Pistillate Flower Production Research Articles

Comparative transcriptome analyses reveal insights into catkin bloom patterns in pecan protogynous and protandrous cultivars.

In perennial plants such as pecan, once reproductive maturity is attained, there are genetic switches that are regulated and required for flower development year after year. Pecan trees are heterodichogamous with both pistillate and staminate flowers produced on the same tree. Therefore, defining genes exclusively responsible for pistillate inflorescence and staminate inflorescence (catkin) initiation is challenging at best. To understand these genetic switches and their timing, this study analyzed catkin bloom and gene expression of lateral buds collected from a protogynous (Wichita) and a protandrous (Western) pecan cultivar in summer, autumn and spring. Our data showed that pistillate flowers in the current season on the same shoot negatively impacted catkin production on the protogynous 'Wichita' cultivar. Whereas fruit production the previous year on 'Wichita' had a positive effect on catkin production on the same shoot the following year. However, fruiting the previous year nor current year pistillate flower production had no significant effect on catkin production on 'Western' (protandrous cultivar) cultivar. The RNA-Seq results present more significant differences between the fruiting and non-fruiting shoots of the 'Wichita' cultivar compared to the 'Western' cultivar, revealing the genetic signals likely responsible for catkin production. Our data presented here, indicates the genes showing expression for the initiation of both types of flowers the season before bloom.

Comparative study on the phenology, gender and yield components of cucumber (Cucumis sativus L.) varieties

The intensity of sex expression is important in crops, including cucumber, since sex form and flowering have direct effects on date of harvesting and yield. Two groups of cucumber comprising a native variety (Odukpani) and elite varieties (Griffaton, Poinsett, Ashley, Marketmore and Monarch) were evaluated in 2015 and 2016 cropping seasons at the Teaching and Research Farm, College of Agriculture, Garkawa, Plateau State; to assess flowering, sex expression and some yield traits. The experimental design was randomized complete block design with the 6 cucumber varieties as the treatments, replicated five times. The two groups differed significantly (p < 0.05) in phonological, flowering and yield traits assessed. The elite varieties were superior with respect to flowering traits, as they flowered early. The intra-population hierarchical cluster analysis of quantitative traits grouped the elite varieties in one main cluster and the native variety (Odukpani) alienated as an outlier. The cluster plots showed that Odukpani was robust in vegetative growth, produced few but larger fruit size and flowering was delayed for over two weeks. Conversely, the elite varieties flowered early, produced more flowers and recorded superior number of fruits per plant. The profuse production of pistillate flowers by the elite varieties had pronounced yield advantage over the local variety. Hybridization between the two groups of cucumber would make a mark in the drive for sustainable cucumber fruit yield.

Exogenous Plant Growth Regulators Show Promise for Management of Alternate Bearing in Pecan

Successful commercial pecan [Carya illinoinensis (Wangenh.) K. Koch] production relies on mitigation of alternate bearing, which is a function of pistillate flower production. Mechanisms of floral initiation in pecan are not well understood. Our objective was to assess the impact of select plant growth regulators (PGRs) on return bloom for commercial application in pecan trees grown in the Southwestern United States. A 2-year study evaluated effects of ethephon, aminoethoxyvinylglycine (AVG), and gibberellin GA3 (GA3) on subsequent season return bloom in fruiting and nonfruiting pecan shoots. Cultivars used were mature Western and immature Western and Pawnee. Effects of PGRs on return bloom of nonfruiting shoots were different from fruiting shoots. As compared with untreated control, a GA3 treatment on fruiting shoots of mature ‘Western’ trees increased the number of flowers per new shoot by 125%. For nonfruiting shoots on the mature ‘Western’ trees, the number of flowers per new shoot decreased significantly by all PGR treatments and as much as 93% for AVG. In previously nonfruiting shoots on the immature ‘Western’ trees, a GA3 treatment reduced the number of flowers per new shoot in the next season by 88.2%. Results from immature ‘Pawnee’ shoots did not show statistically significant differences. The effects of these PGRs on subsequent season flowering in pecan are complex. This study suggests that PGRs can be used to increase or decrease cropload through effects on return bloom and therefore have potential uses for mitigating alternate bearing.

