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     

Synthetic Auxin 3,5,6-TPA Provokes <Emphasis Type="Italic">Citrus clementina</Emphasis> (Hort. <Emphasis Type="Italic">ex</Emphasis> Tan) Fruitlet Abscission by Reducing Photosynthate Availability

The aim of this study was to determine the effects of the synthetic auxin 3,5,6-trichloro-2-pirydiloxyacetic acid (3,5,6-TPA) on photosynthetic activity, photosynthate transport to the fruit, and fruitlet abscission to further explain the physiological basis of auxin-mediated citrus fruit thinning. Applying 15 mg l⁻¹ 3,5,6-TPA to trees during the fruit cell division stage significantly increased fruitlet abscission of Clementine mandarin. On treated trees, abnormal foliar development and photosynthetic damage were observed at the same time as 3,5,6-TPA reduced fruitlet growth rate. Briefly, treatment reduced chlorophyll and carotenoid concentrations and modified chlorophyll a fluorescence parameters, that is, reduced the quantum yield (ФPSII) of the noncyclic electron transport rate, diminished the capacity to reduce the quinone pool (photochemical quenching; q_p), and increased nonphotochemical quenching (q _N), thereby preventing the dissipation of excess excitation energy. In addition, the net photosynthetic flux (μmol CO₂ m⁻² s⁻¹) and leaf photosynthate content decreased in treated trees. As a result, the 3,5,6-TPA treatment significantly reduced the photosynthate accumulation in fruit from day 3 to day 8 after treatment, thus reducing fruitlet growth rate. Hence, treated fruitlets significantly increased ethylene production and abscised. Twenty days after treatment, chlorophyll a fluorescence parameters and fruitlet growth rate were reestablished. Accordingly, the thinning effect of 3,5,6-TPA may be due to a temporarily induced photosynthetic disorder that leads to reduction in photosynthate production and fruitlet uptake that temporarily slows its growth, triggering ethylene production and fruitlet abscission. Afterward, the remaining treated fruit overcame this effect, increased growth rate, and reached a larger size than control fruit.     

Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L. : II. Pathway and Turgor Sensitivity

Phloem import and unloading in perfused bean (Phaseolus vulgaris L.) seed coats were investigated using steady-state labeling. Though photosynthate import and unloading were significantly reduced by perfusion, measurements of photosynthate fluxes in perfused seed coats proved useful for the study of unloading mechanisms in vivo. Phloem import was stimulated by lowered seed coat cell turgor, as demonstrated by an increase in tracer and sucrose import to seed coats perfused with high concentrations of an osmoticum. The partitioning of photosynthates between retention in the seed coat and release to the perfusion solution also was turgor sensitive; increases in seed coat cell turgor stimulated photosynthate release to the apoplast at the expense of photosynthate retention within the seed coat. There was no evidence of a turgor-sensitive sucrose uptake mechanism in perfused seed coats. Thus, the turgor sensitivity of photosynthate partitioning within perfused seed coats was consistent with a turgor-sensitive efflux control mechanism. Measurements of tracer equilibration and sugar partitioning in perfused seed coats provided strong evidence for symplastic phloem unloading in seed coats.     

The Growth of Apple Fruitlets, and the Effect of Early Thinning on Fruit Development: With fourteen Figures in the Text

The development of fruitlets of the apple varieties Cox's OrangePippin and Miller's Seedling was investigated with particularreference to rate and duration of cortical cell division andexpansion. The fruitlet weight increases slowly for the first few daysafter pollination, then exponentially at a rapid rate for about3 weeks, and then at a declining rate until harvest. The rate of cell division is very rapid during the exponentialphase of fruitlet growth, and then the rate declines, but divisioncontinues until about 12 weeks after full bloom in Miller'sSeedling and for at least 6 or 7 weeks after pollination inCox's Orange Pippin. The cells expand exponentially until about 7 weeks after pollination,then continue to expand at a diminishing rate until harvest. Heavy pre-blossom thinning increased fruit size and weight,affecting the rate of both cell division and cell expansion;the differences in fruit weight showed by the 4th week and incortical cell number and cell diameter by the 5th week afterfull bloom.     

