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Transgenic 'Duncan' grapefruit (Citrus paradisi Macf.) and 'Valencia' sweet orange (Citrus sinensis [L.] Osbeck) plants ectopically expressing C. sinensis (cv. Washington navel orange) APETALA1 (CsAP1) or LEAFY (CsLFY) genes under control of the Arabidopsis thaliana stress-inducible promoter AtRD29A flowered under non-inductive (warm temperature, well-watered) greenhouse conditions, whereas their wild-type (WT) counterparts did not. The transgenic plants that flowered exhibited no altered morphological features, except the lack of thorns characteristic of juvenile WT plants. The most precocious T0 line, 'Duncan' grapefruit (Dun134-3) expressing the CsAP1 gene, flowered and fruited when it was 4.5 years old and the T1 siblings from this line flowered and fruited when they were just over 18 months old. In contrast, T1 seedlings from three lines of 'Duncan' grapefruit expressing the CsLFY gene flowered within 3 months after germination, but were unable to support fruit development. Transcript levels of corresponding transgenes in leaves were not correlated with earliness of flowering. To further study the activity of AtRD29A, leaves from three 'Carrizo' citrange (C. sinensis × Poncirus trifoliata) rootstock seedlings transformed with the green fluorescent protein (GFP) gene under regulation of the AtRD29A promoter were subjected to drought stress or well-watered conditions. Expression of GFP was not stress-dependent, consistent with the observation of flowering of CsAP1 and CsLFY transgenic plants under non-inductive conditions. Taken together, the results suggest that AtRD29A is constitutively expressed in a citrus background. Despite the loss of control over flowering time, transgenic citrus lines ectopically expressing C. sinensis AP1 or LFY genes under control of the A. thaliana RD29A promoter exhibit precocious flowering, fruit development and viable transgenic seed formation. These transformed lines can be useful tools to reduce the time between generations to accelerate breeding.

Low temperature (LT) and gibberellic acid (GA3) were used, respectively, to promote and inhibit flowering in ‘Washington’ navel orange to identify relationships between floral gene expression and floral intensity. All trees were maintained under warm temperature (WT) (24/19 °C, day/night) from April to September. For trees receiving 11 additional weeks of WT, buds expressed low levels of FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), APETALA1 (AP1) and APETALA2 (AP2), but never expressed SEPALLATA1 (SEP1), PISTILLATA (PI) and AGAMOUS (AG) and did not flower. In contrast, trees transferred to LT (15/10 °C, day/night) for 8 weeks followed by WT through week 11 had significantly greater bud expression of FT and SOC1 in week 2, AP1 and AP2 during weeks 8 through 10, and SEP1, PI and AG after transfer to WT; 84% of buds flowered. Foliar-applied GA3 (50 mg L−1) in weeks 2 through 8 of LT treatment did not affect FT, SOC1 or LFY expression, but significantly reduced transcripts of AP1 by week 8, AP2, SEP1, PI and AG after transfer to WT and inflorescence number to values equal to 11-week WT-treated trees. Delaying GA3 application to weeks 4 through 8 of LT treatment increased AP1, AP2, SEP1, PI and AG expression equal to LT-treated trees and significantly greater than WT-treated trees; inflorescence number was significantly greater than trees in the WT treatment or receiving seven GA3 applications, but less than LT-treated trees. The results provide evidence suggesting that in adult citrus buds determinacy and subsequent floral development are controlled by AP1 and AP2 transcript levels, which regulate downstream floral organ identity gene activity and the effect of GA3 on citrus flower formation.

Optimizing ‘Hass’ avocado (Persea americana Mill.) tree nutrient status is essential for maximizing productivity. Leaf nutrient analysis is used to guide avocado fertilization to maintain tree nutrition. The goal of this research was to identify a ‘Hass’ avocado tissue with nutrient concentrations predictive of yields greater than 40 kg of fruit per tree. This threshold was specified to assist the California avocado industry to increase yields to ≈11,200 kg·ha−1. Nutrient concentrations of cauliflower stage inflorescences (CSI) collected in March proved better predictors of yield than inflorescences collected at full bloom (FBI) in April, fruit pedicels (FP) collected at five different stages of avocado tree phenology from the end of fruit set in June through April the following spring when mature fruit enter a second period of exponential growth, or 6-month-old spring flush leaves (LF) from nonbearing vegetative shoots collected in September (California avocado industry standard). For CSI tissue, concentrations of seven nutrients, nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), zinc (Zn), and copper (Cu) were predictive of trees producing greater than 40 kg of fruit annually. Conditional quantile sampling and frequency analysis were used to identify optimum nutrient concentration ranges (ONCR) for each nutrient. Optimum ratios between nutrient concentrations and yields greater than 40 kg per tree were also derived. The high nutrient concentrations characterizing CSI tissue suggest current fertilization practices (timing or amounts) might be causing nutrient imbalances at this stage of avocado tree phenology that are limiting productivity, a possibility that warrants further investigation. Because CSI samples can be collected 4–6 weeks before full bloom, nutritional problems can be addressed before they affect flower retention and fruit set to increase current crop yield, fruit size, and quality. Thus, CSI nutrient analysis warrants further research as a potential supplemental or alternative tool for diagnosing ‘Hass’ avocado tree nutrient status and increasing yield.

