Distribution of xylem hydraulic resistance in fruiting truss of tomato influenced by water stress

In this study xylem hydraulic resistances of peduncles (truss stalk), pedicels (fruit stalk) and the future abscission zone (AZ) halfway along the pedicel of tomato (Lycopersicon esculentum L.) plants were directly measured at different stages of fruit development, in plants grown under two levels of water availability in the root environment. The xylem hydraulic connection between shoot and fruits has previously been investigated, but contradictory conclusions were drawn about the presence of a flow resistance barrier in the pedicel. These conclusions were all based on indirect functional measurements and anatomical observations of water-conducting tissue in the pedicel. In the present study, by far the largest resistances were measured in the AZ where most individual vessels ended. Plants grown at low water availability in the root environment had xylem with higher hydraulic resistances in the peduncle and pedicel segments on both sides of the AZ, while the largest increase in hydraulic resistance was measured in the AZ. During fruit development hydraulic resistances in peduncle and pedicel segments decreased on both sides of the AZ, but tended to increase in the AZ. The overall xylem hydraulic resistance between the shoot and fruit tended to increase with fruit development because of the dominating role of the hydraulic resistance in the AZ. It is discussed whether the xylem hydraulic resistance in the AZ of tomato pedicels in response to water stress and during fruit development contributes to the hydraulic isolation of fruits from diurnal cycles of water stress in the shoot.     

The regulation of sweet cherry fruit abscission by polar auxin transport

Inconsistency of cropping is an important problem for UK sweet cherry production. Premature fruit abscission in Prunus can reduce yields severely, however, the environmental cues and hormonal signals that trigger abscission have not been identified. Auxin (IAA) is known to delay abscission by reducing the sensitivity of cells in the abscission zone to ethylene, a promoter of abscission. Therefore, the capacity for polar auxin transport (PAT) through sweet cherry pedicels was examined in relation to fruit abscission. Cherry ‘spurs’ (short shoots) with similar leaf areas and different fruit numbers were phloem-girdled to restrict assimilate movement. Abscission from spurs with many fruit (eight or more) occurred within 14 days of girdling, whereas abscission from spurs with few (two) fruit was minimal. The pedicels’ capacity for PAT in spurs with different fruit numbers was determined 1, 3 and 9 days after girdling (DAG). Fruit were analysed for endogenous IAA concentration 3, 5, 7 and 9 DAG. PAT inhibitors 2,3,5-triiodobenzoic acid or 1-N-naphthylphtalamic acid were applied to pedicels of fruit not expected to abscise, i.e. on spurs with few fruit. The effect of these inhibitors on fruit abscission was determined 14 DAG. The proportion of the transported [³H]-IAA was lower from the outset in pedicels from spurs with many fruit. By 9 DAG, symptoms of fruit abscission were apparent and 40% less [³H] -IAA was transported through pedicels on spurs with many fruit. Fruit endogenous IAA concentrations were similar in the two groups of spurs. Application of PAT inhibitors shortly after girdling increased fruit abscission by 30%. The results suggest that although a decline in PAT is not the only cause of fruit abscission, the maintenance of PAT contributes to fruit retention.     

Ethylene-promoted tomato flower abscission and the possible involvement of an inhibitor

The abscission zone in tomato (Lycopersicon esculentum (L.) Mill. flower pedicels is morphologically distinguishable prior to separation and is delineated by an indentation of the epidermis. Exposure of excised pedicels with the flower attached to ethylene results in abscission within 12 h and this can be accelerated by flower removal. Abscission of excised pedicels with the flower removed takes place in the absence of exogenous ethylene but this is delayed by pretreatment with aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis. The data presented support the hypothesis that flower tissue is the source of an abscission inhibitor.Abbreviations AVG aminoethoxyvinyl glycine - IAA indole-3-acetic acid     

Gibberellins and <Emphasis Type="Italic">N</Emphasis>-(2-chloro-4-pyridyl)-<Emphasis Type="Italic">N′</Emphasis>-phenylurea improve retention force and reduce water core in pre-mature fruit of Japanese pear cv. Housui

A study of the effect of a gibberellin A₃ + A₄ mixture (GAs) on pre-harvest fruit drop of Japanese pear cv. Housui is reported. The GAs was applied alone or in combination with N-(2-Chloro-4-pyridyl)-N′-phenylurea (CPPU) in a lanolin paste to the abscission zone at the spur-end of the pedicel. The results showed that the GAs and CPPU combination treatment increased the pedicel–spur retention force relative to that of the untreated controls. Histological studies showed an accelerated rate of cambium division and the development of secondary xylem in the abscission zone near the spur-end of pedicels treated with the GAs plus CPPU paste, which delayed the formation of the abscission zone. Fruit quality (sugar, acid, firmness, color) was not adversely affected by the application of GAs plus CPPU, although the application of GAs alone promoted ripening. In contrast, the addition of CPPU to the two GAs delayed fruit ripening, which was measured as ethylene efflux. All treatments were without adverse effects on return bloom, measured as bud size. The CPPU plus GAs treatment also suppressed the incidence of water core, whereas the application of the GAs alone accelerated water core in this water core susceptible pear variety.     

