Fruitlet Abscission Research Articles

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.

Girdling increases carbohydrate availability and fruit-set in citrus cultivars irrespective of parthenocarpic ability

SummaryThe effects of girdling performed at various dates were evaluated during two consecutive years in high- and low-bearing commercial orchards of ‘Fortune’ mandarin and ‘Clausellina’ Satsuma mandarin. The time-dependent response was evaluated through fruitlet abscission, final fruit-set and yield as related to carbohydrate contents in developing fruitlets. A few days after treatment, girdling increased the soluble sugars content (SSC) in fruitlets, reduced the daily fruit drop, and thereby diminished abscission. Application of girdling to low-bearing ‘Fortune’ mandarin orchards was most effective 15 d before anthesis (DBA) and 35 d after anthesis (DAA). It increased yield by 125%. In high-bearing orchards, the best results were achieved by girdling 35 DAA, which increased yield by 28%. In both orchards, the effect of girdling declined gradually from 35 DAA to the end of fruit drop. SSC values were found to be positively correlated with fruit-set at 25 DAA, but this tendency was reversed by 50 DAA. This study showed that the effects of girdling are time-dependent, and that yield can be improved in cultivars of different parthenocarpic ability.

FRUIT ABSCISSION AS RELATED TO FRUIT QUALITY

Many fruit species bear an abundance of flowers which produce a surplus of fruits that the tree is unable to support. In anticipation of this, the major fruit species developed an immature fruit (fruitlet) physiological drop as a self regulatory mechanism. From a horticultural point of view, this self regulating mechanism may be too strong or entirely insufficient. In order to overcome these shortcomings bioregulators are currently used. A more precise understanding of the molecular mechanism underlying natural fruitlet abscission would help in setting up more efficient control strategies. In this context apple fruitlets were assumed as study model. Fruitlet abscission is a highly coordinated event. It involves multiple changes leading to cell separation occurring in specific tissues designated as abscission zones (AZ). Ethylene and IAA regulate abscission. The general interaction between the two hormones is manifested in their antagonistic relationship, where the IAA status of the tissues controls their sensitivity to ethylene.[...]

Environmental Effects on the Performance of Foliar Applied Plant Growth Regulators: A Review Focusing on Tree Fruits

Plant response to foliar application of plant growth regulators (PGRs) is often variable, in part due to environmental factors. Weather prior to application is thought to influence cuticle development and thus PGR uptake. For example, in growth chamber studies foliar uptake of 1-naphthaleneacetic acid (NAA) is sometimes increased when fruit trees are placed in low temperature and high humidity several weeks prior to application. Environmental conditions over an extended period of time after application may influence PGR conversion to active form (e.g., ethephon), PGR metabolism, or metabolic factors that affect PGR activity in the plant. The effects of environmental conditions on PGR uptake have been investigated extensively in laboratory studies. In many cases, uptake is clearly increased by high temperatures immediately after application. Laboratory studies report a linear positive correlation between temperature and uptake and greater temperature response above 25 °C (77.0 °F). High humidity and longer drying time often are also reported to increase PGR uptake in laboratory studies. These results are consistent with many grower observations on effects of weather on chemical thinning and have been incorporated into many product labels and extension recommendations. However, relatively few field experiments have been reported in which the relationship between PGR response and environmental conditions were assessed. Wash-off studies have demonstrated that rain shortly after application may reduce efficacy of NAA. Several studies demonstrate environmental interaction with metabolic activity involved in PGR action. For example, shading after thinner application is reported to increase fruitlet abscission and enhance effectiveness of some thinning agents. Chemical thinning of apples (Malus ×domestica) with ethephon is reported to correlate strongly with temperature in the days after application, while studies suggest that higher temperatures after aminoethoxyvinylglycine (AVG) application may reduce control of preharvest drop. However, the stage of fruitlet development at apple thinning often appears to be more important than environmental conditions at the time of PGR application. In addition, field experiments indicate that longer drying times at lower temperatures seem to largely compensate for greater uptake rates at higher temperatures. This paper discusses data from published and previously unpublished experiments in order to understand the effects of environment on PGR response variability.