For ‘Hass’ avocado (Persea americana Mill.), low temperature is known to induce flowering, but effects of other environmental factors remain equivocal. In addition, documentation of interactions among environmental factors, floral gene expression and subsequent flower development is limited. Thus, in this research, the effects of environmental factors on the temporal expression patterns of genes related to flowering time, floral meristem identity and floral organ identity were quantified in buds relative to inflorescence number. ‘Hass’ avocado trees were subjected to four different environmental conditions: optimal growth condition (OGC) (warm temperature, well-watered, photosynthetic active radiation > 900 μmoles m–2 s–1), low temperature (LT), low soil moisture (LSM) or low light intensity (LLI) for 8 weeks, followed by OGC for 6 weeks. Bud expression of homologs of Arabidopsis thaliana flowering genes FLOWERING LOCUS T (FT), LEAFY (LFY), APETALA1 (AP1), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA.1 (PI.1), AGAMOUS.1 (AG.1) and AGAMOUS.3 (AG.3) were quantified over time. Only buds of LT-treated trees flowered. At the start of the experiment (week 0), buds of all trees expressed PaLFY, PaAP1 and PaAP2. After 4 weeks of treatment, bud PaAP1 expression was greater than in week 0 and continued to increase through week 8 only in LT-treated trees, whereas by week 8 PaLFY expression increased in LT- and LSM-treated trees. By week 10 (2 weeks after transfer to OGC), bud PaFT expression increased only in LT-treated trees, followed by activation of downstream floral organ identity genes PaAP3, PaPI.1, PaAG.1 and PaAG.3 by week 12. For OGC-, LSM- and LLI-treated trees, bud PaAP1, PaFT, PaAP3, PaPI.1 and PaAG.1 expression remained low, with PaAG.3 transcripts undetected at week 12. These trees did not flower. Among environmental factors tested, only LT increased bud expression of both PaLFY and PaAP1, which likely conferred floral meristem determinacy since transfer of LT-treated trees to OGC did not prevent flowering but activated PaFT, downstream floral organ identity genes, and flowering. The results suggest ‘Hass’ avocado floral development is promoted by low temperature-dependent floral induction and warm temperature-dependent floral organogenesis.

Effects of nitrogen (N) fertilizer application times and rates on ‘Hass’ avocado (Persea americana) yield and fruit size were determined to resolve whether a single dose of soil-applied N [1x N (25 lb/acre)] at each of the five key stages of tree phenology (January, April, July, August, and November) (control) was as efficacious as soil-applied 2x N (50 lb/acre) at one or two key stages or soil- or foliar-applied 3x N (75 lb/acre) at only one stage. All trees received soil-applied N at 125 lb/acre as ammonium nitrate (NH4NO3) annually; trees receiving 2x or 3x N received the remaining N divided evenly at the same phenological stages (months) as trees receiving five 1x N applications. The importance of supplying N during the summer, when June drop, exponential fruit growth, vegetative shoot growth, and floral initiation occur, was determined by testing soil-applied 0.8x N in July plus August only (40 lb/acre N as NH4NO3 annually). Application time proved an important determinant of total yield. Yield of commercially valuable size (CVS) fruit was correlated with total yield (r = 0.80, P < 0.0001). Four-year cumulative total yields were equal for trees receiving soil-applied 1x N at five key phenological stages and trees receiving soil-applied 2x N in April and 18.75 lb/acre N at the four other stages (months). However, trees receiving soil-applied 2x N in April plus November and only 8.3 lb/acre N in the three other months, in particular July and August, had significantly lower 4-year cumulative total yields (P = 0.0362). Additional evidence of the importance of meeting avocado tree N demand in the summer is that trees receiving only 40 lb/acre N split in July plus August produced 4-year cumulative total yields equal to trees receiving 25 lb/acre N at the five key phenological stages; lower annual N would reduce fertilizer expense and protect the environment.

Antioxidant compounds and reactive oxygen-scavenging systems were investigated in ‘Hass’ avocado fruit set by determinate (DFS) and indeterminate floral shoots (IFS). During early development, DFS produced the largest fruit, IFS the smallest. Normal-phenotype (large) fruit (NPF) from DFS and IFS were faster growing, greater in length, diameter and weight and contained more protein than small-phenotype fruit (SPF) from DFS and IFS, resulting in SPF having greater ascorbic acid and total phenol concentrations and antioxidant capacity per g fresh weight. In contrast, NPF had greater catalase, ascorbate peroxidase, and glutathione reductase activities per g fresh weight than SPF on a greater number of sample dates; SPF had greater superoxide dismutase activity. Per mg protein, ascorbic acid and phenol concentrations, antioxidant capacity, peroxidase, polyphenol oxidase, catalase and ascorbate peroxidase activities were greater in SPF than NPF on more dates and at maturity. Thus, SPF would be a healthier food choice due to their greater antioxidant compounds and capacity compared to NPF.

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.