Contrasting physiological effects of partial root zone drying in field-grown grapevine (Vitis vinifera L. cv. Monastrell) according to total soil water availability

Different spatial distributions of soil moisture were imposed on field-grown grapevines by applying the same irrigation volumes to the entire (DI; deficit irrigation) or part of the (PRD; partial root zone drying) root zone. Five treatments were applied: controls irrigated at 60% ETc (crop evapotranspiration) for the whole season (308 mm year(-1)); DI-1 and PRD-1 that received the same irrigation as controls before fruit set, 30% ETc from fruit set to harvest and 45% ETc post-harvest (192 mm year(-1)); and DI-2 and PRD-2 that were the same, except that 15% ETc was applied from fruit set to harvest (142 mm year(-1)). Compared with DI-1, PRD-1 maintained higher leaf area post-veraison and increased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, but decreased intrinsic gas exchange efficiency without causing differences in leaf xylem abscisic acid (ABA) concentration. Compared with DI-2, PRD-2 increased leaf xylem ABA concentration and decreased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, mainly at the beginning of PRD cycles. Distinctive PRD effects (e.g. greater stomatal closure) depended on the volumetric soil water content of the wet root zone, as predicted from a model of root-to-shoot ABA signalling.     

Studies on the abscission of flowers and fruits of cotton (Gossypium barbadense L.)

Effects of exogenous application of auxin, GA3, abscisic acid, ethrel, methionine and α-alanine to the cut ends of the pedicels of flower buds, flowers and fruits on their abscission behaviour were studied. Fruit pedicels required more time for abscission compared with flower and flower bud pedicels. NAA inhibited abscission of all types of pedicels and the inhibition was maximum in matured fruit pedicels and minimum in flower bud pedicels. Flower pedicels were more sensitive towards the abscission promotive effects of GA3, abscisic acid and ±-alanine and the flower bud pedicels towards ethrel and methionine. The duration of Stage-I of abscission was maximum in cut pedicels of fruit and minimum in those of flower buds. Biochemical analyses revealed greater quantities of endogenous amino acids in the epicalyx of flowers with the exception of methionine and aspartic acid which were found to be present in higher quantities in the epicalyx of flower buds. Levels of IAA-like compounds were maximum in the epioalyx of flower buds and minimum in the epicalyx of flowers. Higher levels of abscisic acid were found in the epicalyx of matured fruits and the epicalyx of flower buds showed a minimum amount of abscisic acid-like compound.     

A morphometric study on the effects of ethylene treatment in promoting abscission of tobacco flower pedicels

The ultrastructural changes observed in ethylene-induced abscission of tobacco flower pedicels (Nicotiana tabacum L. `Little Turkish') were studied by the techniques of morphometric analysis. The surface area of the membranes, relative volume of the organelles, and the number of organelles were determined for both ethylene-treated and control cells. In pedicels exposed to ethylene for 4.5 to 5 hours, abscission was evident within the separation zone. The most significant change in cell structure was observed in the surface area of the rough endoplasmic reticulum which more than doubled with ethylene treatment of the tissue.     

水分和盐分胁迫对番茄植株生长和木质部水力特性的影响

木质部是植株体内水分传输的主要通路,其水力特性的变化会影响植株的水分关系和果实的水分积累。目前关于番茄植株木质部解剖结构和水力特性对水分和盐分胁迫的响应及其与植株生长和果实含水量之间的关系尚不明确。本研究通过日光温室番茄盆栽试验,设置3个处理:对照,土壤含水量(θ)为75%～95%田间持水量(FC),初始电导率(EC)为0.398 dS·m^-1;水分胁迫,开花前θ为75%～95% FC,开花后至成熟期θ为45%～65% FC,EC为0.398 dS·m^-1;盐分胁迫,θ为75%～95% FC,EC为1.680 dS·m^-1,研究了樱桃型番茄(红宝石)和中果型番茄(北番501)植株在水分和盐分胁迫下的植株生长、果实含水量以及木质部水力特性的变化。结果表明: 与对照相比,水分和盐分胁迫下茎秆横截面积和木质部导管直径分别减小了22.0%～40.7%和10.0%～18.3%,茎秆比导水率和桁架柄比导水率分别降低了8.8%～41.1%和12.9%～28.4%,抑制了植株生长,减少了地上部鲜重、果实大小、果实鲜重和含水量,且与樱桃型番茄相比,中果型番茄的降幅更大。此外,果实含水量分别与茎秆和桁架柄比导水率呈显著正相关。综上,番茄植株在水分和盐分胁迫下木质部水力特性指标减小,生长被抑制,果实鲜重显著降低,最终导致产量降低。其中,中果型番茄相较于樱桃型番茄对水分和盐分胁迫更敏感。     

