Carbohydrate and ethylene levels related to fruitlet drop through abscission zone A in citrus

Citrus fruits have two abscission zones (AZ), named A (in the pedicel) and C (in the calyx). Early fruitlet abscission takes place exclusively through AZ A, while at June drop it is progressively inactivated and AZ C begins to operate. In previous work, it has been demonstrated that carbohydrate and ethylene regulate fruit drop through abscission zone C. In this paper, we have analysed the effect of these two factors in developing fruitlets of Satsuma mandarins (Citrus unshiu [Mak.] Marc.) cv. Okitsu to elucidate their involvement on abscission through AZ A. The data indicated that ACC content and ethylene production of fruitlets paralleled abscission rates. Sucrose supplementation increased fruit set, although did not counteract the abscising effect induced by ACC. Branch girdling of terminal fruitlets carrying several leaves significatively reduced ethylene production and abscission rates, and increased sugar content. Pedicel girdling showed the opposite. Taken together, the results revealed that the carbohydrate content may be a biochemical signal involved in the mechanisms controlling abscission through AZ A. The evidence also showed parallelisms between ethylene and its activation. As the induction of higher ethylene levels after the period of AZ A activity, however, was not able to promote fruit drop, it is also concluded that solely ethylene is not sufficient to activate abscission.     

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.     

Biosynthesis and Contents of Gibberellins in Seeded and Seedless Sweet Orange (<Emphasis Type="Italic">Citrus sinensis</Emphasis> L. Osbeck) Cultivars

In this work, we study the capacity to biosynthesize gibberellins (GA) of ovules (either fertilised or unfertilised), developing seeds and pericarp from fruitlets and their relation with fruit set capacity. Experiments were performed in adult, 12-year-old trees of seeded (Pineapple) and seedless parthenocarpic (Washington navel) sweet orange [Citrus sinensis L. Osbeck] cultivars. The activity of GA20-, GA3- and GA2-oxidases and gibberellin levels were measured in the ovules and pericarp of fruitlets in different development states. The results indicate that ovules are the main sites of gibberellin synthesis in fruitlets during the post-anthesis period. The most intense GA₁ synthesis—coincident with the highest expression of GA20ox2, GA3ox1 and GA2ox1—was detected in the ovules of the seeded cultivar, probably induced by fecundation and associated with low early fruitlet abscission rates. By contrast, the low activity detected in the sterile cultivar appears to be rather developmentally or constitutively regulated. As a fruitlet develops, the GA₁ concentration is augmented in the pericarp in comparison to ovules or developing seeds, and levels therein did not exhibit noticeable differences between varieties. Furthermore, developing seeds from pineapple had higher GA₁ content than the unfertilised abortive ovules from Washington navel. Taken together, data suggest a main role for this hormone in the control of fruitlet abscission, and also demonstrate a function in seed development.     

植物落花落果的分子机理研究进展

落花落果是花、果实、种子从母体脱落的一种普遍存在的自然现象。发生器官脱落的区域为离区(abscission zone,AZ)。离区分化形成离层,离层与脱落息息相关。离层的发育和功能行使是多酶、多激素、多基因参与调控的复杂而精确的过程。落花落果不仅是作物栽培和育种中的典型农艺性状,而且是植物器官脱落的主要形式之一。减少植物落花落果或控制某些植物适度落花落果,提高作物和果蔬类植物的产量和品质,是人类在作物驯化上努力的目标。该文基于前人对植物器官脱落的生理生化和分子生物学机制的研究,主要从植物落花落果的细胞学基础、生理生化机制、遗传学规律、分子生物学和相关基因定位、转录组分析方面阐述落花落果分子机理,重点从落花落果的分子生物学和相关基因定位两个方面进行剖析落花落果的作用机制,以便为作物遗传育种研究提供理论指导。     

Hormonal regulation of fruitlet abscission induced by carbohydrate shortage in citrus