‘Hass’ avocado (Persea americana Mill.) is characterized by distinct periods of excessive flower and fruit drop. Determining the cause(s) of these abscission events is critical for the development of strategies to increase fruit set and yield. Therefore, the objectives of this research were to determine pollination and fertilization rates of abscising flowers and to quantify the viability of developing ovules (seeds) and hormone concentrations of abscising versus persisting fruit through early and June drop of ‘Hass’ avocado. Experiments were conducted during sequential off- and on-crop years. Pollination, pollen germination, pollen tube growth and ovule viability of abscising flowers were determined by microscopic analysis. During early and June drop, cellular deterioration of developing ovules of abscising and persisting fruit was visualized with Evan’s blue stain. Abscisic acid (ABA), indole-3-acetic acid (IAA), and isopentenyladenine (IPA) concentrations were determined by radio-immuno assay (RIA) during the on-crop year only. On average ≥ 70% of abscising ‘Hass’ flowers were pollinated but due to the failure of the pollen grains to germinate and produce pollen tubes, fertilization never occurred and the ovule deteriorated. Fruit abscission during early and June drop was also due primarily to a lack of fertilization. Fruit abscission was associated initially with deterioration of the nucellus within the developing ovule in early drop, and subsequently with deterioration of the integument (seed coat) in June drop; both occurred more frequently in abscising than persisting fruit. Abscising fruit had greater ABA concentrations during early drop, June drop and fall fruit drop compared to persisting fruit; IAA and IPA were greater in abscising fruit during June drop, but not early drop or fall fruit drop. Only 25% of the total number of fruit that abscised per crop year were fertilized. Taken together, these results provide strong evidence that the majority of flowers and fruit of ‘Hass’ avocado abscise due to a lack of pollen germination and subsequent fertilization. In addition, the results suggest that ABA accumulation is related to ovule (seed) abortion and reduced fruit growth in abscising fruit.

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.

The effect of harvest date on nutritional compounds and antioxidant activity (AOC) in avocado ( Persea americana Mill. cv Hass) fruit during storage was determined. The fruits were harvested at seven different dates and ripened at 25 °C following 21 or 35 days of cold storage. The results indicated that the phenolic and glutathione contents were increased and the ascorbic acid content was not significantly different in early harvested fruit (January to March), and the phenolic, ascorbic acid and glutathione contents were increased slightly and then decreased on late harvested fruit (April to June). Similar trends were observed in the changes of AOC. Furthermore, AOC in early harvested fruit after storage for 35 days was much higher than that in late harvested fruit after storage for 21 days. Therefore, avocado can be harvested earlier for economic benefits according to the market and can keep high nutritional value for human health benefits.

Despite problems of low fruit set, small fruit size and alternate bearing, the Hass cultivar dominates commercial avocado production worldwide. To increase yield and fruit size, gibberellic acid (GA 3 ) (25 mg L −1 ) was applied at different stages of ‘Hass’ avocado tree phenology: ( i ) mid–late April (flower abscission), end of June–beginning of July (fruit abscission and beginning of the exponential phase of fruit growth), and mid-January (beginning of pre-harvest fruit drop); ( ii ) end of June–beginning of July; and ( iii ) mid-September (near the end of the major fruit abscission period; period of exponential fruit growth). In both years of the research, applications of GA 3 in April and June–July were within the periods of intense flower and fruit abscission, respectively; fruit abscission was low in September and January. Maximum air temperature was not related to flower or fruit abscission. In the on-crop year (391 fruit per untreated control tree), a single application of GA 3 at the end of June–beginning of July significantly increased total yield (kilograms only) and yield of commercially valuable fruit (178–325 g/fruit) (as kilograms and number per tree) compared with the control ( P 3 applied in September increased total yield (kilograms only) and yield of commercially valuable fruit (kilograms and number per tree) to values intermediate to and not significantly different from all other treatments, except trees receiving multiple applications of GA 3 . This treatment reduced total yield and yield of commercially valuable fruit (kilograms and number per tree) relative to all treatments ( P ≤ 0.0002). In contrast, during the off-crop year (32 fruit per control tree), no GA 3 treatment had a significant effect on yield or fruit size compared with the control and all other GA 3 treatments. For ‘Hass’ avocado, there was no negative effect from applying GA 3 at the end of June–beginning of July in both the off- and on-crop years; 2-year cumulative total yield and yield of commercially valuable fruit were increased by 27 kg (128 fruit) and 22 kg (101 fruit) per tree, respectively, above the yield of untreated control trees ( P

Worldwide, gibberellic acid (GA3) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA3 concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2 mm) of ‘Nules’ Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard. GA3 applied during the period of intense flower abscission failed to reduce the total number of abscised flowers in both the light off- and heavy on-bloom years. No GA3 treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA3 in the high yield on-crop year had fewer abscised fruit than untreated control trees (P = 0.0188) and GA3 applied 10 days after 75% petal fall and in July increased the number of fruit retained on tagged branches >20% compared to control trees (P = 0.0005). Maximum air temperature was not related to flower or fruit abscission. In the off-crop year (548 fruit per untreated control tree), it was necessary to apply 15 or 25 mg L−1 GA3 at 60% bloom, 90% bloom, 75% petal fall and 10 days after 75% petal fall to significantly increase the number of fruit per tree and yield of commercially valuable fruit (kilograms and number per tree) (P

In most prokaryotes and many eukaryotes, synthesis of carbamoylphosphate (CP) by carbamoylphosphate synthetase (CPSase; E.C. 6.3.5.5) and its allocation to either pyrimidine or arginine biosynthesis are highly controlled processes. Regulation at the transcriptional level occurs at either CPSase genes or the downstream genes encoding aspartate carbamoyltransferase (E.C. 2.1.3.2) or ornithine carbamoyltransferase (E.C. 2.1.3.3). Given the importance of pyrimidine and arginine biosynthesis, our lack of basic knowledge regarding genetic regulation of these processes in plants is a striking omission. Transcripts encoding two CPSase small subunits (MtCPSs1 and MtCPSs2), a single CPSase large subunit (MtCPSl), ACTase (MtPyrB), and OCTase (MtArgF) were characterized in the model legume Medicago truncatula. Quantitative real-time PCR data provided evidence (i) that the accumulation of all CPSase gene transcripts, as well as the MtPyrB transcript, was dramatically reduced following seedling incubation with uridine; (ii) exogenously supplied arginine down regulated only MtArgF; and (iii) mRNA levels of both CPSase small subunits, MtPyrB, and MtArgF were significantly increased after supplying plants with ornithine alone or in combination with uridine or arginine compared to plants treated with only uridine or arginine, respectively (P< or =0.05). A proposed novel, yet simple regulatory scheme employed by M. truncatula more closely resembles a prokaryotic control strategy than those used by other eukaryotes.

