Free amino Acid content and metabolic activities of setting and aborting soybean ovaries

Fruits of soybean (Glycine max [L.] Merr.) that are destined to abscise shortly after anthesis grow more slowly than fruits that will be retained. In this work, amino acid composition, protein metabolism, and nucleic acid metabolism were studied in setting and abscising soybean ovaries from anthesis to 6 days after anthesis to provide additional evidence of chemical processes associated with abscission. Principal free amino acids were asparagine, aspartic acid, glutamic acid, serine, and glutamine. Percent aspartate and glutamate declined as the ovaries grew, with aspartate declining more in abscising and glutamate more in setting ovaries. Percent glutamate was positively correlated to percent abscission throughout the period. Proline, serine, and leucine were positively correlated to abscission from 0 to 2 days after anthesis, whereas significant negative correlations were observed at these ages for ethanolamine and arginine. ⁷⁵Se fed as selenate and ¹⁴C fed as sucrose, glycine, and alanine were readily incorporated into soluble and insoluble proteins in a 24-hour in vitro incubation. Radioactivity of total proteins, expressed on a perovary basis, was negatively correlated with percent abscission and positively correlated with ovary weight. [¹⁴C]Glutamine and serine followed the opposite pattern, with greater protein labeling in abscising than in setting ovaries. When data were expressed as disintegrations per minute per milligram ovary fresh weight, protein labeling from alanine was seen to be significantly greater in abscising ovaries at anthesis and throughout the sampling period. Nucleic acid labeling from uridine was highly correlated to ovary weight; labeling from thymidine was greater in setting than abscising ovaries at anthesis and in abscising ovaries at later stages of development. It is concluded that abscising ovaries can continue amino acid metabolism almost up to the date of separation from the raceme, and that the involvement of alanine, glutamine, aspartate, glutamate, and other amino acids in soybean flower abortion deserves further study.     

Delay of early fruitlet abscission by branch girdling in citrus coincides with previous increases in carbohydrate and gibberellin concentrations

Current evidence in citrus indicates that gibberellins (GAs) are main determinants of early fruit set while subsequent growth of developing fruits is mostly dependent upon carbohydrate availability. In this work, branch girdling performed at anthesis in Satsuma mandarin (Citrus unshiu (Mak.) Marc.) cv. Okitsu transitorily reduced early abscission rates (12–32 days after anthesis, DAA) delaying initially the process of natural fruitlet drop. The effects of girdling on growth, gibberellin (GA) and carbohydrate concentrations in developing ovaries and fruitlets were assessed during this initial growth stage (0–69 DAA). In girdled branches, abscission rate reduction was preceded by elevated concentrations of carbohydrate and GA in developing ovaries and fruitlets. Girdling at anthesis stimulated higher hexose (21 DAA) and starch (6–20 DAA) concentrations and also higher GA₁ (6 DAA), GA₁₉ (13–20 DAA) and GA₂₀ (6–20 DAA). The results established a relationship between the reduction of early abscission rates and higher concentrations of carbohydrates and GAs induced by girdling in developing fruitlets. These findings revealed that girdling certainly increased GA concentration and strongly suggested that its effect on early fruitlet abscission delay is likely mediated by both GA and carbohydrates.     

Pistil Starch Reserves at Anthesis Correlate with Final Flower Fate in Avocado (Persea americana)

A common observation in different plant species is a massive abscission of flowers and fruitlets even after adequate pollination, but little is known as to the reason for this drop. Previous research has shown the importance of nutritive reserves accumulated in the flower on fertilization success and initial fruit development but direct evidence has been elusive. Avocado (Persea americana) is an extreme case of a species with a very low fruit to flower ratio. In this work, the implications of starch content in the avocado flower on the subsequent fruit set are explored. Firstly, starch content in individual ovaries was analysed from two populations of flowers with a different fruit set capacity showing that the flowers from the population that resulted in a higher percentage of fruit set contained significantly more starch. Secondly, in a different set of flowers, the style of each flower was excised one day after pollination, once the pollen tubes had reached the base of the style, and individually fixed for starch content analysis under the microscope once the fate of its corresponding ovary (that remained in the tree) was known. A high variability in starch content in the style was found among flowers, with some flowers having starch content up to 1,000 times higher than others, and the flowers that successfully developed into fruits presented significantly higher starch content in the style at anthesis than those that abscised. The relationship between starch content in the ovary and the capacity of set of the flower together with the correlation found between the starch content in the style and the fate of the ovary support the hypothesis that the carbohydrate reserves accumulated in the flower at anthesis are related to subsequent abscission or retention of the developing fruit.     