Testing individual inter-annual variations in flower production by means of retrospective analysis of meristem allocation in two tree species

High inter-annual variations in flower and seed production have been frequently reported for wind-pollinated tree species. Such variations have been generally studied from a population perspective. The present study focuses on the meristematic cost of flower production in main branches of Nothofagus obliqua and N. nervosa through a retrospective analysis of flower production over 4–5 consecutive years. In both species, the percentage of flowering nodes in one year varied between 0 and 55 %. In N. obliqua, total flower production was bimodally distributed: years of high and low production were more common than years of intermediate production. Bimodality was not significant either for total flower production in N. nervosa or for each flower type separately in both species. Flower production affected branching negatively, which meant a meristematic cost. Periodicity in flower production was uneven among co-specific trees; more than two consecutive years of high flower production were rare in all trees. Inter-annual variations in the production of staminate and pistillate flowers were synchronized. In N. nervosa, the production of pistillate flowers tended to be more variable among years than the production of staminate flowers. The applied technique helps in the assessment of individual variations of flower production and contributes to understanding factors promoting seed-production variations in tree species at population scale.

Effects of virus infection on pollen production and pollen performance: Implications for the spread of resistance alleles.

Studies over the past 25 years have shown that environmental stresses adversely affect male function, including pollen production and pollen performance (germination and pollen tube growth rate). Consequently, genetic variation among plants in resistance to a stress has the potential to impact pollen donation to conspecifics and, if deposited onto a stigma, the ability of the pollen to achieve fertilization. We examined the effects of a nonlethal virus epidemic on pollen production and pollen performance in a population of susceptible and resistant (transgenic) wild squash (Cucurbita pepo subsp. texana). We grew 135 susceptible and 45 virus-resistant wild squash plants in each of two 0.4-ha fields, initiated a zucchini yellow mosaic virus (ZYMV) epidemic, and recorded staminate and pistillate flower production per plant over the field season and the total number of mature fruit. We also assessed pollen production per flower on ZYMV-infected and non-infected plants and the ability of pollen from flowers on infected and non-infected plants to achieve fertilization under competitive conditions. ZYMV infection reduced flower and fruit production per plant and pollen production per flower. Pollen from infected plants was also less likely to sire a seed under competitive conditions. ZYMV infection adversely impacts the amount of pollen that can be donated to conspecifics, and pollen competition within the styles increases the probability that the ovules are fertilized by pollen from plants that are thriving when challenged by a viral disease.

Experimental defoliation affects male but not female reproductive performance of the tropical monoecious plant Croton suberosus (Euphorbiaceae).

Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant's pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest. Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined. Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation. The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant's pollination success in the face of leaf herbivory stress.

Maximum Potential Vegetative and Floral Production and Fruit Characteristics of Watermelon Pollenizers

Triploid (seedless) watermelon [Citrullus lanatus (Thunb.) Matsum. and Nak.] pollen is nonviable; thus, diploid (pollenizer) watermelon cultigens are required to supply viable pollen for triploid watermelon fruit set. The objective of this research was to characterize maximum potential vegetative growth, staminate and pistillate flower production over time, and measure exterior and interior fruit characteristics of pollenizer cultigens. Sixteen commercially available and numbered line (hereafter collectively referred to as cultigens) pollenizer and two triploid cultigens were evaluated in 2005 and 2006 at Clayton, NC. Vegetative growth was measured using vine and internode length, and staminate and pistillate flower development was counted weekly. Fruit quality and quantity were determined by measuring individual fruit weights, soluble solids, and rind thickness. Based on vegetative growth, pollenizer cultigens were placed into two distinct groups. Pollenizers, which produced a compact or dwarf plant were ‘Companion’, ‘Sidekick’, ‘TP91’, ‘TPS92’, and ‘WC5108-1216’. Pollenizers having a standard vine length were ‘Jenny’, ‘High Set 11’, ‘Mickylee’, ‘Minipol’, ‘Pinnacle’, ‘Summer Flavor 800’ (‘SF800’), ‘Super Pollenizer 1’ (‘SP1’), and ‘WH6818’. Cultigens with compact growth habit had shorter internodes and vine lengths compared with the cultigens with standard growth habit. Cultigens with the greatest quantity of staminate flower production through the entire season were ‘Sidekick’ and ‘SP1’. The lowest number of staminate flowers was produced by ‘TP91’ and ‘TPS92’. Based on fruit quality characteristics and production, pollenizers currently or possibly marketed for consumption include ‘Mickylee’, ‘SF800’, ‘Minipol’, ‘Jenny’, and ‘Pinnacle’. The remaining cultigens evaluated in this study should be used strictly as pollenizers based on fruit quality. Arrangement of diploid pollenizers in a commercial planting of triploid watermelons is an important consideration depending on plant vegetative development. Based on staminate flower production, cultigens with higher staminate flower production are potentially superior pollenizers and may lead to improved triploid quality and production. Furthermore, pollenizer selection by fruit characteristics should include a rind pattern easily distinguished from triploid fruit in the field.

