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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.

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.

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.

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.

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 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 +

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.

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.

Root infection by the citrus nematode (Tylenchulus semipenetrans Cobb) at population densities above the economic threshold caused altered patterns of root and shoot growth of 20-year-old ‘Washington’ Navel orange trees (Citrus sinensis L. Osbeck) budded on Troyer citrange rootstock (Poncirus trifoliata L. Raf. × C. sinensis), compared with control trees protected from nematode infection by oxamyl (1.1 kg/ha/3 weeks) and permitted to recover from prior nematode infection. Nematode-infected trees initiated new roots every 4–6 weeks, beginning as early as midMarch through as late as mid-January. Each root flush was equal to or greater in magnitude (number of new roots per gram roots) than those observed from oxamyl-treated trees. Oxamyl significantly reduced the population of female nematodes. Oxamyl-treated trees produced 4 relatively small root flushes, typical of healthy trees and exhibited a net increase in root biomass; nematode-infected trees failed to increase root biomass despite repeated root initiation. Oxamyl-treated trees also produced more new shoots and exhibited a greater increase in length per shoot and mean number of leaves per unit shoot length than untreated trees. Measurements of the levels of available glucose, sucrose, and starch provided evidence that nematode-infected trees were carbohydrate depleted. Starch levels were typically 70% higher in the canopy and 96% higher in the roots of oxamyl-treated trees compared to untreated. Root initiation appears to be the first response of the trees penetrated by citrus nematode larvae; repeated root initiation in response to the many generations of larvae that hatch each year is probably a key factor contributing to the depletion of carbohydrate in nematode-infected trees.

The relative P dependency obtained for 3 citrus rootstock cultivars was rough lemon [Citrus limon (L)] > Carrizo citrange [Poncirus trifoliate × C. sinensis (L.)] > trifoliate orange [P. trifoliate (L.)]. After only 12 weeks of P deprivation, the youngest, fully expanded leaves of rough lemon (about 21 days old) had amounts of total N, nitrate, and NH3 that exceeded the levels in +P leaves of the same age by 8.4 mg total N, 2.6 mg nitrate, and 0.6 mg NH3 per gram of dry weight. It took 7 months for similar levels of these compounds to accumulate in the youngest, fully expanded leaves of Carrizo citrange (a less P-dependent rootstock) when grown under low P conditions. After 12 weeks of P deficiency, the incorporation of NaH14CO3 into arginine plus urea was 13-, 7.4-, and 4.7-fold greater in P-deficient leaves than P-sufficient leaves for rough lemon, Carrizo citrange, and trifoliate orange, respectively. Concomitantly, arginine accumulated in — P leaves to a concentration 4.2-, 2.1-, and 1.4-fold greater than in +P leaves, respectively, for the same 3 rootstocks. The data clearly demonstrate that relative P dependency of citrus rootstocks influences their N metabolism and support the hypothesis that a more P-dependent rootstock will accumulate total N, nitrate, and NH3 sooner or to a greater extent, exhibit a greater rate of de novo arginine biosynthesis, and accumulate a higher level of arginine than a less P-dependent rootstock. Thus, calculation of the N:P ratio during leaf nutrient analysis may be useful in evaluating the nutritional status of citrus trees. Whenever N:P ratios were >20, de novo arginine biosynthesis and arginine accumulation increased significantly, indicative of ammonia detoxification. The N:P ratio of P-sufficient plants was always

The capacity of ‘Washington’ navel orange fruit [Citrus sinensis (L.) Osbeck] to synthesize and catabolize purines and pyrimidines was assessed. De novo biosynthesis of purine nucleotide was demonstrated by [14C] bicarbonate incorporation into purine nucleotides, blockage of this process by four known inhibitors, and assimilation of radiolabeled carbon from formate, both carbons of glycine, and carbon-3 of serine into the adenine ring. De novo synthesis of pyrimidines via the orotate pathway in young fruit was demonstrated by incorporation of [14C] bicarbonate and [6-14C]orotic acid into uridine nucleotides, release of 14CO2 from [7-14C]orotic acid, and blockage of these processes by 6-azauridine. Synthesis of purine and pyrimidine nucleotides via salvage reactions was demonstrated by incorporation of radiolabeled bases and ribonucleosides into nucleotides and into nucleic acids. Release of 14CO2 from radiolabeled adenine, adenosine, hypoxanthine, and xanthine, uric acid, urea (purines), uracil, and uridine (pyrimidines) provided evidence the pathways for catabolism (degradation) of purines and pyrimidines in navel orange fruit are similar to those found in microorganisms and animal tissues. To the best of our knowledge, this report is the first to assess the capacity of anabolic and catabolic pathways of purine and pyrimidine nucleotide metabolism in fruit of any species. De novo synthetic activities in orange fruit permit increases in the pools of purine and pyrimidine nucleotides using simple precursors. Further, the patterns of salvage and catabolism suggest riboside pools are reused predominantly as nucleotides, while the majority of base pools are degraded to permit recycling of carbon and nitrogen into other metabolites.

Changes in the capacity of ‘Washington’ navel orange [Citrus sinensis (L.) Osbeck] fruit to synthesize (de novo or by salvage) pyrimidine nucleotides, but not purine nucleotides, appears to be related to the stage of fruit development. De novo pyrimidine synthesis in whole-fruit tissue increased 6-fold during Stage I of development (cell division phase), from 10 nmol [14C]bicarbonate incorporated into uridine nucleotides during 5 hr per g dry weight whole-fruit tissue from ovaries harvested at flower petal drop to 57 nmol for 2-month-old fruit. Capacity of peel tissue to synthesize pyrimidine nucleotides de novo decreased following completion of Stage I, from 43 nmol [14C]bicarbonate incorporated into uridine nucleotides during 5 hr per g dry weight of peel tissue from 2-month-old fruit to 11 nmol for 5-month-old (Stage II) fruit. This decrease was not offset by increased salvage of uridine. Capacity of whole-fruit tissue to synthesize purines de novo increased 3-fold during Stage I. Synthetic capacity of peel tissue from Stage I fruit was half that observed for whole-fruit tissue and did not decrease significantly during Stages II (cell enlargement phase) and III (maturation phase). These observations suggest purine synthetic capacity may not be related to stage of development. Changes in protein or glucose contents, or respiratory activity of peel tissue, could not account for the observed reduction in pyrimidine synthetic capacity. Thus, the reduction observed in synthetic activity was specific for pyrimidine nucleotides. The capacity of fast-growing, 1-month-old fruit (high potential to set) to synthesize or catabolize either pyrimidine or purine nucleotides did not differ from that of slow-growing fruit (low potential to set), suggesting that nucleotide synthesis is not limiting to growth.