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.     

Changes in distribution of cell wall polysaccharides in floral and fruit abscission zones during fruit development in tomato (Solanum lycopersicum)

After fruit development has been triggered by pollination, the abscission zone (AZ) in the pedicel strengthens its adhesion to keep the fruit attached. Unpollinated flowers are shed at their respective AZs, whereas an enlargement of the same tissue is observed in pollinated flowers. After the fruit has developed and is fully ripened, shedding occurs easily at the AZ, indicating an acceleration of abscission. Cell wall degradation and synthesis may play important roles in these processes; however, little is understood. In this report, we have visualized changes in polysaccharide distribution in the AZs of pollinated versus unpollinated flowers and in the ripened fruits using immunohistochemistry. During floral abscission, a large increase was observed in LM15 labeling of xyloglucan specifically at the AZ in the abscising pedicel. LM5 and LM6 labeling of galactan and arabinan, respectively, also increased—LM5 throughout the pedicel and LM6 at the basal side of the AZ. The results suggest that xyloglucan, pectic galactan and arabinan play key roles in the abscission process. During fruit abscission, unlike in floral abscission, no AZ-specific cell wall polysaccharide deposition was observed; however, high autofluorescence was seen in the AZ of over-ripe fruit pedicels, suggesting secondary cell wall synthesis and lignification of the AZ prior to fruit abscission.     

Polygalacturonase expression during leaf abscission of normal and transgenic tomato plants

Polygalacturonase (PG, EC 3.2.1.15), an enzyme commonly found in ripening fruit, has also been shown to be associated with abscission. A zone-specific rise in PG activity accompanies the abscission of both leaves and flowers of tomato (Lycopersicon esculentum Mill.) plants. Studies of transgenic plants expressing an antisense RNA for fruit PG indicate that although the enzyme activity in transgenic fruit is < 1 % of that in untransformed fruit, the PG activity in the leaf abscission zone increases during separation to a similar value to that in untransformed plants. The timing and rate of leaf abscission in transgenic plants are unaffected by the introduction of the antisense gene. A polyclonal antibody raised against tomato fruit PG does not recognise the leaf abscission protein. Furthermore a complementary DNA (cDNA) clone (pTOM6), which has been demonstrated to code for fruit PG, does not hybridise to mRNA isolated from the abscission-zone region of tomato leaves. These results indicate that the PG protein in abscission zones of tomato is different from that in the fruit, and that the gene coding for this protein may also be different.Abbreviation PG polygalacturonase The authors of this paper are grateful to David Jackson of the John Innes Institute, Norwich, UK for his assistance with the in-situ hybridisation work. This research was supported by an Agricultural and Food Research Council Post-Doctoral award to J.E.T., and by a grant to D.G. from the Science and Engineering Research Council Biotechnology Directorate in association with ICI seeds. The work was carried out under Ministry of Agriculture, Food and Fisheries licences.     

Water Transport Properties of the Grape Pedicel during Fruit Development: Insights into Xylem Anatomy and Function Using Microtomography