 The hormonal signals controlling fruitlet abscission induced by sugar shortage in citrus were identified in Satsuma mandarin, Citrus unshiu (Mak.) Marc, cv. Clausellina and cv. Okitsu. Sugar supply, hormonal responses and fruitlet abscission were manipulated through full, partial or selective leaf removals at anthesis and thereafter. In developing fruitlets, defoliations reduced soluble sugars (up to 98%), but did not induce nitrogen and water deficiencies. Defoliation-induced abscission was preceded by rises (up to 20-fold) in the levels of abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC) in fruitlets. Applications to defoliated plants showed that ABA increased ACC levels (2-fold) and accelerated fruitlet abscission, whereas norflurazon and 2-aminoethoxyvinyl glycine reduced ACC (up to 65%) and fruitlet abscission (up to 40%). Only the full defoliation treatment reduced endogenous gibberellin A₁ (4-fold), whereas exogenous gibberellins had no effect on abscission. The data indicate that fruitlet abscission induced by carbon shortage in citrus is regulated by ABA and ACC originating in the fruits, while gibberellins are apparently implicated in the maintenance of growth. In this system, ABA may act as a sensor of the intensity of the nutrient shortage that modulates the levels of ACC and ethylene, the activator of abscission. This proposal identifies ABA and ACC as components of the self-regulatory mechanism that adjusts fruit load to carbon supply, and offers a physiological basis for the photoassimilate competition-induced abscission occurring under natural conditions. Received: 19 February 1999 / Accepted: 14 August 1999     

Anatomical Investigation of Abscissed Avocado Flowers and Fruitlets

The anatomy of abscissed avocado fruitlets of the Hass varietywas compared with that of actively growing control fruitletsto investigate the reason for abscission. Control fruitletshad a small embryo (4–14 cells) and numerous endospermcells by seven days after pollination. Cotyledon developmenthad commenced by 21 days and a root/shoot axis was present at28 days. Over 90 per cent of the fruitlets which abscissed during thefirst week after the end of flowering were unfertilized and18 per cent were abnormal. By the fourth week after the endof flowering all abscissed fruitlets were fertilized and noneabnormal. Abnormalities included ovaries with more than oneembryosac or ovule, with an immature embryo-sac, with an ovulein an abnormal position or with a deficiency in ovule structure.In a few fertilized abscissed fruitlets either the embryo orendosperm had ceased development first. In the majority of casesembryo and endosperm were anatomically normal. The majorityof fertilized abscissed fruitlets had reached a stage correspondingto 14 days after pollination of the control tree. Degenerationwas often observed in both unfertilized and fertilized abscissedfruitlets. It is suggested that this degeneration occurred inthe period between the cessation of development and abscissionof the fruit. This period was approximately one week. The proportion of abnormal ovaries was too low to significantlyaffect yields, and sufficient flowers had been fertilized togive an adequate crop. No anatomical reason for the high rateof fruitlet abscission was observed. Persea americana Mill, avocado, abscission, embryo-sac, embryo, endosperm     

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     

Is uronic acid oxidase involved in the hormonal regulation of abscission in explants of citrus leaves and fruits?

The role of uronic acid oxidase in abscission was studied in explants of citrus ( Citrus sinensis L. Osbeck; var. Shamouti) leaves and fruits. In leaf explants, activity of uronic acid oxidase prior to onset of abscission and the rate of abscission were markedly accelerated by ethylene and delayed by 2,4-dichlorophenoxyacetie acid. Similar results were obtained for uronic acid oxidase activity in the exocellular fraction of young fruit explants. In mature fruit explants, treated with ethylene, an immediate increase in activity was evidenty in the non-active shoot/peduncle abscission zone, whereas in the calyx abscission zone the rise in activity occurred after a prolonged exposure to ethylene, when most of the fruits had already abscised. Whenever ethylene enhanced uronic acid oxidase activity, 2,4-dichlorophenoxyacetic acid delayed it. A gradient of decreasing activity or uronic acid oxidase was recorded from both sides of the abscission zone in leaves and fruits toward the separation line, where activity was the lowest as compared with the activity found in adjacent tissues. It is suggested that uronic acid oxidase is involved in senescence and cell wall degradation. However, it is yet questionable whether this enzyme is directly related to the control mechanism of abscission.     

Dry matter accumulation in citrus fruit is not limited by transport capacity of the pedicel