The role of polar auxin transport through pedicels of Prunus avium L. in relation to fruit development and retention.

It was investigated whether premature fruit abscission in Prunus avium L. was triggered by a reduction in polar auxin transport (PAT). The capacity of pedicels to transport tritiated IAA ([3H]-IAA) via the PAT pathway was measured at intervals throughout flower and fruit development. The extent of passive diffusion, assessed by concurrent applications of [14C]-benzoic acid ([14C]-BA), was negligible. Transported radioactivity recovered from agar blocks eluted at the same retention time as authentic [3H]-IAA during HPLC fractionation. The capacity for PAT was already high 7 d before anthesis and increased further following the fertilization of flowers at anthesis. PAT intensity was greatest immediately following fertilization and at the beginning of the cell expansion phase of fruit growth; the transport intensity in fruitlets destined to abscind was negligible. The amount of endogenous IAA moving through the PAT pathway was greatest during the first 3 weeks after fertilization and was again high at the beginning of the fruit expansion stage. IAA export in the phloem increased following fertilization then declined below detectable levels. ABA export in the phloem increased markedly during stone formation and at the onset of fruit expansion. TIBA applied to pedicels of fruit in situ promoted fruitlet abscission in 2000 but not in 2001, despite PAT capacity being reduced by over 98% in the treated pedicels. The application of TIBA to pedicels did not affect fruit expansion. The role of PAT and IAA in relation to the development and retention of Prunus avium fruit is discussed.

Evaluation of Systems for Timing of Fungicide Sprays for Control of Postbloom Fruit Drop of Citrus in Brazil.

Postbloom fruit drop (PFD) of citrus, caused by Colletotrichum acutatum, infects petals of citrus flowers and produces orange-brown lesions that induce the abscission of young fruitlets and the retention of calyces. Proper timing of fungicide applications is essential for good disease control. Different systems for timing of fungicide applications for control of PFD in a major citrus-growing region in southern São Paulo state in Brazil were evaluated from 1999 to 2002. The following programs were compared to an unsprayed control using counts of diseased flowers, persistent calyces, or fruit: (i) a phenology-based program currently recommended in Brazil with one application at early and another at peak bloom; (ii) the Florida PFD model; (iii) the post-bloom fruit drop-fungicide application decision system (PFD-FAD), a new computer-assisted decision method; and (iv) grower's choice. In 1999, no disease developed, sprays applied with the phenology-based program had no effect, and the Florida PFD model saved two sprays compared with the phenology-based program. In 2000, PFD was moderate and the phenology-based and grower's choice treatments had a significantly lower number of persistent calyces and higher fruit numbers than the control, but no differences were found between those treatments and the PFD model. In 2001, PFD was severe with considerable yield loss. The PFD model, the phenology-based program, and the grower's choice reduced flower blight and the number of persistent calyces, and improved fruit yields with two to three applications, but the PFD-FAD achieved comparable yields with only one spray. In 2002, the disease was mild, with no yield loss, and the Florida PFD model and the PFD-FAD saved one spray compared with the other systems. The PFD model and the PFD-FAD were equally effective for timing fungicide applications to control PFD in Brazil. Scouting of trees is simpler with PFD-FAD; therefore, this system is recommended and should eliminate unnecessary sprays and reduce costs for growers.

