Pollen Tube Growth and Seed Set of Banksia coccinea R.Br. (Proteaceae)

Controlled hand pollinations and field observations were usedin conjunction with fluorescence and scanning electron microscopyand fruit and seed set to investigate the timing of stigma receptivity,pollen tube growth and self-incompatibility in relation to fertilityin B. coccinea. The species showed both protandry and partialself-incompatibility. Peak stigma receptivity as measured bypollen germination was recorded at 3 d after anthesis and maximumproduction of stigmatic exudate at 6 d. Pollen tubes reachedthe base of the style by 6 d after pollination. A 5 x 5 diallelexperiment was conducted and the results measured by pollentube growth. Self-pollinations generally resulted in poorertube growth than crosses and there was significant specificand general combining ability as well as reciprocal effects.Cross-pollination resulted in improved fruit set and seed toflower ratio over both selfing and open pollination. Spatiallimitations to fertility due to infructescence size were alsorecorded, but the combination of outcrossing mechanisms andspatial limitation did not entirely account for the low fertility.It is suggested that environmental conditions and the availabilityof resources may also exert an influence. Banksia coccinea R.Br., scarlet banksia, Proteaceae, pollination, stigma receptivity, pollen tube growth, self-incompatibility, breeding system, seed set     

Effects of Seed Number on Competition and Dominance among Fruits in Capsicum annuum L.

The effects of seed number on set, development and growth ofa fruit, and on inhibition of later-developed fruits were studiedby varying the pollen load on the stigma of sweet pepper flowers(Capsicum annuum L.). Despite much variation, a linear increasein individual fruit weight with seed number could be observed.Seed number affected the growth rate rather than the growingperiod of fruit. When seed numbers were low, the probabilityof fruit setting was positively related to seed number. However,a relatively low seed number (50–100 seeds/fruit: 20–30%of the maximum seed number) was sufficient for maximal fruitset. An increase in seed number increased the inhibitory effect ofa fruit on set and growth of later-developing fruits. As a result,when pollination treatments were applied to all the flowersof a plant, results could be quite different to those obtainedwhen only a limited number of flowers were treated. Fruit setof the second fruit was reduced by the application of a highpollen load to the first flower, even when the first fruit abortedbefore it had accumulated much dry matter. Our results suggestthat growth inhibition of the second fruit by seed number ofthe first fruit is controlled both by competition for limitedassimilates, as well as by dominance due to the production ofplant growth regulators by the developing fruit. Sweet pepper; Capsicum annuum L.; pollination; fruit set; abortion; abscission; fruit growth; first-fruit dominance; sink strength     

Self-incompatibility, Inbreeding Depression and Crossing Potential in Five BrazilianPleurothallis (Orchidaceae) Species

Intra- and interspecific experimental pollinations were madeto determine the mating systems and the interspecific crossingpotential in individuals from 24 populations of five Pleurothallisspecies. Pleurothallis johannensis, P. ochreata and P. teresshow weak or partial self-incompatibility while P. adamantinensisand P.fabiobarrosii are more strictly self-incompatible. Wefound no differences in fruit set between intra- and interpopulationintraspecific crosses, and there was no correlation betweenfruit set and genetic variability or inbreeding in these species.All species are interfertile and showed no correlation betweengenetic similarity and crossing potential. We scored fruitsfor seed viability and observed a strong inbreeding depressionin all populations; however, there was no difference in seedviability among intrapopulation, interpopulation intraspecificand interspecific cross-pollinations. These species are pollinatedby flies with a behavioural pattern that facilitates self-pollination.Self-incompatibility and inbreeding depression are apparentlyimportant in the maintenance of the unusually high levels ofgenetic variability found in these species. As generally observedin other orchid species, barriers to hybridization between thesePleurothallis species are at the level of pollination. Copyright2001 Annals of Botany Company Pleurothallis, Orchidaceae, mating system, self-incompatibility, inbreeding depression, fruit set, seed set, polyembryony, interspecific compatibility     

Pollination and Resource Constraints on Fruit Set and Fruit Size ofPersoonia rigida(Proteaceae)