The vascularization of the pedicel in Marisol clementine (Citrus clementina Hort. ex Tanaka) has been characterized in relation to fruit growth. Phloem and xylem formation occurred during the first half of the period of fruit growth. Phloem cross-sectional area reached its maximum value by the end of fruitlet abscission, 78 d after anthesis (DAA), shortly after the rate of accumulation of dry matter in fruitlets reached its maximum value. Secondary xylem formation occurred until day 93, well after the end of fruitlet abscission. At fruit maturity, xylem accounted for 42-46 % of the cross-section of the pedicel. Vessels differentiated in this late-formed xylem. Formation of phloem and early xylem was directly related to fruitlet size (and growth rate). Differences in the rate of formation of conductive tissues in the pedicel of the developing fruitlets followed rather than preceded the differences in growth rate. Specific mass transfer (SMT) in the phloem was highest in the fastest growing fruitlets, and peaked during the late stages of fruitlet abscission (72-78 DAA) and during the main period of fruit growth (107-121 DAA). Application of a synthetic auxin to developing fruits, either at the end of flowering (2,4-D) or by day 64 after flowering (2,4-DP), increased the growth rate of the fruit and fruit size at maturity (8-13 % increase in fruit diameter at maturity). These auxin applications also enhanced the formation of conductive tissues in the pedicel, with a specific effect on phloem formation. Applying auxin at flowering resulted in a reduction in the phloem SMT by days 72-78, whereas auxin application on day 64 increased this parameter. Despite this difference in behaviour, which resulted from the different time-course of the growth response of the fruit to auxin applications, these applications increased fruit size to a similar extent. Severing 37 % of the phloem of the pedicel during the main period of fruit growth resulted in an increase in the specific mass transfer in the phloem but had no influence on fruit growth. These observations demonstrate that the transport capacity in the phloem of the pedicel does not limit fruit growth and, within the limits of our experiments, an increase in demand by the fruit appeared to be matched by an increase in SMT. The dependence of late xylem formation (after the period of fruitlet abscission) on fruitlet growth was demonstrated in Salustiana orange [Citrus sinensis (L.) Osbeck] by means of controlling fruit growth through the manipulation of leaf area. Fruit growth at this time was more closely related to leaf area than to carbohydrate levels, suggesting that it may be limited by current photosynthesis.     

The free movement of14C-labelled organic compounds from intact apple seeds to growing fruitlets and shoots

Indole-3-acetic acid (IAA-1¹⁴C, IAA-2¹⁴C) and gamma-aminobutyric acid labelled with¹⁴C were applied in lanoline to the surface of intact seeds or inserted into seeds of growing apple fruitlets or to the cut surface of the pericarp. Their translocation in trees was checked by means of autoradiography or by a low background Geiger-Müller counter. Auxin applied to the top of intact seeds, or inserted into the seeds was translocated and distributed within the pericarp, spur tissues and shoots below. The translocation of gamma-aminobutyric acid from seeds suggests that the capacity of apple seeds for extensive interchange with the surrounding tissues of the fruit and the spur is more general than suspected. A much poorer or no translocation to the spur was found when radioactive compounds were applied to the cut surface of the pericarp, while the seeds were left untouched. The results suggest two mechanisms of metabolite translocation in the apple fruit: two-directional for seeds and one-directional for the pericarp.     

Expression of MdCAS1 and MdCAS2, encoding apple beta-cyanoalanine synthase homologs, is concomitantly induced during ripening and implicates MdCASs in the possible role of the cyanide detoxification in Fuji apple (Malus domestica Borkh.) fruits

Fruit ripening involves complex biochemical and physiological changes. Ethylene is an essential hormone for the ripening of climacteric fruits. In the process of ethylene biosynthesis, cyanide (HCN), an extremely toxic compound, is produced as a co-product. Thus, most cyanide produced during fruit ripening should be detoxified rapidly by fruit cells. In higher plants, the key enzyme involved in the detoxification of HCN is β-cyanoalanine synthase (β-CAS). As little is known about the molecular function of β-CAS genes in climacteric fruits, we identified two homologous genes, MdCAS1 and MdCAS2, encoding Fuji apple β-CAS homologs. The structural features of the predicted polypeptides as well as an in vitro enzyme activity assay with bacterially expressed recombinant proteins indicated that MdCAS1 and MdCAS2 may indeed function as β-CAS isozymes in apple fruits. RNA gel-blot studies revealed that both MdCAS1 and MdCAS2 mRNAs were coordinately induced during the ripening process of apple fruits in an expression pattern comparable with that of ACC oxidase and ethylene production. The MdCAS genes were also activated effectively by exogenous ethylene treatment and mechanical wounding. Thus, it seems like that, in ripening apple fruits, expression of MdCAS1 and MdCAS2 genes is intimately correlated with a climacteric ethylene production and ACC oxidase activity. In addition, β-CAS enzyme activity was also enhanced as the fruit ripened, although this increase was not as dramatic as the mRNA induction pattern. Overall, these results suggest that MdCAS may play a role in cyanide detoxification in ripening apple fruits.