Source/Sink relations of abscising and nonabscising soybean flowers

The partitioning of recently fixed ¹⁴C to setting and abscising flowers within the axillary raceme of `Clark' isoline E1t soybeans (Glycine max L. Merr) was examined as a function of time after anthesis of individual flowers. In such racemes, the first four flowers showed a 17% abscission while the next four flowers showed 47% abscission.

Source/sink relations of flowers I-IV (normally setting) were compared to those of flowers V-VIII (normally abscising) by pulse labeling source leaves with ¹⁴CO₂ and determining the radioactivity of individual flowers after a 4-hour chase period. The relative specific activity (RSA;% disintegrations per minute per% dry weight), sink strength (% disintegrations per minute), and its components, sink size (milligrams dry weight) and sink intensity (% disintegrations per milligram dry weight) were then calculated as a function of days after anthesis.

Sink intensity (i.e. the competitive ability to accumulate photoassimilate per unit mass) was very high prior to anthesis of both setting and abscising flowers. Sink intensity then became very low for the first 3 days following anthesis after which it recovered in normally setting flowers, but failed to recover in normally abscising flowers. It is concluded that soybean reproductive abscission is determined at or very near the day of anthesis.

     

Fruit Abscission in Cowpea, Vigna unguiculata (L.) Walp: I. DISTRIBUTION OF 14C-ASSIMILATES IN THE INFLORESCENCE, AND COMPARATIVE GROWTH OF OVARIES FROM PERSISTING AND ABSCISING OPEN FLOWERS

One of the hypotheses proposed for the abscission of flowersand fruits from the cowpea was that the oldest fruits at raceme1 on the peduncle monopolize the mobile nutrients availableto the whole inflorescence. Consequently, the distal young fruitsand flowers starve, abort, and are shed. This hypothesis wastested through autoradiographic studies of the distributionof ¹⁴C-assimilates on the cowpea inflorescence and the comparativegrowth of ovaries from persisting and abscising open flowersand young fruits. ¹⁴C-asaimilates were distributed to the fruits at raceme 1 andto flower buds, open flowers, and young fruits distal to raceme1. Therefore, the hypothesis that the fruits at raceme 1 monopolizethe mobile assimilates cannot be maintained. At the time of opening the persisting flowers of raceme 1 hadheavier ovaries than the aborting flowers of raceme 3 althoughboth types of flowers were visually alike. During the initial12 h following opening, the difference in ovary size was maintainedand often accentuated. The ovary of the persisting flower increasedin both fresh and dry weights. The growth of the ovary of theaborting flower was variable; the fresh weight often decreased,although it also increased in some experiments; the dry weightremained unchanged or decreased or increased remarkably. The availability of nutrients was considered to be of secondaryimportance in the chain of events leading to fruit abscission.Other internal factors which may control vital processes relatedto embryo development appear to be of primary, underlying importance.     

Ovary starch reserves and flower development in apricot (Prunus armeniaca)

In histerant species where flowering takes place prior to leaf emergence, a flower lifespan occurs in the absence of new photoassimilates and at the expense of pre-stored reserves either in the plant as a whole or in the flower itself. In the present study, the role that the photoassimilates stored in the flowers might play in flower development from anthesis to fertilization in Prunus armeniaca L. (apricot), a histerant species, was explored. Starch content in individual flowers was measured with the help of an image analysis system. Starch content decreased from its highest value at anthesis and disappeared from the ovary 9 days later. This decrease was inversely related to an increase in ovary size and in cell number in the pericarp, suggesting an intraflower, self-supported development. This process is conserved in both pollinated and nonpollinated flowers and therefore seems to be inherent to the flower at anthesis. The onset of fruiting is preceded by the establishment of large differences among ovaries; while some experience continuous growth, others stop growing and eventually drop. Interestingly, large differences in starch content are found among flowers at anthesis. These results are discussed in terms of the possible implications of pre-stored starch in the flower supporting initial flower development.     