Flower and immature fruit abscission can be caused by proximatefactors preventing the development of all flowers into maturefruits. Two potential limiting factors, pollen supply and carbohydrateavailability, were assessed as relative constraints on accumulationof fruit dry matter inPersoonia rigida(Proteaceae). Naturalpollen transfer was highly efficient, and no evidence of self-incompatibilitywas found. Levels of fruit set (67.4% following open-pollinationand 50.4–58.8% following hand-pollinations) may be thehighest recorded for a species of Proteaceae with hermaphroditicflowers. Fruit abscission occurred principally in a single phasefrom 4–10 weeks post-anthesis, with a peak during theeighth week post-anthesis, and this early abscission was theresult of fertilization failure in some flowers. Manipulationof carbohydrate availability by girdling and/or defoliationof branches provided evidence that carbohydrate supply affectedfruit set only when supply was very low. Fruit abscission dueto very low carbohydrate availability on defoliated brancheswas evident during the early abscission phase but was more pronouncedduring the second half of the fruit development period (>17weeks post-anthesis), coinciding with the period of greatestdry matter demand by the fruits. Minimal fruit abscission occurredduring this later period on leaf-bearing branches. Increasesin carbohydrate supply had no effect on fruit set, but fruitsize was highly sensitive to carbohydrate availability. Subtendingand adjacent leaves were identified as major contributors tothe carbohydrate supply of fruits, although some carbohydratecould be obtained from beyond the fruiting branch. Despite thehigh levels of natural fruit set, both pollen supply and resourceavailability were indicated as potential constraints on accumulationof fruit dry matter inP. rigida.Copyright 1999 Annals of BotanyCompany. Persoonia rigidaR.Br., Proteaceae, pollination, carbohydrates, fruit set, fruit size, abscission, girdling.     

Vegetative Parthenocarpy in the Cactus Pear Opuntia ficus-indica (L.) Mill.

The dependence of fruit development on fertilization was studiedin two clones of Opuntia ficus-indica, Ofer and BSI. Fruitsof the clone Ofer bear fully developed seeds, whereas fruitsof the clone BSI contain only degenerated seeds and it was suspectedthat BSI is parthenocarpic. The two clones differed in theirpattern of fruit development. The increase in fruit fresh weightin Ofer was a result of both peel and pulp growth, whereas inBSI fruit growth was mainly due to pulp growth and the peelhad reached its final weight almost at anthesis. Pulp growthin BSI commenced earlier and was faster than in Ofer, but thefinal pulp weight in BSI was only 64% of that in Ofer. Seedgrowth in BSI was limited to the development of semilignifiedand lignified seed coats, whereas in Ofer 43·5% of theseeds were completely developed, the remainder being similarto those found in BSI. Fruits bearing well-developed seeds werealso found in a 'mutant' of BSI that, like Ofer, contained smallerovules at anthesis than flower of the regular BSI genotype.Germination in vivo and in vitro were similar in BSI and Ofer,but pollen tubes failed to reach the ovules in the regular BSIgenotype, while penetrating the ovules in Ofer and the small-ovuledgenotype of BSI. Good-quality fruits similar to Ofer fruitsin weight but with a higher peel to pulp ratio developed inBSI after flower-sterilization in the spring and in the autumn.In Ofer, sterilized flowers failed to develop fruits in springand partially set fruit in the autumn; the fruits consistedalmost exclusively of peel tissue. It was concluded that BSIis a vegetative parthenocarpic clone, i.e., that pollinationis not required for fruit set and development.Copyright 1993,1999 Academic Press Opuntia ficus-indica, cactus pear, fruit development, parthenocarpy, pollen-tube-growth, fluorescence microscopy     

Purine Alkaloids of the Fruits of Camellia sinensis L. and Coffea arabica L. during Fruit Development

The amounts of two purine alkaloids, caffeine and theobromine,in the fruit of tea (Camellia sinensis L.) increased markedlyduring the growing season until the fruit was full-ripened anddried. In the dry fruit, the pericarp contained the most alkaloids,but there were also considerable amounts in the seed coat and,to a lesser extent, the fruit stalk and the seed. The shed seedsalso contained significant amounts of the alkaloids, especiallyin the seed coats. In contrast with the dry fruit of tea, seedsand pericarp of coffee (Coffea arabica L.) fruit contained aconsiderable amount of caffeine and a small amount of theobromide.A small amount of theophylline was also present in the pericarpof the ripened fruit. Relationships between growth and purinealkaloid content in tea and coffee fruits and their roles duringseed formation are discussed. Camellia sinensis L., tea, Coffea arabica L., coffee, purine alkaloids, fruit development, seed, seed coat, caffeine, theobromine, theophylline     

Effect of Sporophytic Heterozygosity on the Male Gametophyte of the Tetraploid Potato (Solanum tuberosum L.)