The objectives of this preliminary study were to optimize a carbon dioxide (CO2) application system for field-grown raspberry (Rubus ideaus) under high tunnels and then to compare plant photosynthesis, growth, and fruit yield with and without CO2. Based on plant photosynthesis measures before CO2 application, we placed the drip irrigation tape to apply CO2 in the middle of the plant canopy at 100 cm aboveground and split daily CO2 application from 0700 to 1100 hr and 1400 to 1800 hr. In the morning hours, CO2 concentration in the tunnel was 18% higher than in the afternoon; wind speed often increased later in the day, which may have moved the CO2 even in the tunnel. We maintained an average CO2 concentration of 436 ppm for 4 months, applying about 25 tons of CO2. In tunnels enriched with CO2, yield and berry size from plots 20 ft in length increased 12% and 5%, respectively, compared with untreated tunnels. We measured no corresponding differential response in the number of 6-oz baskets harvested from the full tunnel, leaf CO2 assimilation, stomatal conductance (gS), or fluorescence, raspberry cane height, pruned cane biomass, cane diameter, or carbohydrate content of the fruit after CO2 application. Documenting potential plant physiological changes should therefore be a focus of future research.

Alternate bearing trees produce a heavy (on) crop followed by a light (off) crop. Whereas it is well documented for citrus that fruit number in the current crop inversely affects flower number in the return bloom, when in the phenology of the tree and how fruit exert an effect on floral intensity the following spring remained unresolved. ‘Pixie’ mandarin (Citrus reticulata) was used as the model system to investigate when and how fruit perpetuate cyclic differences in floral intensity. Parent shoots (current spring flush shoots) were tagged on on-crop trees and fruit were removed from individual shoots or whole trees. The number of summer and fall (summer/fall) vegetative shoots that developed on parent shoots with and without fruit and the contribution of spring shoots (floral and vegetative) made by parent shoots alone (now 1 year old) and by their summer/fall shoots to return spring bloom was quantified. Removal of fruit from individual shoots on on-crop trees in June or July had no effect on the number of flowers contributed by parent (current spring) shoots to return bloom, but increased total flower number 4-fold because summer/fall shoot number increased more than 8-fold. Removal of fruit from individual shoots of on-crop trees after July had no effect on flower number. In the whole tree experiment, parent + summer/fall shoots of off-crop trees produced more flowers the following spring than on-crop trees due to greater flower production by both parent shoots and their greater number of summer/fall shoots. Removal of all fruit in July from on-crop trees resulted in 2-fold more flowers in spring compared with off-crop trees due to the increased number of flowers contributed by both parent shoots (75% of the total) and the increased number of summer/fall shoots. The importance of summer/fall shoots to return bloom was confirmed by removing all summer/fall shoots from off-crop trees. This reduced floral intensity to that of on-crop trees. Removing all fruit from on-crop trees in December increased the percentage of budbreak in spring and flower number on parent shoots to that of off-crop trees, whereas the number of summer/fall shoots and the number of flowers the parent shoots contributed to bloom were both less than that of off-crop trees. For the branch and whole tree experiments, flower number was significantly correlated with the percentage of spring budbreak on parent + summer/fall shoots (r 2 = 0.88, P ≤ 0.0001 and r 2 = 0.71, P ≤ 0.0001; respectively). Taken together, the results of this research provide evidence that fruit of the ‘Pixie’ mandarin reduce floral intensity of the return bloom by inhibiting budbreak, which reduces summer/fall shoot growth and thus the number of nodes that can bear inflorescences and development of spring shoots, which are predominantly floral.

Hass' avocado (Persea americana Mill.) is characterized by excessive flower and fruit abscission, resulting in extremely low fruit set. Low outcrossing rates might be a factor contributing to low yields. It is hypothesized that self-fertilized flowers and resulting fruit abscise at a much higher rate than fruit that are the product of outcrossing. However, significant relationships between outcrossing rates and yields have only been established in a few avocado studies. The objective of this research was to investigate the importance of outcrossing to yield in a commercial 'Hass' orchard containing 'Bacon', an effective pollinizer of 'Hass'. Microsatellite markers were used to determine the rate of outcrossing of fruit persisting to harvest on 'Hass' trees. Experiments were conducted during sequential on- and off-crop years. During both years, outcrossing rates were not related to yield or alternate bearing. These results indicate that outcrossing was not the primary factor affecting flower and fruit persistence and ultimately yield in this orchard for the two sequential years of this research. Avocado and many other subtropical fruit tree crops are characterized by excessive flower and fruit abscission. Thou- sands of avocado flowers and immature fruit abscise in a single season (Cameron et al., 1952; Garner and Lovatt, 2008; Inoue and Takahashi, 1990; Lahav and Zamet, 1975; Slabbert, 1981), resulting in extremely low fruit set (

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.