Changes of Carbohydrates in Pepper (Capsicum annuum L.) Flowers in Relation to Their Abscission Under Different Shading Regimes

Abscission of pepper flowers is enhanced under conditions oflow light and high temperature. Our study shows that pepperflowers accumulate assimilates, particularly in the ovary, duringthe day time, and accumulate starch, which is then metabolizedin the subsequent dark period. With the exception of the petals,the ovary contains the highest total amounts of sugars and starch,compared with other flower parts and contains the highest totalactivity, as well as activity calculated on fresh mass basis,of sucrose synthase, in accordance with the role of this enzymein starch biosynthesis. Low light intensity or leaf removaldecreased sugar accumulation in the flower and subsequentlycaused flower abscission. The threshold of light intensity fordaily sugar accumulation in the sink leaves was much lower thanin flowers, resulting in higher daytime accumulation of sugarsin the sink leaves than in the adjacent flower buds under anylight intensity, suggesting a competition for assimilates betweenthese organs. Flowers of bell pepper cv. ‘Maor’and ‘899’ (sensitive to abscission) accumulatedless soluble sugars and starch under shade than the flowersof bell pepper cv. ‘Mazurka’ and of paprika cv.‘Lehava’ (less sensitive). The results suggest thatthe flower capacity to accumulate sugars and starch during theday is an important factor in determining flower retention andfruit set. Pepper; Capsicum annuum L.; abscission; shading; pepper flowers; ovary; leaves; sugars; starch; acid invertase; sucrose synthase     

Carbohydrate metabolism during early fruit development of sweet melon (Cucumis melo)

In sink tissues of cucurbits, including sweet melon fruits, the galactosyl-sucrose oligosaccharides, stachyose and raffinose, together with sucrose, are the major translocated carbohydrates. In the present study we investigated the carbohydrate metabolism of young melon ( Cucumis melo L. cv. C-8) fruit during the period of initial fruit set and development, from 3 days prior to anthesis until 20 days after anthesis (DAA), prior to the onset of sucrose accumulation. The enzymes assayed could be classified into two categories according to developmental patterns. Two of the enzymes, alkaline α -galactosidase I [EC 3.2.1.22], which hydrolyzes both raffinose and stachyose, and acid invertase [EC 3.2.1.26] either increased or remained stable during the first 10 DAA. The remaining measured enzymes (the stachyose-specific alkaline α -galactosidase form II, acid α -galactosidase, alkaline invertase, sucrose synthase [EC 2.4.1.13], galactokinase [EC 2.7.1.6], UDP-Gal PPase [EC 2.7.7.10], UDP-Glc-4 epimerase [EC 5.1.3.2], UDP-Glc PPase [EC 2.7.7.9], phosphoglucomutase [EC 5.4.2.2] and phosphoglucoisomerase [EC 5.3.1.9]) all showed a similar developmental pattern of steady decrease in activity following anthesis. We also compared the saccharide metabolism of pollinated and non-pollinated ovaries during the initial days following anthesis. In the absence of pollination, ovary growth dramatically decreased by the first DAA and was accompanied by a sharp decrease in the activity of UDP-Glc PPase. Other enzymes in the pathway, including the enzymes of stachyose and raffinose hydrolysis, did not decrease in activity until 2 or 4 DAA, after ovary growth was affected. These results provide information to assess the possible regulating enzymes in cucurbit ovary development and fruit set.     