Effect of modified endogenous ethylene production on sex expression, bisexual flower development and fruit production in melon (Cucumis melo L.)

Members of the Cucurbitaceae family display a range of sexual phenotypes including various combinations of male, female, or bisexual flowers. Ethylene appears to be a key hormone regulating the sex determination process. Application of ethylene, or inhibition of ethylene action, increases or decreases the number of pistil-bearing buds, respectively. Elevated levels of ethylene production and expression of genes for ethylene biosynthesis, have been correlated with pistillate flower production. In this study, we sought to determine the effect of modified endogenous ethylene production on sex expression by constitutively expressing ACS (1-aminocyclopropane-1-carboxylate synthase), the first committed enzyme for ethylene biosynthesis, in transgenic melons (Cucumis melo L.). Most melon genotypes are andromonoecious, where an initial phase of male flowers is followed by a mixture of bisexual and male flowers. ACS melon plants showed increased ethylene production by leaves and flower buds, and increased femaleness as measured by earlier and increased number of bisexual buds. ACS melons also had earlier and increased number of bisexual buds that matured to anthesis, suggesting that ethylene is important not only for sex determination, but also for development of the bisexual bud to maturity. Field studies showed that ACS melons had earlier mature bisexual flowers, earlier fruit set, and increased number of fruit set on closely spaced nodes on the main stem. These results provide a direct demonstration of the importance of endogenous ethylene production for female reproductive processes in melon.

Prior Selfing and Gynomonoecy in Silene noctiflora L. (Caryophyllaceae): Opportunities for Enhanced Outcrossing and Reproductive Assurance

The production of seed by both selfing and outcrossing, or mixed mating, may be selected for under conditions of variable pollinator availability. Seed production was examined following experimental manipulations of perfect and pistillate flowers of the gynomonoecious plant Silene noctiflora. Style excision experiments in a greenhouse demonstrated that autonomous selfing occurs in the perfect flowers prior to their opening and continues as the flower ages; 45% of ovules were fertilized prior to flower opening and 95% by the third night of opening. Field experiments conducted in four populations showed that autonomous selfing of perfect flowers also occurs in the field. Perfect flowers excluded from pollinators had similar seed set to perfect flowers that were open to pollinators, and emasculated flowers had very low levels of seed set. These results are concordant with the hypothesis that selfing may be selected to confer reproductive assurance when pollinator visitation limits seed set. Pistillate flowers also showed significant pollinator limitation because hand‐pollinated pistillate flowers had significantly higher fruit set than those open to pollinators. Pollinator visitation, while low, occurs during both day and night, even though flowers are only fully open at night. The production of perfect flowers capable of autonomous selfing, together with the plastic production of pistillate flowers, confers reproductive assurance and enhances the likelihood of producing outcrossed seed.

Herbivore effects on developmental instability and fecundity of holm oaks.