Quantitative variation for pollen stainability was examinedin both tetraploid x diploid and tetraploid x tetraploid familiesof potato (Solanum tuberosum L.). The diploid parents, whichwere highly heterozygous, produced first division restitution(FDR) 2n pollen. The families used were not affected by cytoplasmic-geneticmale sterility. Significant differences between families werefound. FDR 2n pollen producing diploid clones had progenieswith a significantly higher pollen stainability than those derivedfrom crosses between tetraploid parents. This was related tothe level of heterozygosity in the sporophyte. In FDR 2n pollen,80% of the heterozygosity and a large fraction of epistasispresent in the diploid parent was transmitted to its tetraploidprogeny. This is important for the production of open pollinated,true potato seed, because pollen stainability is significantlycorrelated with fruit set (r = 0·85).Copyright 1994,1999 Academic Press FDR 2n pollen, gametophytic heterozygosity, true potato seed     

Studies on the Development of the Desiccation-sensitive (Recalcitrant) Seeds of Avicennia marina (Forssk.) Vierh.: The Acquisition of Germinability and Response to Storage and Dehydration

Germinability and responses to storage and dehydration werestudied throughout the development of the desiccation-sensitiveseeds of Avicennia marina. Seeds acquired the ability to produceroots at 55 d after fruit set (DAFS) which is shortly afterhistodifferentiation, but the capacity for full germinability(seedling establishment) was not attained until 70 DAFS, whichis midway through the phase of growth and reserve accumulation.Pre-mature seeds showed a germination lag that was equivalentto the period between harvest and full maturity, but, followingshort-term storage, this was reduced to that of mature seeds.At no stage, however, would seeds with an intact pericarp germinate. Once seeds were fully germinated, storage lifespan under non-desiccatingconditions was independent of developmental stage, but was considerablyreduce by the presence of the pericarp, probably because offungal contamination. Prior to the acquisition of full germinationcapacity, the seeds were unable to tolerate any dehydrationbut became tolerant to slight water loss once they became fullygerminable, after which desiccation sensitivity was not influencedby the stage of development. If rapidly dried, excised axesof germinable seeds survived to lower water contents than didaxes removed from seeds following slower drying.Copyright 1993,1999 Academic Press Desiccation-tolerance/sensitivity, germination, mangrove, recalcitrant, seed development, seed storage     

Effects of varying pollen load on fruit set,seed set and seedling performance in apple and pear

Two apple crosses and one pear cross were carried out using three different pollen densities. In one of the apple crosses, both fruit and seed set decreased when a mixture of pollen and powder (diluted pollen treatment) was used for pollination. Also, seedlings from the undiluted treatment were more vigorous than those originating from pollination with diluted pollen. In the other apple cross, however, no effect on fruit set, seed set or growth in the first year was found. Mean diameter after the second year differed among treatments, but did not seem to be related to pollen density. For pear, the number of good seeds decreased and the number of deaf, probably unfertilized, seeds increased with decreasing pollen density. However, there was no apparent significant effect of pollen density on seedling growth. With respect to scab resistance and mildew attack in both apple crosses, no differences among treatments were found. The results indicate that gametophytic competition may occur in apple crosses, but that it is probably weaker in pear.     

Reproductive Biology ofDalbergia miscolobiumBenth. (Leguminosae-Papilionoideae) in SE Brazil: The Effects of Pistillate Sorting on Fruit-set

The breeding system ofDalbergia miscolobiumBenth. was investigatedby means of controlled pollinations and studies of post-pollinationevents in pistils. This species does not have homomorphic self-incompatibility,but crossed flowers always have higher fruit set than selfs,despite the fact that embryo development may be initiated inselfed pistils. It is suggested that deleterious recessive alleles,which lead to abortion of most selfed seeds, together with somematernal resource limitation, result in low fruiting success.Copyright1998 Annals of Botany Company Dalbergia miscolobiumBenth., Leguminosae, breeding system, pistillate sorting, fruiting success, seed set.     