Pollination Increases Gibberellin Levels in Developing Ovaries of Seeded Varieties of Citrus

Reproductive and vegetative tissues of the seeded Pineapple cultivars of sweet orange (Citrus sinensis L.) contained the following C-13 hydroxylated gibberellins (GAs): GA53, GA17, GA19, GA20, GA1, GA29, and GA8, as well as GA97, 3-epi-GA1, and several uncharacterized GAs. The inclusion of 3-epi-GA1 as an endogenous substance was based on measurements of the isomerization rates of previously added [2H2]GA1. Pollination enhanced amounts of GA19, GA20, GA29, and GA8 in developing ovaries. Levels of GA1 increased from 5.0 to 9.5 ng/g dry weight during anthesis and were reduced thereafter. The amount of GA in mature pollen was very low. Emasculation reduced GA levels and caused a rapid 100% ovary abscission. This effect was partially counteracted by either pollination or application of GA3. In pollinated ovaries, repeated paclobutrazol applications decreased the amount of GA and increased ovary abscission, although the pattern of continuous decline was different from the sudden abscission induced by emasculation. The above results indicate that, in citrus, pollination increases GA levels and reduces ovary abscission and that the presence of exogenous GA3 in unpollinated ovaries also suppresses abscission. Evidence is also presented that pollination and GAs do not, as is generally assumed, suppress ovary abscission through the reactivation of cell division.     

Ovary starch reserves and pistil development in avocado (Persea americana)

In avocado, only a very small fraction of the flowers are able to set fruit. Previous work in other woody perennial plant species has shown the importance of carbohydrates accumulated in the flower in the reproductive process. Thus, in order to explore the implications of the nutritive status of the flower in the reproductive process in avocado, the starch content in the pistil has been examined in individual pollinated and non‐pollinated flowers at anthesis and during the days following anthesis. Starch content in different pistilar tissues in each flower was quantified with the help of an image analysis system attached to a microscope. Flowers at anthesis were rich in highly compartmentalized starch. Although no external morphological differences could be observed among flowers, the starch content varied widely at flower opening. Starch content in the ovary is largely independent of flower size because these differences were not correlated with ovary size. Differences in the progress of starch accumulation within the ovule integuments between pollinated and non‐pollinated flowers occurred concomitantly with the triggering of the progamic phase. The results suggest that starch reserves in the ovary could play a significant role in the reproductive process in avocado.     

Changes in Abscisic Acid and Indoleacetic Acid before and after Anthesis Relative to Changes in Abscission Rates of Cotton Fruiting Forms

Cotton (Gossypium hirsutum L.) fruiting forms exhibit pronounced changes, with age, in their probability of abscission. Large floral buds rarely abscise, but after anthesis the young fruits (bolls) have a high probability of abscising. Abscission rate reaches a peak about 5 to 6 days after anthesis and then gradually decreases. An experiment was conducted to try to determine the reason for the rapid and pronounced increase in probability of abscission just after anthesis. Cotton was grown in the field and fruiting forms of various ages from 9 days before to 9 days after anthesis were all harvested the same day and subsequently analyzed for ABA and IAA. The concentration of ABA decreased slightly at anthesis and increased gradually thereafter. In contrast, the concentration of IAA was high before anthesis and then decreased at anthesis to about one-fifth the previous concentration. IAA remained low for at least 4 days after anthesis and then increased rapidly between 7 and 9 days after anthesis. The high concentration of IAA in floral buds before anthesis is probably a major factor in their resistance to abscission. Likewise, the low concentration of IAA at anthesis and for about 4 days thereafter may promote fruit abscission during the young boll stage.     

SECONDARY-FRUIT ONTOGENY IN NAVEL ORANGE

Ontogeny of the secondary fruit of navel orange (Citrus sinensis [L.] Osbeck) was studied using flower buds and fruit from groves in north central Florida. The secondary fruit, or navel, develops as a whorl of secondary-carpel primordia within the primary-carpel whorl when the flower bud is 1.5 to 2.0 mm in length. A complete secondary gynoecium with fused ovary but separate styles and stigmas develops within the primary gynoecium before anthesis. The stigma and style of secondary carpels are not as distinct as those of primary carpels. Three types of tissue protrusions from the secondary fruit into primary-fruit locules were abnormal placentae, free secondary carpels, and secondary-carpel outgrowths, the last being the most common. An abscission layer is present in the central axis of the primary fruit near the base of the secondary ovary. This layer is indistinguishable from adjacent tissues prior to commencement of secondary fruit abscission. Parenchyma cells in the abscission layer, however, contain large numbers of starch grains during abscission. Secondary and primary fruit have similar sigmoid growth patterns; however, onset of the cell-enlargement stage is approximately 2 wk later in secondary fruit.     