Gibberellins (GAs) are plant hormones with diverse roles in plant growth and development. Male sterility is one of several GA responses. In an effort to understand the involvements of GAs in sunflower male reproductive development, physiological, biochemical and molecular studies of GA-induced and cytoplasmic male sterility were performed. Male sterility was induced by exogenous application of gibberellic acid (GA3) to the fertile inflorescence apex in early reproductive development. Using different experimental approaches: hormonal quantitative analyses (gas-liquid chromatography), protein profiles (SDS PAGE) and transcripts assays (RT-PCR) revealed a similarity in expression of cytoplasmic (CMS) and induced (IMS) male sterility. Our results provide the first evidence that expression of CMS-specific orfH522 gene can be induced by GA3 in plant with original fertile cytoplasm.

Mexican montane rainforests and adjacent disturbed areas were studied for disturbance-related spatio-temporal changes to the arbuscular mycorrhizal fungal (AMF) community and soil glomalin concentration. The AMF community functions to both improve plant growth and soil conditions and is thus an important component to the restoration of this forest type to disturbed areas. The study areas included mature rainforests that were converted to pine forests, milpas, pastures and shrub/herbaceous plant communities via burning and logging. Seasonal patterns in AMF spore species richness and sporulation significantly differed across disturbance types at two of the three sites surveyed. Contrasting patterns of sporulation among AMF families across different disturbance types helped to explain how species richness and composition were maintained despite dramatic changes to the host plant community. Meaning, in most cases, disturbance induced changes in when different AMF taxa sporulated but not what taxa sporulated. Only conversion from mature pine‐oak‐Liquidambar‐Persea forests to pine-dominated stands severely reduced AMF spore richness and total sporulation. Surprisingly, in pine-dominant stands no concomitant negative impacts on soil glomalin (MAb32B11 immunoreactive soil protein) concentrations were detected. However, soils of mature forests containing no pines had the highest concentration of glomalin. Conversion to pasture and milpa (diverse cornfield) had a strong negative impact on the concentration of soil glomalin concentrations. In sharp contrast, the same disturbance types improved AMF sporulation and AMF spore richness. It appears that disturbance type, and not AMF community measures used herein, best predicts changes in soil glomalin concentration. # 2007 Elsevier B.V. All rights reserved.

Resumen es: Esta investigacion fue desarrollada durante 1998-2003 en huertos comerciales de aguacate 'Hass' en el clima semicalido del estado de Nayarit, Mexico. Los...

Michoacán and Nayarit are, respectively, the largest and second largest avocado-producing states in Mexico. The main harvest of the ‘Hass’ avocado in both states is concentrated during November to December, which saturates the market and reduces the price of fruit and grower income. The goal of this research was to manipulate vegetative and reproductive growth of the ‘Hass’ avocado with properly timed foliar-applied plant bioregulators (PBRs) to shift the date of flowering and harvest to the period before or after the main harvest. Effects of canopy sprays of gibberellic acid (GA3) or prohexadione calcium (a gibberellic acid biosynthesis inhibitor) applied at different stages of tree phenology on inflorescence development, time of anthesis, date of legal maturity for harvest of ‘Hass’ avocado fruit, yield, and fruit size were quantified. No PBR treatment influenced the time of anthesis. A single or double foliar application of GA3 (50 mg·L−1) ≈4 months (July) before the expected date of main harvest (November) resulted in ‘Hass’ avocado fruit reaching legal maturity (mesocarp dry matter 21.5% or greater) 24.8 to 28.2 d earlier than those of untreated control trees with no negative effect on yield or fruit size.

Functional differences between the arbuscular mycorrhizal fungi Glomus intraradices Schenk and Smith and Scutellospora heterogama Nicolson and Gerdemann as they affect Persea americana Mill. growth, glomalin, and fungal sporulation were examined by varying the composition and relative density of the two fungi over a gradient of available phosphorus (P). The plant benefit provided by these mycorrhizal fungi together was not a simple sum of the benefits provided by each fungus in monoculture at its respective density. Glomus intraradices and S. heterogama interacted to reduce plant growth rates and uptake of P, zinc (Zn), and iron (Fe) relative to plants inoculated with G. intraradices alone. Thus, for plant growth and nutrition, no evidence for functional complementarity was detected. Instead, interspecific interactions between mycorrhizal fungi resulted in a negative feedback on plants. Under high available P, fungal functional differences were reduced, whereas the overall difference between mycorrhizal and nonmycorrhizal plants was greatest. Overall, S. heterogama produced more glomalin than did G. intraradices. In a mixture, sporulation of the inferior mutualist, S. heterogama, was lower than that of the superior mutualist, G. intraradices, but interspecific fungal interactions increased the sporulation of both fungi. Despite the negative impact of interspecific interactions on plants, supporting multiple arbuscular mycorrhizal fungi was of greater benefit than being nonmycorrhizal.