Plants are able to compensate for loss of tissue due to herbivores at a variety of spatial and temporal scales, masking detrimental effects of herbivory on plant fitness at these scales. The stressing effect of herbivory could also produce instability in the development of plant modules, and measures of such instability may reflect the fitness consequences of herbivory if instability is related to components of plant fitness. We analyse the relationships between herbivory, developmental instability and production of female flowers and fruits of holm oak Quercus ilex trees by means of herbivore removal experiments. Removal of leaf herbivores reduced herbivory rates at the tree level, but had no effect on mean production of female flowers or mature fruits, whereas herbivory tended to enhance flower production and had no effect on fruit abortion at the shoot level. Differences in herbivory levels between shoots of the same branch did not affect the size and fluctuating asymmetry of intact leaves. These results indicate compensation for herbivory at the tree level and over-compensation at the shoot level in terms of allocation of resources to female flower production. Removal of insect herbivores produced an increase in the mean developmental instability of leaves at the tree level in the year following the insecticide treatment, and there was a direct relationship between herbivory rates in the current year and leaf fluctuating asymmetry the following year irrespective of herbivore removal treatment. Finally, the production of pistillate flowers and fruits by trees was inversely related to the mean fluctuating asymmetry of leaves growing the same year. Leaf fluctuating asymmetry was thus an estimator of the stressing effects of herbivory on adult trees, an effect that was delayed to the following year. As leaf fluctuating asymmetry was also related to tree fecundity, asymmetry levels provided a sensitive measure of plant performance under conditions of compensatory responses to herbivory.

Effect of cobalt nitrate, penshibao and Fantac on flowering and yield of musk melon (Cucumis medo L.)

The investigation carried out on musk melon revealed that pre treatment of seeds with Cobalt Nitrate at I ppm showed significant influence on vegetative, flowering and yield characters. Seed treatment with 100 ppm Penshibao had significant influence in delaying the emergence of male flower at higher nodes Whereas foliar application of Penshibao at the same concentration had significant influence only on yield characters. The foliar application of Fantac (500 ppm) showed significant influence on leaf area, fruit set per cent and fruit weight. Plants raised from seeds treated in Cobalt Nitrate 1.0 ppm and received three applications of Fantac (500 ppm) gave the highest yield per vine (3.47 kg). This increase in yield was attributed to more leaf area, advancement of female flowering, increased pistillate flower production, narrow sex ratio, increased fruit set per cent, bigger sized fruits and fruit weight.

The Influence of Fruit on the Regulation of Internal Ethylene Concentrations and Sex Expression inCucurbita texana

Plants with labile sex expression often regulate the production of pistillate and staminate flowers on the basis of developmental and environmental conditions. In many monoecious plant species, developing fruits inhibit pistillate flower production and promote staminate flower production. Because the phytohormone ethylene promotes pistillate flower production in a variety of wild and cultivated plant species, we examined the influence of fruit development on internal ethylene production and their combined effects on sex expression in field‐grown Cucurbita texana (Cucurbitaceae). We compared the sex expression of plants with fruit and without fruit, and we measured internal ethylene concentrations within the hollow internodes of their branches. Plants without fruit had more than twice as many pistillate flowers than plants with fruit. Ethylene was greatest within internodes just below the tips of branches compared with more basal internodes (2–21 internodes from the tip) in both fruiting and fruitless treatments. Ethylene concentrations within branches with fruit were highest 18 d after pollination, just as fruits reached their maximum volume. Branches with two or more developing fruits had the lowest ethylene concentrations, whereas ethylene levels were significantly higher on fruitless branches and one‐fruited branches. Our results indicate that young developing fruits may send a signal that inhibits ethylene production, leading to the promotion of staminate flowers, and that, when fruits are no longer strong resource sinks, ethylene concentrations increase and promote pistillate flower production.

Environmental and Physiological Effects on Pistillate Flower Production in Silene noctiflora L. (Caryophyllaceae)

Environmental and physiological elements influencing pistillate flower production are described for the gynomonoecious annual Silene noctiflora L. (Caryophyllaceae). We tested two environmental (temperature/physical disturbance) and two hormonal (gibberellin/ethylene) factors in greenhouse experiments. The effect of ethylene on rosette-stage seedlings was also tested in the field. A 6⚬C increase in average greenhouse temperature (19⚬/25⚬C) caused a significant increase in the production of pistillate flowers, whereas the removal of buds, flowers, or branches did not cause any significant changes. There was a trend, however, for increased pistillate flower production as more biomass was removed. Of the two hormones tested, only ethylene caused a significant increase in pistillate flower production. This effect was more prominent when applications were given to rosette-stage seedlings in the field, compared with newly emerging buds in the greenhouse, probably because sex determination occurs early in bud development. The results of this study show that plasticity in pistillate flower production in S. noctiflora is influenced by changes in temperature and ethylene. Such temperature-related plasticity may represent an adaptive mechanism to increase outcrossing when conditions are more favorable for pollinator activity.