Spermidine and related-metabolic inhibitors modulate sugar and amino acid levels in Vitis vinifera L.: possible relationships with initial fruitlet abscission

The relationships between free polyamines (PAs), sugar and amino acid status were investigated in cuttings from two cultivars of Vitis vinifera L., Pinot noir (PN), a low abscising cultivar and Merlot (MRT), a high abscising one. In both cultivars free PAs decreased in inflorescences, but more drastically in MRT plants. Upon anthesis, this was associated with a decreased sugar content, especially sucrose, and an increase in total free amino acids. Thereafter, sucrose and amino acids showed opposite trends. In addition, darkening the PN plants at full flowering resulted in a dramatic decrease of PAs and sugars in inflorescences, but an increase in amino acid content, followed by high abscission. The concept that initial fruitlet abscission might be regulated by free PAs through changes in primary metabolites was hypothesized. Hence, the application of exogenous spermidine (Spd), but not putrescine (Put), prior to flowering markedly inhibits abscission. The Spd treatment also increased soluble sugar content but reduced amino acids in both leaves and inflorescences, while Put had no significant effect. By contrast, cyclohexylamine and beta-hydroxyethylhydrazine, as potent inhibitors of Spd synthase and PA-oxidases, respectively, exerted inverse effects on sugar, amino acid and abscission levels. Sucrose and free proline seemed to be highly sensitive to these treatments. This study suggests that Spd could regulate fruitlet abscission in grapevine by modulating, in a reverse way, the levels of sugars and amino acids in inflorescences.     

Effect of shading individual soybean reproductive structures on their abscisic Acid content, metabolism, and partitioning

Pod set in soybean is related to carbon partitioning and may be, at least partially, regulated by abscisic acid (ABA) concentrations. The studies reported here examine the relationship between carbon and ABA partitioning, reproductive abscission and ABA metabolism. The partitioning of radiolabeled ABA and photoassimilates from leaves to flowers and endogenous ABA concentrations were determined in shaded and unshaded reproductive structures. Aluminum foil was gently placed over individual soybean reproductive structures for 48 hours at 0, 4, 12, 17, and 22 days after anthesis (DAA). Shading of flowers at 12, 17, and 22 DAA resulted in significantly reduced concentration of ABA. However, shading had no effect on the catabolism of exogenously supplied [³H] ABA. The shading treatment on the first four of the five dates reduced partitioning of photoassimilates and ABA from the subtending leaf to the flower. Shading of reproductive structures also caused a significant reduction in the amount of assimilate exported from the subtending leaf, at 17 DAA. We conclude that shade-induced premature reproductive abscission in soybean is not stimulated by high levels of ABA within reproductive structures, but that ABA may inhibit abscission of reproductive structures by playing a role in preferential assimilate partitioning.     