THINNING OF 'JONAGOLD' AND 'ELSTAR' APPLES WITH THE COMBINATION OF ETHEPHON AND CPPU

The combined spraying of ethephon with synthetic cytokinin forchlorfenuron (CPPU) was investigated on seven-year-old ‘Jonagold’ and ‘Elstar’ apple trees / M.9. The application of ethephon a few times after petal fall (PF) at low concentration was aimed to enhance return bloom without affecting fruitlet abscission. The problems of small fruit in high cropping trees should be overcome by spraying of CPPU since its application in general increased fruit size. Successive applications of ethephon at 30 and 50 ppm, alone or in combination with CPPU 5 ppm, were done after PF at weekly intervals. The single application of CPPU at 5 ppm at PF resulted in strong thinning response of ‘Jonagold’ and non significant thinning of ‘Elstar’ apple trees. The yield of fruit from bigger size class was enhanced on both cultivars after CPPU 5 ppm single application. Postbloom ethephon at 200 ppm alone spraying (PF) and/or five weekly applications of ethephon at 30 ppm or 50 ppm (starting at PF) thinned ‘Jonagold’ trees but no significant additional fruit growth was observed. Three weekly applications of ethephon at 30 ppm or 50 ppm (starting at PF) did not affect fruit set or fruit growth of ‘Jonagold’ trees. Moderate thinning of fruitlets was seen on ‘Elstar’ with all ethephon alone treatments (200 ppm, 3x30 ppm, 5x30 ppm, 3x50 ppm, 5x50ppm, weekly intervals starting at PF) but the average fruit weight was not enhanced by treatment. When 50 ppm ethephon was sprayed at five weekly intervals starting at 18 mm fruitlet diameter of ‘Jonagold’ and ‘Elstar’ trees, a weak thinning and no better fruit growth occurred. The combination spraying of ethephon 3x30 ppm + CPPU 5 ppm or ethephon 5x50 ppm + CPPU 2x5 ppm (at weekly intervals starting at PF) resulted in strong thinning response and in enhanced fruit growth for both cultivars. Return bloom was increased significantly and satisfactorily on well thinned ‘Jonagold’ trees, while on ‘Elstar’ no treatment affected better return bloom even in the case of pronounced thinning.

Mobilisation and distribution of starch and total N in two grapevine cultivars differing in their susceptibility to shedding.

As a part of a project aimed at elucidating the causal relationship between reserve mobilisation and the extent of shedding in Vitis vinifera L., we compared storage and fate of carbon (C) and nitrogen (N) reserves in two varieties differing in their susceptibility to fruitlet abscission. Merlot (susceptible) and Pinot Noir (P. Noir, not susceptible) vines were grown in trenches under semi-controlled conditions over a 3-y period after planting. Mobilisation of stored C and N, distribution of reserve materials within the vines and 15N uptake were followed particularly during the spring growth flush and floral development in the third year. At dormancy, starch levels in the perennial tissues (roots, trunk, canes) were higher in Merlot than in P. Noir. During the spring growth flush, starch level decreased markedly in the roots of both cultivars until early bloom. At that time, starch started to accumulate in P. Noir but not in Merlot. Similar variations were found with total N. Accordingly, 15N analysis showed that translocation of storage N to the annual tissues was nearly achieved at early bloom in P. Noir while it continued until pea berry size in Merlot. In parallel, N uptake increased during the spring growth flush, and it was higher in P. Noir than in Merlot. These results indicate that transition between heterotrophic (root) and autotrophic (leaf) mode of nutrient allocation towards the developing inflorescences occurs earlier in P. Noir. Possible consequences are discussed in relation to the susceptibility of each cultivar to shedding.

Fruit set dependence on carbohydrate availability in citrus trees

We generated source-sink imbalances by defoliation and sucrose supplementation by stem injection, to investigate effects of carbohydrate availability on fruitlet growth and abscission in cv. Okitsu of Satsuma mandarins (Citrus unshiu (Mak.) Marc.). Partial defoliation promoted fruitlet abscission, whereas sucrose supplementation increased citrus fruit set by more than 10%. Moreover, when applied together, sucrose supplementation counteracted the effect of partial defoliation on fruit set. When sucrose was supplied continuously from flowering until harvest, it increased the concentrations of soluble and insoluble sugars in fruits. We conclude that fruit set in citrus is highly dependent on carbohydrate availability.

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.