Symplastic Solute Transport and Avocado Fruit Development: A Decline in Cytokinin/ABA Ratio is Related to Appearance of the Hass Small Fruit Variant

Studies on the effect of fruit size on endogenous ABA and isopentenyladenine(iP) in developing avocado (Persea americana Mill. cv. Hass)fruit revealed that ABA content was negatively correlated withfruit size whilst the iP/ABA ratio showed a linear relationshipwith increasing size of fruit harvested 226 d after full bloom.The effect of this change in hormone balance on the relationshipbetween symplastic solute transport and appearance of the smallfruit variant was examined following manipulation of the endogenouscytokinin (CK)/ABA ratio. Application of ABA caused seed coatsenescence and retarded fruit growth but these effects wereabsent in fruit treated with equal amounts of ABA plus iP. Thus,the underlying physiological mechanisms associated with ABA-inducedretardation of Hass avocado fruit growth appeared to be inextricablylinked to a decline in CK content and included: diminution ofmesocarp and seed coat plasmodesmatal branching, gating of mesocarpand seed coat plasmodesmata by deposition of electron densematerial in the neck region, abolishment of the electrochemicalgradient between mesocarp and seed coat parenchyma, and arrestof cell-to-cell chemical communication. (Received February 25, 1998; Accepted July 28, 1998)     

Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativum L.)

The amino acid composition of the EDTA-induced phloem exudatereaching the fruit and the seed, and of the solutes releasedby the seed coat during fruit development were determined inglasshouse-grown pea (Pisum sativum L. cv. Finale) suppliedeither with nitrate-free nutrients (nodulated plants) or withcomplete medium (non-nodulated plants). The EDTA-promoted exudationtechnique was used supposedly to collect phloem sap and theempty seed technique supposedly to collect the solutes secretedby the seed coat to the embryo sac cavity. In young seeds embryosac liquid was sampled directly from the embryo sac. The maincarbohydrate transported and secreted was sucrose. The mainamino acids reaching the fruit were asparagine, glutamine, andhomoserine. Their proportions were steady during a day-nightcycle and throughout fruit development. Amino acid compositionchanges occurred first in the pathway from fruit stalk to seedfunicle, due to the formation of threonine (probably from homoserine)and in the seed coat due to production of glutamine, alanineand valine which, together with threonine were the main secretedamino acids. The temporary nitrogen reserves of the pod walland seed coat were remobilized as asparagine during senescence.Phloem exudate of nodulated plants showed a higher (about twice)proportion of asparagine but lower proportions of homoserineand glutamine than in EDTA-induced phloem exudate of nitrate-fedplants. The two types of nitrogen nutrition also produced somechanges in relative proportions of threonine and homoserinesecreted by the seed coat. Key words: Pisum sativum, phloem, amino acids, pod wall, seed coat     

Sequential variation in the components of reproductive success in the distylousJasminum fruticans (Oleaceae)

A 2-year study of three natural populations of the distylousJasminum fruticans showed that mean fruit and seed production were significantly greater in shortstyled plants (thrums) than in long-styled plants (pins). In this study, we investigated the role of four sequential factors which may differentially influence fruit and seed set in the two floral morphs: (1) differences in flowering phenology, (2) a limitation of pollen transfer towards pins, (3) a differential capacity of the two morphs to act through famale and male function and (4) differential fruit abortion in the two morphs. Fruit set was significantly influenced by differences in flowering phenology although there were no differences in flowering time between the two morphs. supplementary pollinations in a natural population significantly increased fruit set and reduced the difference in fruit set between the two morphs in relation to controls, indicating a limitation on pollen transfer which was most severe towards pin stigmas. In reciprocal crosses, seed set was significantly dependent on the paternal and maternal identity of the pin parent. There was no significant variation among thrums for their performance as male or female parent. Furthermore, individual pin plants with relatively high percentage seed set as female parents gave poor seed set as male parents and vice-versa. Whereas fruit removal had no effect on seed number in thrum plants, a greater proportion of viable seeds were produced on pin plants which were left to naturally mature their fruits than on pins which had fruits artificially removed, suggesting the occurrence of selective fruit abortion in pins but not in thrums. The initially greater maternal fitness of thrums due to their greater success as pollen recipients may thus be opposed by increased viable seed set in the pins due to factors acting after the pollination stage. The relative reproductive success of floral morphs in the distylousJ. fruticans is thus differentially influenced by ecological factors occurring at different stages of the reproductive process.     

Limitations to reproductive output and genetic rescue in populations of the rare shrub Grevillea repens (Proteaceae)

Background and Aims

When conserving rare plant species, managers are often faced with small and/or isolated populations displaying low levels of sexual reproduction and genetic variation. One option for reinvigorating these populations is the introduction of genetic material from other sites, but in some cases fitness may be reduced as a result of outbreeding depression. Here the pollination biology of the rare shrub Grevillea repens is studied across its natural range and reproductive responses following cross-pollination among populations are examined to determine factors that may be limiting sexual reproduction and the potential for genetic rescue.