Several studies were undertaken in commercial nonirrigated `Hass' avocado orchards under the subhumid semiwarm subtropical climate of the state of Nayarit, Mexico, with the following objectives: 1) to determine the frequency and intensity of vegetative shoot flushes and their contribution to the production of floral shoots, 2) to quantify the effect of tree fruit load on the occurrence of vegetative shoot flushes during the year and the relationship between vegetative and reproductive shoot number during flowering, and 3) to determine the time when apical buds borne on the major vegetative shoot flushes reached irreversible commitment to flowering (floral determination) through the use of shoot defoliation and girdling. Data trees were selected in two orchards based on their current crop load. Four to five branches per tree were tagged, and the number and intensity of vegetative flushes that developed during 2 years, as well as the type of growth produced by apical buds of shoots of different ages, were recorded at the end of the winter bloom periods for two separate years, 1999 and 2001. In a separate experiment using a different set of trees, winter and summer flush shoots were defoliated (year 1) or defoliated and girdled (year 2) at different stages of bud development from September to January in each case. Four vegetative flushes occurred each year. The winter flush that emerged in Feb. 1998 made the greatest contribution to the 1999 winter bloom—76.5% of the shoots produced floral shoots. Contributions of the summer 1 (late July 1998), summer 2 (early Aug. 1998), and summer 3 (late Aug. 1998) flushes to flowering were intermediate. A total of 30.6%, 36.4%, and 19% of the shoots produced floral shoots respectively. All four vegetative flushes produced a similar number of vegetative shoots during winter bloom. Evaluation of the 2001 winter bloom for trees with high (>95 kg fruit/tree) and low (

Homologues of the floral meristem identity genes LEAFY (LFY) and APETALA1 (AP1) were isolated from the hybrid perennial tree crop `Washington' navel orange (Citrus sinensis) and designated CsLFY and CsAP1, respectively. Citrus has an extended juvenile period unlike herbaceous plants and responds to different floral stimuli than herbaceous plants or deciduous tree species. Despite these differences, the deduced amino acid sequences of CsLFY and CsAP1 genes are at least 65% identical with their Arabidopsis thaliana L. Heynh counterparts and share even greater sequence similarity to LFY and AP1 from the deciduous woody perennials, Populus balsamifera Bradshaw and Populus tremuloides Michaux, respectively. Like A. thaliana LFY (AtLFY) and AP1 (AtAP1), CsLFY and CsAP1 expression was restricted almost exclusively to reproductive tissues, but observed expression of CsAP1 in the fourth whorl carpel tissue of mature flowers was distinct from other plant AP1 genes. Transgenic A. thaliana plants ectopically expressing CsLFY or CsAP1 showed early-flowering phenotypes similar to those described for overexpression of AtLFY and AtAP1. In addition, the 35S:CsLFY and 35S:CsAP1 transgenes partially complemented the lfy-10 and ap1-3 mutants, respectively. The severity of the overexpression phenotypes correlated with the accumulation of CsLFY or CsAP1 transcripts. LFY is a single-copy gene in flowering plants but consistent with its hybrid origin, the genome of C. sinensis `Washington' has two easily distinguishable CsLFY and CsAP1 alleles derived from it's parental genotypes, C. maxima L. Osbeck (pummelo) and C. reticulata Blanco (mandarin). Allelic polymorphism at both the CsLFY and CsAP1 loci was restricted to the 5′- and 3′-flanking regions.

Seedlings of Petunia x hybrida ‘Orchid’ treated with the ethylene-releasing compound ethephon at 0.9, 1.7, and 3.5 mM evolved ethylene at a higher rate as the concentration of ethephon increased. Regardless of the concentration of ethephon applied, ethylene evolution peaked 6 to 8 h following application. Evidence that ethephon application decreased apical dominance included an increase in the number of new nodes on the main stem and a sustained increase in the length of new and existing lateral shoots compared to the control (no ethephon). Plants treated with 3.5 mM ethephon developed mild chlorosis, whereas a concentration of 1.7 mM ethephon decreased apical dominance without phytotoxic effects. The auxin/cytokinin ratio decreased in the apical shoot section as early as 1 h after ethephon treatment. In contrast, a decrease in the ratio in the subapical shoot section was not detected until 24 h after ethephon application. Reduction in auxin/cytokinin ratio was a result of a decrease in indole-3-acetic acid (IAA) and an increase of zeatin riboside (ZR), but not isopentenyladenosine (iPA). These results suggest that exposing ‘Orchid’ petunia seedlings to ethylene via ethephon lowers the auxin/cytokinin ratio, thereby promoting the outgrowth of lateral shoots.

The effects of GA3 (25 or 100 mg×liter-1) canopy sprays applied to Hass avocado (Persea americana Mill.) trees in a commercial orchard in the months preceding an “off” or “on” bloom year on inflorescence and vegetative shoot number and yield were quantified. GA3 (100 mg×liter-1) applied in September (early stage of inflorescence initiation) reduced inflorescences number both years, but not yield. GA3 (25 or 100 mg×liter-1) applied in November (early stage of inflorescence development) before the “on” bloom reduced inflorescence number with a concomitant increase in vegetative shoot number and a 47 % yield reduction. GA3 in January (initial development of the perianth of apical flowers) or March (cauliflower stage of inflorescence development) caused precocious development of the vegetative shoot apex of indeterminate inflorescences, but had no effect on inflorescence or shoot number either year. GA3 (25 mg×liter-1) applied in March of an “off” bloom year increased production (2-fold) of commercially valuable fruit (213-269 g per fruit) and delayed skin blackening. This paper provides basic information that can be used to develop strategies for manipulating yield and evening out alternate bearing in avocado.

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.

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.

