Perfect Syncarpy in Apple (Malus x domestica ‘Summerland McIntosh’) and its Implications for Pollination, Seed Distribution and Fruit Production (Rosaceae: Maloideae)

Abstract

Perfect Syncarpy in Apple (Malus x domestica ‘Summerland McIntosh’) and its Implications for Pollination, Seed Distribution and Fruit Production (Rosaceae: Maloideae)

CORY S. SHEFFIELD1,2,*, ROBERT F. SMITH1 and PETER G. KEVAN2

1 Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, Nova Scotia, Canada B4N 1J5 and 2 Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1

* For correspondence. E-mail SheffieldC@agr.gc.ca

Received: 26 July 2004    Returned for revision: 22 September 2004    Accepted: 29 October 2004    Published electronically: 20 January 2005

 

Background and Aims The gynoecium of the domestic apple, Malus x domestica, has been assumed to be imperfectly syncarpic, whereby pollination of each stigmatic surface can result in fertilization within only one of the five carpels. Despite its implied effect on fruit quantity and quality, the resulting influence of flower form on seed set and distribution within the apple fruit has seldom been investigated. Instead, poor fruit quality is usually attributed to problems with pollination, such as low bee numbers and/or ineffective pollinators within apple agro-ecosystems. The objective of this study was to determine the true nature of gynoecial structure and its influence on fruit production in the apple cultivar ‘Summerland McIntosh’.

Methods A stigma-excision method was used to determine the effects of uneven pollination among the five stigmas on fruit quantity (as measured by fruit set), and quality (seed number and distribution). In addition, flowers were examined microscopically to determine pollen tube pathways.

Key Results Fruit set, seed number, seed distribution, and the microscopic examination of flower gynoecial structure reported in this study indicated that the gynoecium of the cultivar Summerland McIntosh is perfectly syncarpic and not imperfectly syncarpic as previously thought.

Conclusions Pollination levels among the five stigmas need not be uniform to obtain full seed development within Summerland McIntosh fruit; even if one stigmatic surface is adequately pollinated, a full complement of seeds is likely. The importance of perfect syncarpy in recognizing true causes of poor fruit quality in apple is discussed. Cory S. Sheffield, Robert F. Smith and Peter G. Kevan For the Department of Agriculture and Agri-Food, Government of Canada

Key words: Malus x domestica, apple, pollination, flower structure, pollen-tube pathway, perfect syncarpy, seed distribution, fruit quality

Annals of Botany 2005 95(4):583-591.


Introduction

Apples have been part of the human diet for thousands of yearsand cultivation practices have existed since at least 1000 BC(Morgan and Richards, 2002Go). Intense management of apple, Malusx domestica Borkh. (Rosaceae: Maloideae), as a horticulturalcrop is more recent, with many advancements for production occurringeven in the last half century (see treatments in Westwood, 1978Go;Childers, 1983Go; Morgan and Richards, 2002Go). The reproductiverequirements of the domestic apple, therefore, have been a topicof horticultural investigation for a long time and thoroughunderstanding has been gained (Brittain, 1933Go; McGregor, 1976Go;Westwood, 1978Go; Pratt, 1988Go; Sedgley and Griffin, 1989Go; Free,1993Go; Delaplane and Mayer, 2000Go). However, despite these investigations,there is still an incomplete understanding of apple flower formand function with respect to the actual pollen tube pathwayand its influence on the formation and distribution of seedswithin the fruit.

The female organ of a typical flower, the gynoecium, consistsof one or more structural units commonly called carpels, eachhaving a stigma, a style, and an ovary containing the ovules(Weberling, 1989Go; Endress, 1994Go; Raven et al., 1999Go). Endress(1982Go, 1994Go) reports that >80 % of taxa have syncarpous gynoeciain which the carpels are congenitally fused; the remaining taxaare split evenly between the apocarpous forms (with separatecarpels) and those with a single carpel (Endress, 1994Go). Thesyncarpous group contains taxa with a range of degrees of inter-carpelcommunication, but most forms have a compitum—a zone ofinter-carpel communication where pollen tubes have the potentialto cross over and distribute evenly among carpels (Endress,1982Go, 1994Go), a condition which Endress (1990)Go termed ‘perfectsyncarpy’. However, ‘syncarpy’ is also usedto describe taxa in which the pollen tube transmitting tissuesof each carpel remain separate throughout their entire lengthdespite the carpels being congenitally fused externally. Ina sense, these taxa have gynoecia that are effectively apocarpous(Carr and Carr, 1961Go; Williams et al., 1993Go). To distinguishthis form of syncarpy, Carr and Carr (1961)Go used the term ‘pseudosyncarpy’, although P. K. Endress (Institute of SystematicBotany, University of Zurich, Switzerland; pers. comm.) suggests‘imperfect syncarpy’ to describe taxa with congenitallyunited carpels with no compitum.