Regulators of Cell Division in Plant Tissues VUI. The Cytokinins of the Apple Fruit

The cytokinin activities of extracts of organs developed from the apple fruit bud were compared using the carrot phloem bioassay, and the identity of the cytokinins in the apple fruitlet was investigated. The activity of apple fruitlet extracts was slightly greater than the activity of pedicel extracts, and considerably greater than the activities of extracts of other organs. Extracts of the developing seeds of fruitlets were much more active than extracts of fruitlet flesh. Apple fruitlet extracts contained three principal cytokinins. One was identified as a 6-(substituted amino)purine and was either zeatin or some very closely related compound. The two other cytokinins exhibited the chromatographic behaviour of zeatin riboside and zeatin ribotide. A cytokinin extracted from vegetative apple shoots was chromatographically indistinguishable from zeatin.     

Short-term control of root: shoot partitioning

We present data showing that the fraction of the available photosynthatepartitioned between the root and the shoot of a barley seedlingis affected by the supply of photosynthate from the source leaf:an increased fraction of the exported photosynthate goes tothe shoot when supply is reduced. Also, if the roots are cooleda short time before reducing the supply of photosynthate, thenthe effect of a reduced supply upon partitioning is reversedwith an increased fraction then going to the root. We concludethat the distribution of available photosynthate between competingsinks is influenced by source supply as well as sink function.The reported source-sink interactions are consistant with thepredictions of a recently pro posed model of source-sink interaction(Minchin et al., 1993). The concept of marginal partitioningis introduced to describe the distribution, between all of thesinks, of a small change in photosynthate supply. Key words: Carbohydrate partitioning, shoot : root ratio, source-sink interactions     

Behavioral responses of leafroller larvae to apple leaves and fruit

Larvae of Epiphyas postvittana and Planotortrix octo were released onto branches cut from apple trees, and allowed to colonize a range of types of artificial nests. Both species exhibited similar strong preferences for nests comprising leaf-leaf or leaf-fruitlet combinations, followed by nests comprising leaf-plastic leaf, leaf-plastic fruitlet or plastic leaf-fruitlet combinations. Nests involving fresh plant material alone (shoot, fruitlet or leaf alone) were also colonized to a lesser extent, but no larvae were found on nests consisting of plastic leaves or fruitlets alone or in combinations of the two. In another experiment, more E. postvittana larvae colonized nests with leaf and fruitlet combinations, compared to leaf and glass ball, or leaf and treated wax ball combinations, where the wax had been in contact with fresh apples or fruitlets. Choice tests, conducted using larval traps, showed that larvae were caught in traps baited with odors collected and released by wax which had been in contact with mature apples and leaves. Chloroform extracts from apple skin also caught larvae in choice tests. These results suggest that both physical and chemical cues are important to leafroller larval establishment.     

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.     

Photoassimilate partitioning in nodulated soybean II. The effect of changes in photoassimilate availability shows that nodule permeability to gases is not linked to the supply of solutes or water

It is concluded that the permeability of the soybean nodule to gases is not linked to the supply of solutes or water via the phloem to the nodule. Nodule respiration and nitrogenase activity were less affected by diel variation and shading treatments than partitioning to the nodule, as assessed using a non-invasive ¹¹C-based technique. Thus C import to the nodule was not matched to C requirement by the nodule. Transit times of tracer to, and within, the nodulated root increased under conditions of reduced photosynthetic rate. The increase in transit time was interpreted as a reduction in the flux of phloem sap. Thus the fluxes of both water and C to the nodule decreased following a reduction in photosynthetic rate. The change in partitioning of recent photosynthate to soybean roots and nodules in response to changes in photoassimilate availability was also used to assess the 'priority' of these sinks. Partitioning from the leaf to the root system was greatly decreased when photoassimilate availability was limited, indicating that root system priority is lower than that of the shoot, as reported for other systems. However, partitioning of tracer arriving in the root system between the nodulated and non-nodulated zones of the root was not affected by changes in photoassimilate availability, as caused by diel change, shading, or steaming of branch roots. Thus although nodules are sinks of high sink 'activity', they have 'priority' equal to that of other root sinks. It is suggested that there are similar phloem unloading kinetics, despite the very different metabolic destiny of the carbohydrate within the two organs.     

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.     