Xylem flow of water into fruits declines during fruit development, and the literature indicates a corresponding increase in hydraulic resistance in the pedicel. However, it is unknown how pedicel hydraulics change developmentally in relation to xylem anatomy and function. In this study on grape (Vitis vinifera), we determined pedicel hydraulic conductivity (k_h) from pressure-flow relationships using hydrostatic and osmotic forces and investigated xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography. Hydrostatic k_h (xylem pathway) was consistently 4 orders of magnitude greater than osmotic k_h (intracellular pathway), but both declined before veraison by approximately 40% and substantially over fruit development. Hydrostatic k_h declined most gradually for low (less than 0.08 MPa) pressures and for water inflow and outflow conditions. Specific k_h (per xylem area) decreased in a similar fashion to k_h despite substantial increases in xylem area. X-ray computed microtomography images provided direct evidence that losses in pedicel k_h were associated with blockages in vessel elements, whereas air embolisms were negligible. However, vessel elements were interconnected and some remained continuous postveraison, suggesting that across the grape pedicel, a xylem pathway of reduced k_h remains functional late into berry ripening.In grape (Vitis vinifera), fruit growth by water accumulation follows a double sigmoid pattern and is influenced by the diurnal and developmental changes in water flow between fruit and the parent plant (Matthews and Shackel, 2005). Until the onset of fruit ripening (i.e. veraison), water enters the fruit predominantly via the xylem and thereafter mainly through the phloem (Greenspan et al., 1994, 1996). Choat et al. (2009) showed that the hydraulic conductance (i.e. 1/resistance) of the grape berry and pedicel declines substantially at later ripening stages predominantly due to a decline in pedicel conductance. Significant developmental changes in pedicel hydraulic properties were also reported for tomato (Solanum lycopersicum) and were found to be associated with xylem anatomical changes (Lee 1989; Van Ieperen et al., 2003; Rancić et al., 2008, 2010). Due to its position along the vascular transport pathway between fruit and the parent plant, the pedicel can play an important role in affecting fruit growth, as in kiwi (Actinidia deliciosa; Mazzeo et al., 2013). However, for grape, it needs to be elucidated how pedicel hydraulic properties change developmentally in relation to xylem anatomy and function.The location and nature of the loss in hydraulic conductance between the parent plant and the fruit is unclear and may differ among fruits. For tomato, Malone and Andrews (2001) showed that most of the loss of hydraulic conductance occurs in the fruit per se, but Van Ieperen et al. (2003) reported important and decreasing hydraulic conductance in the pedicel abscission zone over fruit development. For Citrus spp., Garcia-Luis et al. (2002) reported that xylem vessels in the pedicel remain largely functional late into fruit ripening. For grape, although vessel breakage in the berry was thought to lead to xylem dysfunction (Coombe and McCarthy 2000), several studies and methods have shown that xylem vessels in the fruit remain functional (Rogiers et al., 2001; Bondada et al., 2005; Chatelet et al., 2008a, 2008b). In line with these findings, data by Keller et al. (2006) suggest that the pedicel xylem also remains at least partially functional in ripening grape berries and can conduct water to and from the parent plant. Nevertheless, a reduction in the ability to transport water during ripening has been reported for grape (Tyerman et al., 2004; Choat et al., 2009) and other fleshy fruits, such as apple (Malus domestica; Lang and Ryan, 1994) and kiwi (Mazzeo et al., 2013), and it still remains unclear what causes this loss in xylem hydraulic conductance. For the grape pedicel, Choat et al. (2009) detected higher concentrations of xylem solutes postveraison and proposed that this is related to the deposition of gels into the xylem vessel lumen. However, direct evidence for the presence of xylem blockage and/or embolism formation in the grape pedicel is missing.This study of the grape ‘Cabernet Sauvignon’ pedicel was conducted with the goal to obtain a comprehensive understanding of how changes in hydraulic properties relate to changes in xylem structure and function over fruit development. Over the course of fruit development from 20 to 90 d after anthesis (DAA), water transport properties of pedicels were investigated under osmotic and hydrostatic driving forces using a modified pressure-probe system. This was combined with analyses of spatial and temporal changes in pedicel xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography (microCT; Brodersen et al., 2010, 2013; Rancić et al., 2010).     

Interactions between jointless and wild-type tomato tissues during development of the pedicel abscission zone and the inflorescence meristem.

The jointless mutation of tomato results in the formation of flower pedicels that lack an abscission zone and inflorescence meristems that revert to vegetative growth. We have analyzed periclinal chimeras and mericlinal sectors of jointless and wild-type tissue to determine how cells in different meristem layers (L1, L2, and L3) and their derivatives interact during these two developmental processes. Cells in the inner meristem layer, L3, alone determined whether the meristem maintained the inflorescence state or reverted to vegetative growth. Moreover, L3 derivatives determined whether a functional pedicel abscission zone formed. Limited and disorganized autonomous development of wild-type L2-derived cells occurred when they overlay mutant tissue. Adjacent mutant and wild-type L3-derived tissues in pedicels developed autonomously, indicating little or no lateral communication. Only the outermost L3-derived cells within the pedicel were capable of orchestrating normal pedicel development in overlying tissues, revealing the special status of those cells as coordinators of development for L1- and L2-derived cells, whereas the innermost L3-derived cells developed autonomously but did not influence the development of other cells.     