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

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

Abscission of mango fruitlets as influenced by biosynthesis of polyamines

SummaryThe role of polyamines in fruitlet abscission of mango (Magnifiera indica L.) was studied by estimating the endogenous polyamines in intact and about-to-abscind fruitlets and their pedicels, and the effect of exogenous applications of polyamines and inhibitors of polyamine biosynthesis. Endogenous free polyamines (putrescine, spermidine and spermine) were estimated in intact and about-to-abscind fruitlets and their pedicels at three developmental stages in ‘Kensington Pride’ and ‘Glen’ mango. Intact fruitlets and their pedicels exhibited 276% and 341% higher total free polyamines than about-to-abscind ones in ‘Kensington Pride’, whilst in ‘Glen’; the increase was 137% and 63%, respectively. Mean spermine concentrations were higher in intact tissues of ‘Kensington Pride’ (fruitlets 220 nmol g–1 FW, pedicels 172 nmol g–1 FW), while spermidine concentrations were greater in ‘Glen’ (fruitlets 244.5 nmol g–1 FW, pedicels 256 nmol g–1 FW). The role of spermine seems to be critical in mango fruitlet abscission as the about-to-abscind fruitlets had very low levels of spermine (10.4% and 19% of intact fruitlets) in ‘Kensington Pride’ and ‘Glen’, respectively. Exogenous applications of polyamines reduced fruitlet abscission during the initial heavy fruitlet abscission period (two weeks after fruit set) and the response was significantly influenced by type, concentration and time of application. Spermidine (0.1 mM) was the most effective of the three polyamines applied at the fruit-set stage, reducing fruitlet abscission by 6.8% compared with the control. Of the four times of application, spermine (0.1 mM) sprayed when the panicle had grown 5–8 cm showed least fruitlets abscission (91.5%) compared with controls (95.5%) after six weeks of fruit set. Higher levels of endogenous polyamines in intact fruitlets and their pedicels, compared with about-to-abscind ones, and the reduction of fruitlet abscission with exogenous application of polyamines, as well as the acceleration with inhibitors of polyamines, demonstrate the key role of polyamines biosynthesis in mango fruitlet abscission.

Fruitlet abscission of mango in relation to ethylene

SummaryThe role of ethylene in mango (Mangifera indica L.) fruitlet abscission was investigated by determining endogenous ethylene concentrations in intact and about-to-abscise fruitlets and their pedicels, and the effects of exogenous applications of ethephon and inhibitors of ethylene biosynthesis. About-to-abscise fruitlets and pedicels at pinhead, pea and marble stages had higher endogenous ethylene production than intact tissues in ‘Kensington Pride’ and ‘Glen’ (191% and 66% more ethylene in ‘Kensington Pride’ in 2001, 53.5% and 101% in ‘Glen’ in 2000, and 135% and 93% in 2001). Exogenous application of ethephon at fruit set, accelerated fruitlet abscission compared with control. Fruitlet abscission was increased with increased rates of ethephon applied during 1999 and 2000. Exogenous spray of ethylene inhibitors at pre-anthesis stage increased fruitlet retention. Spray of STS or CoSO4 (200.mg l–1) resulted in the highest fruitlet retention (7.7 and 7.6 fruit per panicle) compared with control (4.9 fruit per panicle). These results suggested that ethylene is involved in mango fruitlet abscission.

Characterization of two putative ethylene receptor genes expressed during peach fruit development and abscission.

Two peach genes homologous to the Arabidopsis ethylene receptor genes ETR1 and ERS1, named Pp-ETR1 and Pp-ERS1 respectively, have been isolated and characterized. Pp-ETR1 and Pp-ERS1 are conserved in terms of exon numbers and intron positions, although the first and fifth introns of Pp-ETR1 have an unusual length. In addition, two putative polyadenylation sites, that may cause an incomplete splicing at the 3' terminus, are present in the fifth intron. A motif of 28 nt, which shows high homology with ethylene responsive elements found in promoters of genes up-regulated by ethylene, is present in the promoter region of Pp-ERS1. Expression analysis, carried out by quantitative RT-PCR, was performed during fruit development and ripening, and leaf and fruitlet abscission. The level of Pp-ETR1 transcripts remained unchanged in all the tissues and developmental stages examined, whereas Pp-ERS1 mRNA abundance increased in ripening mesocarp, in leaf and fruitlet activated abscission zones, and following propylene application. 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, did not affect Pp-ETR1 transcription, while it down-regulated Pp-ERS1. A rise in ethylene evolution, accompanied by an increase of Pp-ERS1 transcript accumulation occurred within 24 h from the end of 1-MCP treatment. These results indicate that Pp-ERS1 might play a role in abscission and ripening.