Reproductive allometry in Malaysian rain forest trees: Biomechanics versus optimal allocation

Quantitative predictions of reproductive allometry in iteroparous plants may be derived from two bodies of theory: biomechanics and optimal allocation theory. Biomechanical theory predicts allometric scaling exponents between reproductive (R) and vegetative (V) biomass in the range of 0.44–1.33, while very general models of life history evolution predictR–V exponents > 1 in all cases. These predictions are examined in light of allometric patterns of flower and fruit production in 32 species of Malaysian rain forest trees. Among these species the mean estimatedR–V exponents are in the range 1.8–2.0 for staminate flower, pistillate flower and fruit production. This range of exponent values provides unambiguous support for some of the general predictions of optimal allocation models, but not for biomechanical theory. Optimal allocation models also predict a positive relationship between species size andR–V slope and a positive relationship between species size andR–V intercept parameters. The latter, but not the former prediction is supported by the data.R–V allometries in sexes of dioecious species were also found to differ in intercept, though not slope, reflecting smaller sizes at reproductive onset in staminate trees. Further critical examinations of reproductive allometry are encouraged as a relatively unexplored avenue for increasing the contact of theory and data in studies of life history evolution in long-lived organisms such as tropical trees.

303 VARIATION IN SUMMER SQUASH FLOWERING IN RESPONSE TO ENVIRONMENT

Research over a two year period assessed the influence of planting date and location on time to flowering and number of flowers produced for five summer squash (Cucurbita pepo L.) cultivars. Heat units (HU) were calculated using a single equation to determine if this approach could account for a significant portion of the variability in time to onset of flowering over the range of environments. Depending on cultivar and flower sex, the number of days to flowering varied as much as 20 days. There were significant cultivar differences in HU required for the onset of both staminate and pistillate flowers. The use of HU instead of days reduced the variability of time to flowering as indicated by regression analyses and mean absolute differences between predicted and observed days to flowering. The total number of staminate flowers produced was more variable than that of pistillate flowers. The ratio of pistillate-to-staminate flowers was stable for two of the five cultivars; however, pistillate flower production for those two cultivars was severely restricted during hot weather. Thus environment has a considerable influence on both the onset of flowering and the number of flowers produced for summer squash.

Staminate and Pistillate Flower Production of Summer Squash in Response to Planting Date

Staminate and pistillate flower production in summer squash (Cucurbita pepo L.) fluctuates readily in response to the various crop production environments throughout the southeastern United States. `Dixie', `Senator `, `Lemondrop', `Meigs', and `Elite' squash were planted at various times over 2 years in Griffin, Ga., to determine the effect of planting date on staminate and pistillate flower counts for the first 2 weeks of flowering. Staminate and pistillate flower counts varied considerably depending on cultivar and time of planting, but no consistent pattern emerged. The production of staminate flowers was generally more variable than that of pistillate flowers. The distillate: staminate flower ratio was generally stable for `Senator' and `Elite', but not for the other cultivars, particularly `Dixie'. `Dixie' produced more distillate than staminate flowers 50% of the time, whereas `Senator' always produced more staminate flowers. Pistillate flower production for `Senator' and `Elite' was restricted during hot weather. These data indicate that staminate and pistillate flower counts of squash fluctuate under varying environmental conditions and that maintaining production over a range of planting dates will depend on careful cultivar selection.

Pollination dynamics of arctic dwarf birch (Betula glandulosa; Betulaceae) and its role in the loss of seed production

At the northern limit of its distribution, arctic dwarf birch, Betula glandulosa, shows nearly complete absence of sexual reproduction (i.e., <;0.5% viable samaras) and maintains its populations vegetatively. To investigate the possible role of pollination dynamics in the loss of sexual reproduction, pollination biology of arctic dwarf birch was compared at two locations: Tarr Inlet on Baffin Island, Northwest Territories near the northern limit of the species, and Kuujjuaq, Quebec in the center of its distribution where sexual reproduction is the primary mode of reproduction. Relative production of staminate and pistillate flowers, pollen rain, pollen viability, and stigmatic pollen loads throughout pollen dispersal were compared. Plants at Tarr Inlet produced 15%–30% of pollen produced at Kuujjuaq, both as a result of a lower density of staminate catkins and less pollen per catkin. Potential seed productivity is limited at the northern limit because pistillate catkins produce 50% fewer flowers in the north than in the south. While stigmatic pollen loads were similar at both sites, lower pollen viability (68% vs. 93%) and a higher probability of geitonogamous pollen due to clonal growth pattern reduced fertilization success at the northern site. These data suggest that lack of sexual reproduction in B. glandulosa at its northern limit is in part due to pollen limitation.