The distribution of xylem hydraulic resistance in the fruiting truss of tomato

The distribution of hydraulic resistances in xylem throughout the pathway leading to the tomato fruit was investigated. Previous work had indicated that there were large resistances within the supporting sections of this pathway (the peduncle and pedicel), perhaps associated with interruptions in the xylem. These high resistances are believed to impede calcium flux into the fruit and thus impair fruit development. It is shown here that fruit on intact plants do not shrink detectably during drought, even when the drought is sufficient to cause marked shrinkage of leaves and visible wilting of the shoot. In explants, it is possible to induce back‐flow from the fruit into the stem (probably via the xylem) but this flow is small and very slow. These observations support the view that there is a large hydraulic resistance in the pathway between fruit and stem. When pulses of water were made available within explants, by scorching of one leaflet, there was a rapid swelling of leaves and sepals. Such rapid fluxes indicate the presence of strong hydraulic (xylem) connections throughout the pathway between leaf and calyx. This shows that there are no significant hydraulic constrictions in the xylem proximal to the calyx. This finding is contrary to some previous conclusions but it is supported by experiments with dyes which showed continuous, functional xylem throughout the peduncle and pedicel. Calculations show that over 90% of the hydraulic resistance between stem and fruit must reside within the fruit pericarp. Implications for calcium nutrition are discussed.     

Hydraulic connections of leaves and fruit to the parent plant in Capsicum frutescens (hot pepper) during fruit ripening

Background and Aims

The hydraulic architecture and water relations of fruits and leaves of Capsicum frutescens were measured before and during the fruiting phase in order to estimate the eventual impact of xylem cavitation and embolism on the hydraulic isolation of fruits and leaves before maturation/abscission.

Methods

Measurements were performed at three different growth stages: (1) actively growing plants with some flowers before anthesis (GS1), (2) plants with about 50 % fully expanded leaves and immature fruits (GS2) and (3) plants with mature fruits and senescing basal leaves (GS3). Leaf conductance to water vapour as well as leaf and fruit water potential were measured. Hydraulic measurements were made using both the high-pressure flow meter (HPFM) and the vacuum chamber (VC) technique.

Key Results

The hydraulic architecture of hot pepper plants during the fruiting phase was clearly addressed to favour water supply to growing fruits. Hydraulic measurements revealed that leaves of GS1 plants as well as leaves and fruit peduncles of GS2 plants were free from significant xylem embolism. Substantial increases in leaf petiole and fruit peduncle resistivity were recorded in GS3 plants irrespective of the hydraulic technique used. The higher fraction of resistivity measured using the VC technique compared with the HPFM technique was apparently due to conduit embolism.

Conclusions

The present study is the first to look at the hydraulics of leaves and fruits during growth and maturation through direct, simultaneous measurements of water status and xylem efficiency of both plant regions at different hours of the day.     

Polyamine-induced modulation of genes involved in ethylene biosynthesis and signalling pathways and nitric oxide production during olive mature fruit abscission

After fruit ripening, many fruit-tree species undergo massive natural fruit abscission. Olive (Olea europaea L.) is a stone-fruit with cultivars such as Picual (PIC) and Arbequina (ARB) which differ in mature fruit abscission potential. Ethylene (ET) is associated with abscission, but its role during mature fruit abscission remains largely uncharacterized. The present study investigates the possible roles of ET and polyamine (PA) during mature fruit abscission by modulating genes involved in the ET signalling and biosynthesis pathways in the abscission zone (AZ) of both cultivars. Five ET-related genes (OeACS2, OeACO2, OeCTR1, OeERS1, and OeEIL2) were isolated in the AZ and adjacent cells (AZ-AC), and their expression in various olive organs and during mature fruit abscission, in relation to interactions between ET and PA and the expression induction of these genes, was determined. OeACS2, OeACO2, and OeEIL2 were found to be the only genes that were up-regulated in association with mature fruit abscission. Using the inhibition of ET and PA biosynthesis, it is demonstrated that OeACS2 and OeEIL2 expression are under the negative control of PA while ET induces their expression in AZ-AC. Furthermore, mature fruit abscission depressed nitric oxide (NO) production present mainly in the epidermal cells and xylem of the AZ. Also, NO production was differentially responsive to ET, PA, and different inhibitors. Taken together, the results indicate that PA-dependent ET signalling and biosynthesis pathways participate, at least partially, during mature fruit abscission, and that endogenous NO and 1-aminocyclopropane-1-carboxylic acid maintain an inverse correlation, suggesting an antagonistic action of NO and ET in abscission signalling.