured in complete Shive's nutrient solution (+K) or in Shive's nutrient solution with potassium omitted (-K) for a period of eight months. Fresh and dry weight of whole -K plants were reduced 4-fold (P<0.01). Nitrogen metabolism was monitored during this period in young, fully expanded leaves. Results showed that leaves of -K plants accumulated 2.5-fold more NH3-NH4 + than +K plants (P<0.01) and exhibited a concomitant increase in both activity of the de novo arginine biosynthetic pathway (2.5-fold) and free-arginine concentration (3.5-fold; P<0.001). Leaf proline content of -K plants increased 1.6-fold (P<0.05), while putrescine content increased 10-fold. Arginine decarboxylase activity was accelerated in -K plants.

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Anwar G. Ali and Carol J. Lovatt

The objective of this study was to test whether a single winter prebloom foliar application of low-biuret urea would increase the yield of 30-year-old `Washington' navel orange trees [Citrus sinensis (L.) Osbeck] on Troyer citrange rootstock [C. sinensis `Washington' × Poncirus trifoliata (L.) Raf.]. All trees received a winter (November to January) soil application of urea (0.5 kg N/tree). Trees were maintained under irrigation or irrigation was withheld from 1 Oct. to 1 Mar. To determine the optimal time for foliar urea application, trees in both irrigation main plots received one application of low-biuret urea in mid-November, mid-December, mid-January, or mid-February applied at a rate of 0.16 kg N/tree. There was a set of control trees that only received the soil application of urea. Trees receiving foliar-applied urea in mid-January or mid-February, independent of irrigation treatment, had significantly greater yield and fruit number per tree each year than the control trees for 3 consecutive years. The number of fruit with diameters of 6.1 to 8.0 cm increased significantly as yield increased (r 2 = 0.88). Withholding irrigation from 1 Oct. to 1 Mar. had a negative impact on yield. Annual winter application of low-biuret urea to the foliage did not significantly increase leaf total N at the end of 3 years.

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Anwar G. Ali and Carol J. Lovatt

The effects of different methods of sample preparation, extraction, and storage on the recovery of the combined pool of ammonia plus ammonium (NH3 + NH4 +) from `Washington' navel orange leaves previously incubated in solutions of increasing NH4 Cl concentrations were assessed. Procedures and instruments for quantifying NH3 + NH4 + were tested for their sensitivity, reproducibility, and freedom from interference by amino acids. Reliable recoveries of NH3 + NH4 + free from amino acid interference, were obtained with oven-dried (60C) leaves ground to pass through a 40-mesh screen, extracted by homogenization in 10% TCA or by shaking in 2% acetic acid, and then filtered and analyzed on the basis of differences in electrical conductance between the sample and the reference cell. Methods measuring NH3 + NH4 + in KCl extracts by reaction with salicylate-nitroprusside in the presence of hypochlorite were compromised by significant color formation due to amino acids. Using fresh or freeze-dried leaf samples resulted in lower recoveries than use of oven-dried samples. Storage at -20C of fresh or oven-dried leaf samples in 10% TCA before or after homogenization and filtration did not alter NH3 + NH4 + levels, whereas storage of these samples at 4C increased NH3 + NH4 + levels.

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Shahzad M.A. Basra and Carol J. Lovatt

Growth-promoting properties of moringa (Moringa oleifera) leaves were investigated for potential use in crop production by comparing the efficacy of bimonthly foliar and root applications of a moringa leaf extract [MLE (3.3% w/v)] with the cytokinins 6-benzyladenine (6-BA) and trans-zeatin (t-Z), each at 25 mg·L−1, to increase plant growth, flowering, yield, fruit size, and fruit quality of ‘Super Sweet 100’ cherry tomato (Solanum lycopersicum). Foliar-applied t-Z and root-applied MLE increased canopy biomass (P ≤ 0.01) and root- and foliar-applied MLE increased lateral vegetative shoot number (P ≤ 0.001) and plant height (P ≤ 0.001) relative to untreated control plants. Only foliar-applied MLE increased floral shoot number compared with untreated control plants (P ≤ 0.001). Plants in all treatments, except root-applied 6-BA, produced more flowers than untreated control plants (P ≤ 0.001). Plants receiving root-applied t-Z produced the greatest number of flowers followed by plants receiving root-applied MLE. Cherry tomato plants treated with root-applied t-Z or MLE produced the greatest number of fruit per plant and significantly more than untreated control plants (P ≤ 0.001). Foliar-applied 6-BA and MLE and root-applied t-Z and MLE increased yield as grams of fruit per plant compared with the untreated control (P ≤ 0.01). Foliar- and root-applied MLE increased fruit concentrations of soluble sugars (P ≤ 0.001), protein (P ≤ 0.001), antioxidants (P ≤ 0.001), and lycopene (P ≤ 0.001) compared with fruit from untreated control plants. Foliar- and/or root-applied MLE resulted in the greatest leaf concentrations of protein (P ≤ 0.01), proline (P ≤ 0.01), arginine (P ≤ 0.01), and total antioxidants (P ≤ 0.05), which were all significantly greater than the concentrations in leaves from untreated control plants. The results of this single experiment provide evidence suggesting that MLE warrants further research as an inexpensive growth promoter for enhancing tomato plant biomass, yield, and fruit quality, especially in organic crop production, which prohibits the use of many commercial synthetic plant growth regulators.

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Lauren C. Garner and Carol J. Lovatt

Despite profuse flowering, ‘Hass’ avocado (Persea americana Mill.) yields are low because of excessive flower and fruit abscission. Whether the dynamics of flower and fruit abscission are influenced by or contribute to alternate bearing, the production of a heavy on-crop followed by a light off-crop that is characteristic of many avocado cultivars, remains unresolved. The objective of this research was to determine whether abscission of reproductive structures from ‘Hass’ avocado trees during specific developmental stages, including flowering, fruit development, and fruit maturity, was influenced by crop status of the current or preceding year. Abscised reproductive structures were collected from commercially bearing trees during two complete crop years. Flower abscission began at about the same time but peaked 1 month later in the off-crop year compared with the on-crop year. Peak abscission rates were lower during the off-crop year than the on-crop year (compare 1836 ± 403 to 5378 ± 856 flowers per day and 50 ± 18 to 280 ± 23 immature fruit per day, respectively). The off- or on-crop status of the tree did not influence the percentage fruit set, average fruit diameter, or biomass of individual fruit that abscised at similar phenological stages. Furthermore, flower and fruit abscission were not influenced by the number of mature fruit from the previous year's crop. In both years of the research, as immature fruit abscission declined, abscission of the preceding year's crop increased, indicating that the processes were controlled independently. During the study, neither weather conditions nor tree nutrient status were associated with key abscission events. Taken together, these results provide evidence that the previous year's yield does not influence flower or fruit abscission and the seasonal abscission of reproductive structures is an independent process that does not contribute to alternate bearing of ‘Hass’ avocado.