Pollination Dynamics of Arctic Dwarf Birch (Betula glandulosa; Betulaceae) and Its Role in the Loss of Seed Production

At the northern limit of its distribution, arctic dwarf birch, Betula glandulosa, shows nearly complete absence of sexual reproduction (i.e., <;0.5% viable samaras) and maintains its populations vegetatively. To investigate the possible role of pollination dynamics in the loss of sexual reproduction, pollination biology of arctic dwarf birch was compared at two locations: Tarr Inlet on Baffin Island, Northwest Territories near the northern limit of the species, and Kuujjuaq, Quebec in the center of its distribution where sexual reproduction is the primary mode of reproduction. Relative production of staminate and pistillate flowers, pollen rain, pollen viability, and stigmatic pollen loads throughout pollen dispersal were compared. Plants at Tarr Inlet produced 15%–30% of pollen produced at Kuujjuaq, both as a result of a lower density of staminate catkins and less pollen per catkin. Potential seed productivity is limited at the northern limit because pistillate catkins produce 50% fewer flowers in the north than in the south. While stigmatic pollen loads were similar at both sites, lower pollen viability (68% vs. 93%) and a higher probability of geitonogamous pollen due to clonal growth pattern reduced fertilization success at the northern site. These data suggest that lack of sexual reproduction in B. glandulosa at its northern limit is in part due to pollen limitation.

SEX RATIOS AND FRUIT CHARACTERISTICS OF CRIMSON SWEET WATERMELONS GROWN UNDER VisPore ROW COVER AND PLASTIC MULCH

Staminate and pistillate flower production and fruiting characteristics of `Crimson Sweet' watermelons were evaluated under VisPore row cover plus clear polyethylene mulch (VCM), VisPore row cover plus black polyethylene mulch (VBM), clear polyethylene mulch (CM), black polyethylene mulch (BM) and bare soil (BS). VCM produced significantly higher numbers of pistillate and staminate flowers than other treatments. All mulched and mulched plus VisPore treatments were significantly different from BS with regards to the 1st nodal position of the staminate and pistillate flowers. Fruit-set among the treatments between 53-55 days after transplanting were: 100%, 75%, 59% and 32% for VCM, VBM, CM and BM, respectively. Average number of fruits per plant were: 4, 3, 3, 3 and 1 for VCM, VBM, CM, BM and BS, respectively.

Within- and Among-Tree Variation in Flower and Fruit Production in Two Species of Carya (Juglandaceae)

The patterns of variability in the production of staminate inflorescences, pistillate flowers, and fruits for Carya ovata (Mill.) K. Koch and C. tomentosa (Poir.) Nutt. at Hutcheson Memorial Forest (Somerset County, New Jersey) were examined over a four-year period. We sought to determine 1) the patterns of variability in flowering and fruiting (within-trees, among-trees, and between-years), 2) if variable flowering might account for observed nut-bearing patterns, and 3) what were the relative contributions of intrinsic (genetic) and/or extrinsic (environmental) factors in determining flower production and fruit set. In general, our fine-scale analysis of shoots within canopies did not reveal a distinct mast fruiting pattern. While the number of male and female flowers varied significantly (among trees and between years), fruit set was not markedly affected. Variability of flowering and fruiting among shoots within trees was low. Both flowering and fruiting were observed to have a strong genetic and/or microenvironmental basis; however, flowering appeared more plastic and thus more sensitive to yearly environmental vagaries than did fruiting. Favorable environmental conditions at the time of flower differentiation may result in increased flower production; but, this does not necessarily result in increased fruit set. Many pistillate flowers abscise around the time of pollination and fertilization, apparently adjusting fruit set to available shoot resources. Approximately 50% of the pistillate flowers of both species fail to set fruit. Overall, individual trees exhibit their own flowering and fruiting schedules, suggesting the need to account for this level of variability in future studies of mast fruiting.