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.

Characterization of a major latex protein (MLP) gene down-regulated by ethylene during peach fruitlet abscission

We report the isolation of a new peach gene, Pp-MLP1, that shows significant similarity to a family of fruit- and flower-specific genes, designated as major latex protein (MLP) homologues. Transcript of Pp-MLP1 highly accumulated in cells of fruit pedicel, similar to lacticifers, adjacent to the abscission zone (non-abscission zone) and, to a lesser extent, in epicotyls, stems and roots, while no accumulation was detected in leaves. In contrast to the MLP homologues isolated so far, the Pp-MLP1 transcript was detected during fruit cells expansion, though its expression appeared unrelated to fruit ripening. Propylene treatment caused a decrease in mRNA accumulation of Pp-MLP1 in all tested tissues. The function of Pp-MLP1, as with all previously described MLP homologues, is unknown. MLPs are associated with fruit and flower development in addition to plant pathogenesis responses. Expression in tissues associated with abscission would be consistent with a role in implementing this aspect of floral development or possibly protective responses to plant pathogens which may infect post-abscission wounds. In addition, the high similarity between proteins encoding by Pp-MLP1 and Csf2, an MLP gene associated with the early development of cucumber fruit, could suggest an alternative developmental role such as cell and tissue expansion.

Activity of Benomyl for Control of Postbloom Fruit Drop of Citrus Caused by Colletotrichum acutatum.

Postbloom fruit drop (PFD) of citrus caused by Colletotrichum acutatum produces orange-brown lesions on petals and induces the abscission of young fruitlets and the retention of the calyces. Despite the fact that C. acutatum is not highly sensitive to benomyl in culture, this fungicide provides good control of the disease under field conditions. This study was undertaken to determine the effect of benomyl on various stages of disease development to understand the basis for its effectiveness in the field. We found that benomyl at 1.0 μg/ml reduced colony area of C. acutatum by about 75% and completely inhibited growth of C. gloeosporioides. Benomyl did not prevent conidial germination even at 100 μg/ml, but reduced germ tube elongation at 10 and 100 μg/ml. When benomyl was applied to flower clusters on screenhouse-grown plants before inoculation, disease severity was greatly reduced. Applications at 24 and 48 h, but not at 72 h, after inoculation reduced PFD severity. Application of benomyl to symptomatic petals not bearing conidia did not prevent or reduce production of inoculum. Application to petals bearing conidia reduced viability of these fungal propagules by only about 50%. The viability of appressoria on mature leaves was not affected by benomyl application. Even when appressoria on mature leaves were stimulated to germinate by treatment with flower extracts, subsequent application of benomyl did not reduce propagule numbers below original levels. Benomyl appears to act by preventing infection and early development of the fungus in petals. However, once symptoms have developed, this fungicide has only minimal effects on further disease development and spread.