Methods

Pollen manipulation treatments (self-, autogamous self-, cross- and open pollination) were applied to flowers to examine the breeding system and fruit and seed production in five populations of G. repens. Pollen production, presentation and viability were investigated and interpopulation crosses of increasing genetic distance performed among the populations.

Key Results

The study species is self-incompatible and displayed very low natural seed set over two seasons, due partly to low pollen viability in one of the populations. Within-population crossing increased fruit and seed production at some sites, indicating pollinator limitation. Interpopulation crosses further increased reproductive output in one population, suggesting mate limitation, and for this site there was a positive relationship between genetic distance among populations and the size of genetic rescue benefits. However, in other populations there was a decrease in fruit and seed set with increasing genetic distance.

Conclusions

The results highlight that management strategies involving interpopulation crosses can improve reproductive output in small, isolated populations of rare plants, but guidelines need to be developed on a population by population basis.Key words: Grevillea repens, Proteaceae, genetic rescue, pollination ecology, self-incompatibility, breeding system, interpopulation cross, outbreeding depression, pollinator limitation, mate limitation, resource limitation     

BREEDING SYSTEM AND POLLINATION ECOLOGY OF TRIENTALIS BOREALIS (PRIMULACEAE)

Selfing in Trientalis borealis is reduced by spatial separation of stigma and anthers during anther dehiscence and by internal self-incompatibility. Artificial self-pollination resulted in low levels of fruit set (2.0%). Crosses made within patches of plants yielded variable (0–72.7%), but generally low fruit set (x̄ = 21.4%). This may be caused by patches often consisting of a single genet. In contrast crosses made among patches resulted in uniformly high levels of fruit set (x̄ = 84.3%). Pollinator activity was higher at open sites than at heavily shaded sites and was correlated with fruit set. Pollinator activity, however, showed no correlation with seed set. We conclude that the primary pollinators, Halictid and Andrenid bees, play an important role in the breeding system of the plant by promoting outcrossing but that availability or efficiency of pollinators may limit seed set.     

Factors Influencing Seed Set in Triploid Musa spp. L. and Production of Euploid Hybrids

Seed set in triploid Musa spp. L. is very low due to high levelsof sterility. This makes cross breeding of plantain and bananadifficult. Nonetheless, several triploid plantain and bananacultivars produce seeds after hand pollination with diploidparents. This paper reports results about the pattern of seedvariation, success of in vitro germination and hybrid productionafter triploid-diploid crosses. Between-and within-clones differencesin seed set were observed. ABB cooking bananas had the highestrates of seed production followed by AAB French plantains. Plantainsproduced more diploid than tetraploid offsprings after triploid-diploidcrosses, which suggests that meiosis resulted in the productionof balanced haploid gametes (n = x) due to normal chromosomebehaviour between homologous genomes. Seed set variation withinclones was influenced by both the production of 2n (= 3x) gametesin the triploid parent and climatic factors. Indeed, seed production,embryo and seed germination success, and euploid and tetraploidhybrid production followed a pattern of seasonal variation reachinga peak in September. High frequency of seed with tetraploidembryos was obtained in the period from January to mid-March.The results demonstrate that production of desirable tetraploidswould be more efficient when hand pollinations are done underhigh temperature, high solar radiation and low relative humidity.These seem to be the most convenient conditions for relativelyhigh production of 2n eggs as compared to haploid gametes.Copyright1995, 1999 Academic Press Banana, fertility, embryo culture, Musa spp. AAA group, Musa spp. AAB group, Musa spp. ABB group, plantain, ploidy, seed set, 2n gametes     

Effects of Parental Embryo and Endosperm Mitotic Cycle Times on Development of Hybrids Between Barley and Rye

Seven different barley/rye crosses were made using genotypeshaving close (predicted compatible) or dissimilar (predictedincompatible) mean cell doubling times. The relative successof the crosses was determined by a cytological study of earlyhybrid seed development and by the yield of 16-day-old hybridembryos. The results support the hypothesis that parental developmentalrates must be similar for successful hybridization. The degenerationof the hybrid endosperm occurred earlier in the predicted incompatiblecrosses than in the more compatible ones. Fewer hybrid embryoswere harvested at day 16 from predicted incompatible crossesthan from compatible crosses. We conclude that development ofhybrid embryos depends on the early stages of endosperm developmentand that mitotic rates in parental endosperms are more importantthan in embryos. Hordeum vulgare L., Secale cereale L., barley, rye, hybrid, mean cell doubling time, embryo, endosperm     