Plant growth regulators (PGRs), both naturally occurring and synthetic, are used extensively in citriculture not only to help growers solve production problems, but also to increase the market value of the crop and to improve grower returns. Yield increases are obtained via an improvement in fruit set, particularly for shybearing cultivars, and/or fruit size, especially for the “small-fruit” group of mandarins. Increased fruit number per tree is achieved indirectly through an increase in flower number as a result of better flower initiation or by a direct effect on fruit set. Improved fruit size is brought about directly by stimulating the growth of fruit tissues or indirectly by reducing fruit number by partial inhibition of flower initiation or by subsequent fruit removal. PGRs are also used to hasten maturity, to delay harvest, and to maintain fruit quality postharvest, resulting in greater profits to the grower. This article provides an overview of current uses of PGRs in citriculture. The re...

The developmental stage at which the shoot primary axis meristem (PAM) of the `Hass' avocado (Persea americana Mill.) is committed to flowering was determined. Three-year-old trees were subjected to low-temperature (LT) treatments at 10/7 °C day/night with a 10-h photoperiod for 1 to 4 weeks followed by 25/20 °C day/night at the same photoperiod. Before LT treatment, apical buds of mature vegetative shoots consisted of a convex PAM with two lateral secondary axis inflorescence meristems lacking apical bracts each associated with an inflorescence bract. Apical buds did not change anatomically during LT treatment. However, the 3- and 4-week LT treatments resulted in inflorescences at 17% and 83% of apical buds, respectively. Trees receiving 2 weeks or less LT, including controls maintained at 25/20 °C, produced only vegetative shoots. Apical buds of 2-year-old trees receiving 3 weeks at 10/7 °C plus 1 week at 20/15 °C produced 100% inflorescences. GA3(100 mg·L-1) applied to buds 2 or 4 weeks after initiation of this LT treatment did not reduce the number of inflorescences that developed. `Hass' avocado apical buds were fully committed to flowering after 4 weeks of LT, but were not distinguishable anatomically from those that were not committed to flowering.

Phaseolus acutifolius A. Gray is a potential source of stress-tolerant traits for Phaseolus vulgaris L. through interspecific hybrids. The objective of this study was to quantify the effects of water-deficit stress on vegetative growth, shoot relative water content (RWC), and leaf concentrations of proline, polyamines, and related metabolites in P. vulgaris compared with P. acutifolius. Stress-induced changes in N metabolism putatively related to stress tolerance have not been investigated previously in P. acutifolius. Replicate pots, each containing three 5-d-old plants in 18.9 L of soil with 4 L of available water, were subjected to water deficit by withholding water (terminal drought) or were maintained under well-watered (control) conditions. Compared with controls, stressed plants of both species accumulated approximately 55% less shoot dry matter. Root dry matter accumulation was inhibited to a greater degree in P. acutifolius (70% for two genotypes) than in P. vulgaris (14 and 27% for two genotypes). P. acutifolius maintained greater shoot RWC than P. vulgaris. In droughted plants of P. acutifolius, leaf arginine and proline concentrations did not change, total polyamine (Σ agmatine + putrescine + spermidine + spermine) concentrations decreased, and ammonia increased compared with controls. In P. vulgaris, water deficit increased concentrations of arginine (>30%) and proline (>300%), whereas total polyamine and ammonia concentrations did not change compared with controls. In all four genotypes examined, proline concentration was inversely related to RWC (R 2 ≥ 0.90). Leaf proline concentration is an indicator of plant water status in Phaseolus but not of tolerance or sensitivity of vegetative growth to water deficit.

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 (r2 = 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.

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.

Inflorescence and flower development of the `Hass' avocado (Persea americana Mill.) were investigated at the macro- and microscopic level with three objectives: 1) to determine the time of transition from vegetative to reproductive growth; 2) to develop a visual scale correlating external inflorescence and flower development with the time and pattern of organogenesis; and 3) to quantify the effect of high (“on”) and low (“off”) yields on the flowering process. Apical buds (or expanding inflorescences) borne on summer shoots were collected weekly from July to August during an “on” and “off” crop year. Collected samples were externally described and microscopically analyzed. The transition from vegetative to reproductive condition probably occurred from the end of July through August (end of shoot expansion). During this transition the primary axis meristem changed shape from convex to flat to convex. These events were followed by the initiation of additional bracts and their associated secondary axis inflorescence meristems. A period of dormancy was not a prerequisite for inflorescence development. Continued production of secondary axis inflorescence meristems was observed from August to October, followed by anthesis seven months later. In all, eleven visual stages of bud development were distinguished and correlated with organogenesis to create a scale that can be used to predict specific stages of inflorescence and flower development. Inflorescence development was correlated with minimum temperature ≤15 °C, whereas yield had little effect on the timing of developmental events of individual inflorescence buds. However, the high yield of the “on” year reduced inflorescence number and increased the number of vegetative shoots. No determinate inflorescences were produced during the “on” year. For the “off” year, 3% and 42% of shoots produced determinate and indeterminate inflorescences, respectively.