Sucrose metabolism and IAA and ethylene production in muskmelon ovaries

Changes induced by the pollination of ovaries may be mediated by phytohormones and involve sudar-mediated by phytohormones and involve sugar-metabolizing enzymes. In order to further explore these relationships, soluble sugars, sucrose-phosphate synthase (EC 2.4.1.14), sucrose synthase (EC 2.4.1.13), acid and neutral invertases (EC 3.2.1.26), indole-3-acetic acid (IAA), and ethylene were investigated in muskmelon (Cucumis melo L.) ovaries sampled before, during, and after anthesis. The fresh weight of ovaries increased 100% within 48 h after pollination, but did not change significantly in the absence of pollination. While sugar content per ovary increased after pollination, sugar content per mg protein was unaffected. Sucrose was not detected in nonpollinated ovaries 48 h after anthesis. Free IAA content was highest in ovaries sampled 48h before anthesis. Pollination had no immediate effect on IAA content per mg protein in postanthesis ovaries. Although detected in all ovaries sampled, ethylene production increased significantly only in nonpollinated ovaries. Activity of sucrose-phosphate synthase was the same at all stages. The specific activities of sucrose synthase and the invertases were highest in nonpollinated ovaries. The increase in rate of sugar import into ovaries following pollination was not accompanied by an increase in the specific activity of any enzyme assayed, but was coincident with an increase in the total activity per ovary of surcose synthase and acid invertase. There appears to be no direct relationship between sucrose-metabolizing enzymes, IAA or ethylene in developing pollinated ovaries but the increase in sucrose cleavage activity in nonpollinated ovaries may be related to the increase in ethylene production.     

The Relationship between Sucrose Supply, Sucrose-cleaving Enzymes and Flower Abortion in Pepper

Abortion of pepper flowers depends on the light intensity perceivedby the plant and on the amounts of sucrose taken up by the flower(Aloni B, Karni L, Zaidman Z, Schaffer AA. 1996.Annals of Botany78: 163–168). We hypothesize that changes in the activityof sucrose-cleaving enzymes within the flower ovary might beresponsible for the changes in flower abortion under differentlight conditions. In the present study we report that the activityof sucrose synthase, but not of cytosolic acid invertase, increasesin flowers of pepper plants which were exposed, for 2 d, toincreasing photosynthetically active radiation (PAR) in therange of 85–400 µmol m^-2s^-1at midday. Sucrose synthaseactivity increased in parallel with the increasing concentrationsof starch in the flower ovary. Feeding flower explants, preparedfrom 3-d-predarkened plants, with 100 mM sucrose for 24 h, causeda 23% increase in reducing sugars and a 2.5-fold increase instarch concentration, compared with explants fed with buffer.Likewise, feeding explants of pepper flowers with sucrose, glucose,fructose and also mannitol increased the sucrose synthase activityin the ovaries. Concomitantly, sucrose, glucose and fructose,but not mannitol, reduced the abortion of flower explants. Itis suggested that sucrose entry into the flower increases theflower sink activity by inhibiting abscission and inducing metabolicchanges, thus enhancing flower set. Pepper; Capsicum annuum L.; abscission; light; pepper flowers; sucrose; glucose; fructose; starch; acid invertase; sucrose synthase     

The roles of abscisic acid and ethylene in the abscission and senescence of cocoa flowers

Cocoa flowers have a limited period of longevity; more than 90% of unpollinated flowers abscised within 32 h after anthesis. Abscisic acid (ABA) levels increased significantly prior to abscission. By 21 h after anthesis, ABA levels had increased almost 10-fold, and by 32 h flowers had 20-fold higher levels of ABA than at anthesis. Fluridone completely inhibited both the increase in ABA, the formation of an abscission zone, and the abscission and senescence of flowers. In contrast, ethylene production increased only slightly 21 h after anthesis and was only 2-fold higher after 32 h. Aminoethoxyvinylglycine (AVG) delayed but did not prevent abscission. In cocoa flowers, ABA is the primary regulator of abscission; ethylene accelerates abscission but only in the presence of ABA. Naphthalene acetic acid (NAA) treatment of flowers at anthesis prevented abscission zone formation and flower abscission, but did not induce fruit set. All parts of the NAA-treated flower except the pedicel senesced after 6 days. NAA+AVG treatment only delayed, whereas fluridone treatment completely prevented flower senescence.     