Carbon import into barley roots: effects of sugars and relation to cell expansion

The import of photosynthate labelled with ¹¹C from a sourceleaf into the two halves of a split root system of an intactbarley plant was studied. When applied to one half of a splitroot system sugars that are absorbed and metabolized reducesubsequent import of ¹¹C into that root half. The non-metabolizedsugar analogue 3-O-methyl glucose has no effect on import, whilstmannose and 2–deoxyglucose inhibit both root elongationand import of ¹¹C. EDTA, PCMBS, and apoplastic pH in the range4–7, have little effect on partitioning. These resultsare interpreted in terms of a suggestion that phloem unloadsdirectly into expanding cells in the elongation zone of roottips. Key words: Carbon partitioning, roots, ¹¹C, sucrose, phloem, sugars, cell expansion     

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     

Competition and fruit set in the Washington navel orange

In the Washington navel orange [ Citrus sinensis (L.) Osbeck] an increase in the number of flowers results in a reduction of flower weight at anthesis and the initial fruit growth rate, and the number of developing fruitlets increases. Most of these fruitlets are shed during post-anthesis, and the final set of fruit is unrelated to the number of flowers and to the total amount of metabolites and mineral elements used up in fructification but appears to be controlled by the capacity of the tree to supply metabolites to the developing fruitlets during post-anthesis. When the number of flowers is too large, there is a reduction both in the number of initially developing fruitlets and in their growth rate. The final set of fruit is reduced through a different mechanism acting at anthesis and involving differences in mineral composition, which impairs the capacity of the fruit to act as a sink.     

Effects of Edaphic Factors on the Soluble Carbohydrate Content of Roots of Lolium perenneL. and Trifolium repens L.

In a 3³x2² factorial experiment with Lolium perenne L. and Trifoliumrepens L. grown at three soil moisture tensions, three levelsof nitrogen and three levels of phosphorus, there was a widerange of percentage soluble carbohydrate (TSC) in the roots.The significant positive correlations between percentage ofTSC and root weight suggest that root growth-rate was proportionalto the concentration of TSC in the roots. In other experiments with Lolium perenne grown at five constantsoil temperatures and factorial combinations with three soilmoisture treatments with and without manure, there was declinein the percentage of TSC with increasing temperature. This impliedthat the respiration sink strength in the roots did not controlthe partitioning of photosynthate to the roots. No firm conclusions can be drawn from this work, but it is inferredthat there is a dynamic balancing mechanism in the foliage whichcontrols the partitioning of photosynthate between foliage androot growth. This mechanism appears to divert carbohydrate toroots in inverse proportion to root activity.     

Changes in the accumulation of flavonoid and isoflavonoid conjugates associated with plant age and nodulation in alfalfa (Medicago sativa)

Carbohydrate and mineral nutrition was studied in relation to abscission in fruitlets from leafy inflorescences of the Washington navel orange ( Citrus sinensis [L.] Osbeck). Differences in the growth rate of the fruitlets permitted to predict abscission several weeks in advance. This allowed characterization of early differences in composition and behaviour of persisting and abscising fruitlets.
Inflorescences with persisting fruitlets accumulated more mineral elements than inflorescences with abscising fruitlets, and for the phloem-mobile elements the excess accumulation was allocated to the fruitlets. Starch accumulated in the inflorescence leaves during early fruitlet growth, and this accumulation was enhanced by the persisting fruitlets despite their higher growth rate and mobilizing ability. The relations between the fruitlets and the inflorescence leaves cannot be explained totally in terms of source sink relationships; a hormonal regulation of the leaves by the fruitlets is postulated.
Acid invertase activities and hexose concentration in the pericarp were higher in the abscising fruitlets. The lower early growth rate of these fruitlets is thus not caused by a limitation in carbohydrate supply. It seems more related to carbohydrate utilization, probably hormonally mediated, as demonstrated by the higher dependence on hormone supply for the growth in vitro of the endocarp explants.     

Enzymatic browning and biochemical alterations in black spots of pineapple [Ananas comosus (L.) Merr.

Penicillium funiculosum Thom. was consistently isolated from pineapple-infected fruitlet (black spots). Polyphenol oxidase, peroxidase, and laccase activities were determined in extracts from contiguous and infected fruitlets. Healthy fruitlets showed a rather high level of polyphenol oxidase (optimum pH 7.0), and this activity was tremendously increased (X 10) in contiguous infected fruitlets. Furthermore, infected fruitlets also exhibited laccase activity (optimum pH 4.0), while peroxidase was rather constant in both fruitlets. Browning reactions were attributed to qualitative and quantitative modifications of the enzymatic equipment (polyphenol oxidase and laccase) (p < 0.0001). In infected fruiltets, sucrose and L-malic acid were present at significantly lower amounts than in healthy ones, likely owing to fungal metabolism (p < 0.0001), whereas cell wall material was three times higher, which could be viewed as a defense mechanism to limit expansion of the mycelium.