The objective of this study was to quantify the effect of salinity on the capacity of leaves of Phaseolus vulgaris L. and P. acutifolius A. Gray to synthesize arginine de novo and to determine the relative ability of the two species to tolerate salinity during the vegetative growth phase. Research into the effect of salinity on P. acuafolius relative to P. vulgaris is limited. Plants were hydroponically cultured until age 5 d and then salinized for 16 d with 40 mM NaCl plus 20 mM CaCl₂. Salination caused greater reduction (P < 0.05) P. acutifolius shoot dry weight (35 and 43% for two lines) than P. vulgaris [

SummaryThe present study aimed to quantify the effect of withholding irrigation during the winter season in southern California on the productivity of ‘Washington’ navel orange and to determine whether the additional expense of irrigating navel orange trees during the winter is a cost-effective production management strategy. Yield and fruit size were quantified for 30 year old ‘Washington’ navel orange scions on Troyer citrange rootstock which were rain-fed from October 1 to March 1 in each of three successive years and for another set of trees which were irrigated during the winter. Supplementing winter rainfall with irrigation significantly increased the weight of fruit per tree in all three years of the study (45 ± 17 kg fruit tree−1, n = 3 years) and number of fruit per tree in the two years in which the rain-fed, –winter irrigation trees had significantly lower predawn water potentials than the +winter irrigation trees. Despite the yield increases which resulted from supplementing winter rain with i...

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 NH4Cl concentrations were assessed. Procedures and instruments for quantifying NH 3 + NH4 + were tested for their sensitivity, reproducibility, and freedom from interference by amino acids. Reliable recoveries of NH 3 + 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 NH 3 + 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 + NH 4 + levels, whereas storage of these samples at 4C increased NH 3+ NH 4 + levels. Concentrations of the combined pool of ammonia plus ammo- nium (NH3 + NH4 + ) in plant tissues are determined with increasing frequency by researchers and commercial laboratories. Total ni- trogen content of plant tissue is routinely used as the basis for nitrogen fertilization recommendations, analysis of the nutritional quality of forage crops, and in research. The standard method for measuring total nitrogen is that of Kjeldahl (Bremner, 1965), which is based on quantitation of ammonium in an acid-digested sample or its distillate. Development of ammoniacal and urea fertilizers for soil and foliar nitrogen fertilization of crops has increased the demand for analysis of NH 3 + NH 4 + in plant tissue samples. Brown and Zhang (1993) assessed ammonium accumu- lation in leaves as a diagnostic measure of optimal nitrogen fertilization of nectarines and almonds. In addition, an increasing number of reports in the literature document that NH3 + NH4 +

Since the review on endogenous growth substances of citrus tissues by Goldschmidt in 1976 (HortScience, 11: 95-99), much information regarding this topic has been published in a wide array of journals. The present review provides a comprehensive overview of published information on endogenous levels of the five classes of plant growth substances (i.e., auxins, cytokinins, gibberellins, ethylene, and abscisic acid), plus polyamines and other endogenous substances that appear to have a role in regulating citrus growth and development. It is the first in a three-part series that next examines hormonal regulation of physiological processes in citrus followed by review of current uses and commercial applications of plant growth regulators in citrus production. In this article, a brief history of the detection and characterization of each class of plant growth substances is given. Following this, variation in endogenous levels associated with different organs (and/or tissues), stages of development, sp...

SummaryThe objectives of the present study were to quantify the relationship between flowering and leaf polyamine content at the initiation of, and during, a low-temperature floral-induction treatment and to test the ability of canopy sprays of L-arginine (50 mM), putrescine (10 and 20 mM), and spermidine (10 and 20 mM) to enhance the flowering response of the ‘Washington’ navel orange (Citrus sinensis L. Osbeck). Five year old container-grown ‘Washington’ navel orange trees were subjected to four weeks of low-temperature treatment of 8 h days at PFD of 500 µmol m−2 s−1 and 10°C and 16 h nights at 7°C. Leaf NH3–NH4+, putrescine and spermidine concentrations were significantly greater at the end of, or one day after, the low temperature floral-induction treatment, while spermine content decreased. Trees receiving putrescine (20 mM) or spermidine (10 mM) had significantly greater leaf concentrations of spermidine at the initiation of the low-temperature floral-induction treatment. Flower number per tree was...

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 (r2 = 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.

Structure and function of the inflorescence of cv. 'Hass' and 'Fuerte' avocado ( Persea americana Mill.) were examined by scanning electron microcopy (SEM) and by porometry. Sepals and petals could not be distinguished by their position in the flower, by visual gross morphology or by microscopic surface structure and were hence designated as tepals. These tepals were arranged in two whorls of three, followed by two whorls of three outer and three inner stamens, each opposite a tepal. The most conspicuous feature of tepals, developing leaves and peduncles was the dense cover of hair which were most frequent on the adaxial tepal surface (925-1200 trichomes mm -2 ), followed by their abaxial surface (625-1000 mm -2 ) and peduncles (375-655 mm -2 ). Stomata were absent from the adaxial surfaces of both tepal and leaves, as well as peduncles. On the tepals, abaxial stomata appeared functional, small (8-9 × 11-13 μm) and scarce with 2·8-3·4 stomata mm -2 , i.e. very low relative to avocado leaves (350-510 stomata mm -2 ) or young fruit (50-75 stomata mm -2 . However, flowers including tepals transpired 1·2-1·3 mmol under field conditions in Southern California (1·6-2 kPa), i.e. in excess of leaves (0·7-1·1 mmol) and peduncles (0·6-0·8 mmolH 2 O m -2 s -1 ). This situation was attributed to the few small but functional abaxial stomata on the tepal, in contrast to 80% closed stomata and dense epicuticular wax cover in form of rodlets on young and dendritic crystals on old leaves including the guard cells, and absence of stomata from the peduncle.

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.