Sucrose metabolism and IAA and ethylene production in muskmelon ovaries

Changes induced by the pollination of ovaries may be mediated by phytohormones and involve sudar-mediated by phytohormones and involve sugar-metabolizing enzymes. In order to further explore these relationships, soluble sugars, sucrose-phosphate synthase (EC 2.4.1.14), sucrose synthase (EC 2.4.1.13), acid and neutral invertases (EC 3.2.1.26), indole-3-acetic acid (IAA), and ethylene were investigated in muskmelon (Cucumis melo L.) ovaries sampled before, during, and after anthesis. The fresh weight of ovaries increased 100% within 48 h after pollination, but did not change significantly in the absence of pollination. While sugar content per ovary increased after pollination, sugar content per mg protein was unaffected. Sucrose was not detected in nonpollinated ovaries 48 h after anthesis. Free IAA content was highest in ovaries sampled 48h before anthesis. Pollination had no immediate effect on IAA content per mg protein in postanthesis ovaries. Although detected in all ovaries sampled, ethylene production increased significantly only in nonpollinated ovaries. Activity of sucrose-phosphate synthase was the same at all stages. The specific activities of sucrose synthase and the invertases were highest in nonpollinated ovaries. The increase in rate of sugar import into ovaries following pollination was not accompanied by an increase in the specific activity of any enzyme assayed, but was coincident with an increase in the total activity per ovary of surcose synthase and acid invertase. There appears to be no direct relationship between sucrose-metabolizing enzymes, IAA or ethylene in developing pollinated ovaries but the increase in sucrose cleavage activity in nonpollinated ovaries may be related to the increase in ethylene production.Mention of a specfic product does not imply an endorsement by the United States Depertment of Agriculture or Texas Aricultural Experiment Station over other suitable products.     

Carbohydrate Availability in Relation to Fruitlet Abscission in Citrus

Abscission of flowers and fruitlets in the Washington navelorange (Citrus sinensis [L.] Osbeck) has been characterizedin relation to carbohydrate availability. A main wave of flowerabscission occurs shortly after anthesis while the carbohydratereserves in the tree are high. Fruitlet abscission starts approx.30 d after the commencement of flowering, while carbohydrates(mainly starch) are being accumulated in the leaves. Flowerand early fruitlet abscission are not caused by carbohydrateshortage. During late fruitlet abscission sucrose concentrationin the leaves falls to a low value demonstrating a limitationin supply and competition among the developing fruitlets forcarbohydrates. Concentrations of sucrose and reducing sugarsin the peel of the fruitlets also fall to low values, and arelationship could be demonstrated between these free sugarlevels and abscission. Ringing increases carbohydrate supplyto fruit and reduces late fruitlet abscission, but only hasa marginal effect on the growth of the fruitlets, which seemsless sensitive than abscission to carbohydrate shortage. Thelimitation of carbohydrate supply to the fruitlets occurs whilestarch levels in the leaves remain high. Slow mobilization ofstarch reserves may be one factor limiting set in Citrus. Copyright2001 Annals of Botany Company Carbohydrate supply, citrus, fruit growth and abscission, ringing, navel orange, starch, sugar metabolism     

Seed Development in a Shrunken Endosperm Barley Mutant

Seg8 is one of eight barley (Hordeum vulgare L.) mutants whoseendosperm development is affected by the maternal plant genotype.This study was initiated to determine the nature and onset ofabnormal development to provide a basis for further studiesaimed at understanding the mechanism of genetic control. Seeddevelopment and synthesis and accumulation of reserve substanceswere compared between seg8 and its normal counterpart, cv. Klages.Light microscopic examination showed that the mutant phenotypeappeared as early as 4 d after anthesis (DAA), and seg8 graindry weight was significantly lower than cv. Klages by 8 DAA.Grain cell number was significantly lower in seg8 by 8 DAA,indicating an early termination of cell division. The mutanthad a lower starch concentration and higher sucrose concentration,also evident at 8 DAA. Rates of [¹⁴C]sucrose incorporation intostarch in excised half seeds were similar in both genotypesat 2 and 4 DAA, but at 8 and 12 DAA seg8 had a lower rate. Totalprotein concentration was not significantly different betweenthe two genotypes throughout endosperm development. These resultsindicate that the mutation affects cell division and starchaccumulation prior to 8 DAA. It is not known if the reductionin starch biosynthesis and accumulation results from a reducedcapacity for starch or a defect in starch biosynthesis. Hordeum vulgare L., barley, shrunken endosperm mutant, endosperm development, starch, protein, endosperm cell number