Floral biology and potential hybridization of three sympatric epiphytic bromeliads in Veracruz,Mexico

In the Bromeliaceae, hybrids have been engineered for horticultural purposes; in addition, hybrids have been observed in natural habitats. We investigated the hybridization potential of three sympatric species of Tillandsia inhabiting secondary forests and shaded coffee plantations in the central mountains of Veracruz, Mexico. The studied species (T. juncea, T. polystachia and T. variabilis) show overlap in flowering phenology and are believed to share pollinators. We compared the floral morphology of the species, performed controlled intra- and interspecific pollinations, and recorded pollen tube growth, fruit and seed set, as well as seed germination. Flower size differed among the species, but stamen and stigma heights were similar between pairs of species, which would facilitate interspecific pollen exchange. Pollen tubes reached the ovary in interspecific crosses. The three species showed interspecific cross-compatibility, as all crosses resulted in development of mature fruit and viable seeds. Tillandsia juncea achieved the highest fruit set, whereas T. variabilis showed the highest seed set. Seed set as a result of agamospermy, autogamy, geitonogamy and interspecific cross-pollination did not differ among the species. The findings suggest that pre- and post-pollination barriers are weak among the three species of Tillandsia, which thus could potentially hybridize in natural sympatric populations.     

Some Morphological and Chemical Characteristics of Developing Fruits of Raphia hookeri

Ndon, B. A. 1985. Some morphological and chemical characteristicsof developing fruits of Raphia hookeri.—J. exp. Bot. 36:1817–1830. Fruits which were at different stages of development were randomlysampled from different inflorescences of Raphia hookeri palms.The morphological characteristics and chemical (the dry matter,lipid and carbohydrate) contents of the exocarp and seeds weredetermined. The results showed that the seed length, circumferenceand volume were optimal at 24 months after pollination whichindicates that Raphia seeds attained maximum size at that period.The seed endosperm was liquid or semi-liquid between 6–18months after pollination but became solid with a prominent embryoat 24 months. The seed dry matter was low at the early stagesof development but there was a rapid increase in seed dry weightat 18–33 months after pollination. The seeds were physiologicallymatured at 30–33 months after pollination, while the exocarpmatured at 24–30 months after pollination. The Raphia seeds were low in lipid (about 2%) compared to theexocarp which had 30–40% lipid at full maturity. Maximumamount of lipid was accumulated within the exocarp at 36–42months after pollination and this period indicates the timefor harvesting Raphia fruits for maximum oil which is probablythe most economic part of the fruit. The total sugar concentration increased in the exocarp withincrease in maturity. Conversely the concentration of sugarsdecreased within the seeds as the fruit matured. Maximum totalsugar concentration (about 309 mg g^–1 dry fat free sample)was found in the exocarp at 36–42 months after pollination.Mature seeds at 48 months after pollination had about 50 mgof total sugars per g of fat free sample. There was insignificantaccumulation of starch in the exocarp. The mature seeds werelow in starch (5–10% of the dry weight). Key words: Raphia hookeri, development, fruit     

Role of Ethylene in Avocado Fruit Development and Ripening: I. FRUIT DROP

A survey of avocado (Persea americana Mill.) fruit drop andethylene production was carried out during fruit development.Natural drop of avocado fruitlets started soon after set (May)and continued at a gradually decreasing rate until September,except for a temporarily increasing rate in late July. Fruitletsweighing up to 0?2 g dropped at a rate of over 30 percent perweek. With larger fruits, the rate was under 1 percent per week.Fruit drop ceased after September, when fruit growth declinedand the seed coat began to shrivel. A positive correlation was found between the rate of fruitletand fruit drop and ethylene production. Fruitlets with defectiveseeds produced ethylene at a very high rate of 7–10 timesmore than apparently normal fruits. The high incidence of defectiveseeds might be the cause of the very high levels of ethyleneproduction by young avocado fruitlets. The seed was found to be the main site of ethylene productionin fruitlets. Abscisic acid (ABA) levels in young abscissing fruits were 7times as high as those in non-abscissing fruits.