The effects of phloem girdling on the abscission of Prunus avium L. fruits

SummaryExcessive premature abscission of developing fruitlets in UK cherry orchards often results in low fruit yields. An improvement in our understanding of the underlying causes of embryo abortion and fruitlet abscission will help rationalize effective remedies to this problem. The aim of this study was to determine the effects of limiting the availability of leaf-derived assimilates, during critical stages of fruitlet development, on the severity of fruitlet abscission. Experimentally, this was achieved by isolating individual “spur units” (short shoots (<10 cm) with leaves and fruit) from the tree by girdling (severing the phloem connections) branches on either side of the unit. In this way, the developing fruitlets within the associated spur would be able to derive their assimilates only from the associated spur leaves. Spur units with different total leaf areas and variable numbers of developing fruitlets were chosen to achieve a wide range of potential source and sink strengths. The spurs analysed varied in leaf number from 4–9 leaves as spur leaf area increased. The largest variability in the spur leaf area number relationship occurred in spurs with 6–7 leaves. When initially determining the total leaf area per spur in May, there was no obvious relationship with fruit number per spur. Subsequent analysis of the relationship between spur leaf area and fruit number per cluster showed that fruit had been lost from spurs with the smallest leaf areas. Spurs girdled later in the season in June also showed no obvious relationship between spur leaf area and fruit number. As with spurs girdled in May, those manipulated in June lost fruit from spurs with small leaf areas. By July, there was a positive curvilinear relationship between spur leaf area and fruit number for girdled spurs. Neither total nor average fruit fresh weight per spur, at harvest, could be related to spur leaf area. The average individual fresh weight of fruit in a spur was, however, limited by the number of fruit within that spur. When spurs were girdled, fruit loss was shown to take place preferentially where the spur leaf area per fruit was low. From this analysis, it was possible to predict which girdled spurs would lose fruit, using the calculated ratio of spur leaf area per fruit. It is concluded that fruit retention, not size, appears to be limited by the availability of leaf-derived assimilates.

Involvement of polyamines in the control of fruitlet physiological abscission in grapevine (Vitis vinifera)

The potential contribution of polyamines (PAs) in the regulation of physiologically induced fruitlet abscission was investigated in cuttings from two cultivars of Vitis vinifera L., Pinot noir (PN) and Merlot (MRT). Abscission was higher in MRT than in PN and was preceded by a decrease in free PA levels. This decline was more pronounced in inflorescences than in leaves of the sensitive cultivar. Soluble conjugated PA showed an opposite trend in both cultivars. This suggests a cause‐effect relationship between free and/or conjugated PA levels in floral organs and susceptibility to abscission. Spermidine (Spd), but not putrescine (Put) or diaminopropane, supplied at 0.5–1 mM to the nutritive medium prior to the anthesis, increased free and conjugated PA levels in the inflorescences and markedly inhibited abscission. α‐Difluoromethylarginine, an inhibitor of arginine decarboxylase, but not α‐difluoromethylornithine, an inhibitor of ornithine decarboxylase, lowered PA levels and increased abscission. Treatment with cyclohexylamine or β‐hydroxyethylhydrazine as potent inhibitors of Spd synthase and PA oxidases, respectively, reduced the Spd and/or spermine levels and enhanced free Put in the inflorescences, inducing an increased abscission of floral organs shortly after anthesis. These data suggest that PAs, particularly Spd, could be involved in the regulation of grapevine fruitlet physiological abscission.

Carbohydrate Availability in Relation to Fruitlet Abscission in Citrus

Abscission of flowers and fruitlets in the Washington navel orange (Citrus sinensis [L.] Osbeck) has been characterized in relation to carbohydrate availability. A main wave of flower abscission occurs shortly after anthesis while the carbohydrate reserves in the tree are high. Fruitlet abscission starts approx. 30d after the commencement of flowering, while carbohydrates (mainly starch) are being accumulated in the leaves. Flower and early fruitlet abscission are not caused by carbohydrate shortage. During late fruitlet abscission sucrose concentration in the leaves falls to a low value demonstrating a limitation in supply and competition among the developing fruitlets for carbohydrates. Concentrations of sucrose and reducing sugars in the peel of the fruitlets also fall to low values, and a relationship could be demonstrated between these free sugar levels and abscission. Ringing increases carbohydrate supply to fruit and reduces late fruitlet abscission, but only has a marginal effect on the growth of the fruitlets, which seems less sensitive than abscission to carbohydrate shortage. The limitation of carbohydrate supply to the fruitlets occurs while starch levels in the leaves remain high. Slow mobilization of starch reserves may be one factor limiting set in Citrus.