Apple flowers are typical for the rose subfamily Maloideae,which have been described as syncarpous (which, in a broadersense, includes imperfect syncarpy) (Pratt, 1988Go; Roher et al.,1994Go). However, within the Maloideae, considerable variationin the extent of connation among carpels has been reported,including apocarpous forms (Roher et al., 1991Go, 1994Go), and formsapparently without a compitum (Gorchov and Estabrook, 1987Go;Grochov, 1988Go). In Malus, each of the five styles bears a singlestigma and is basally fused with the other styles for a portionof its length. The styles are the solid type with a core oftransmitting tissue through which the pollen tubes grow inter-cellularly(Cresti et al., 1980Go; Sedgley, 1990Go). The gynoecium of appleis believed to be imperfectly syncarpous (Carr and Carr, 1961Go;Cresti et al., 1980Go; Anvari and Stösser, 1981Go; Pratt, 1988Go;Weberling, 1989Go) and, like most maloids (Cambell et al., 1991Go;Rohrer et al., 1994Go), each carpel contains two ovules whichhave the potential to form two seeds or ten seeds per fruit,although there are differences among cultivars (McGregor, 1976Go;Westwood, 1978Go; Faust, 1989Go; Free, 1993Go). Therefore, to producean apple with a full complement of seeds, it has been assumedthat at least two viable pollen grains must be transferred froma compatible cultivar to each of the five receptive stigmaticsurfaces (Torchio, 1985Go).

Because of imperfect syncarpy in apple, differences in the levelsof pollination among the five stigmas should have a direct effecton fruit quality and quantity due to variable production anddistribution of seeds (Carr and Carr, 1961Go). The number anddistribution of seeds within a developing apple affects itsshape and weight (Brittain, 1933Go; Brittain and Eidt, 1933Go; Free,1993Go; Brault and de Oliveira, 1995Go; Keulemans et al., 1996Go).Furthermore, flowers and developing fruit that are not pollinatedor that are poorly fertilized usually drop soon after bloom(Free, 1993Go). Most dropped apples collected during June andJuly have fewer developing seeds than those that stay on thetree (Brittain and Eidt, 1933Go; Brain and Landsberg, 1981Go). However,Lee (1988)Go suggests caution in interpreting the relationshipbetween seed number and fruit drop due to intra-plant variationin spur quality, stating that the same tree may have both many-seededfruit which drop and few-seeded ones which remain. In addition,Ward et al. (2001)Go also found that the date of drop was notrelated to the number of seeds of dropped fruit in some cultivars.

Several factors may result in pollination differences betweenthe stigmas. Normally the subdivided styles of apple flowersare the same length which places the stigmatic surfaces on thesame plane for visitation by pollinators (Fig. 1). The stigmaticsurfaces and styles are often tightly arranged into a column,which increases the likelihood of bees successfully pollinatingall five surfaces during a single visit. Occasionally the stigmasmay be spread apart or the sexual column may be damaged or deformed,which could result in asymmetric fertilization and seed distribution,leading to early fruit drop or misshapen, inferior fruit.

The findings of Beaumont (1927)Go and Visser and Verhaegh (1987)Gosuggest that the flowers of some cultivars of apple may havea compitum. Unfortunately their findings have been largely overlookedand most pomologists assume that apple flowers have an imperfectlysyncarpous gynoecium. Ward et al. (2001)Go, for example, investigatedthe relationships of seed number and fruit weight to day ofdrop in the cultivars ‘Smoothee Golden Delicious’,‘Redchief Delicious’ and ‘Commander York’.In that study, a stigma-excision technique similar to that ofBeaumont (1927)Go and Visser and Verhaegh (1987)Go was used to prescribeseed number in the resulting fruit—it was assumed thatat most two seeds would result for every stigma left intactand pollinated. Although seeds per fruit and seed distributionfor each stigma treatment was not presented in that study (Wardet al., 2001Go), R. P. Marini (Virginia Polytechnic Instituteand State University, VA; pers. comm) indicated that in somefruit more seeds were present than expected.

The objective of the present study was to determine the truenature of gynoecial structure in the apple cultivar ‘SummerlandMcIntosh’, specifically by comparing the effects of unevenpollination among the five stigmas on fruit quantity (as measuredby fruit set), and quality (seed number and distribution). Inaddition, flowers were examined microscopically to determinethe location and structure of the pollen transmitting tissue.


Materials and Methods

 

 
Collection of pollen for hand pollination and determination of pollen viability
In both years of study (2002 and 2003), branches with several flower clusters from different apple Malus x domestica Borkh. (Rosaceae, Maloideae) cultivars, maintained as breeding stock at the Atlantic Food and Horticulture Research Centre, Kentville, Nova Scotia (45°05'N, 64°28'W), were collected and placed into a greenhouse compartment with the temperature maintained continuously above 18°C and ambient light conditions. Branch ends were snipped diagonally and quickly placed into 4-L jars of water. At the balloon stage of flowering, petals were peeled away and the swollen anthers were removed from the filaments by rubbing the open flower on wire mesh, following the procedures of Galletta (1983)Go. Anthers were collected in a glass Petri dish and allowed to dehisce for 24–48 h. Dehisced anthers were placed into a glass vial and crushed to release more pollen. The vial was sealed, placed in a jar with anhydrous calcium sulfate (DrieriteTM) and kept in a cool, dry place until used (within 1–5 d).

In 2003, viability of a pollen sub-sample was determined using the MTT [3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide; Sigma M 2128] (Rodriguez-Riano and Dafni, 2000Go) and DAB (Sigma FastTM 3,3'-diaminobenzidine tablets; Sigma D-4168) techniques outlined by Dafni (2001)Go. A control of non-viable pollen was prepared for viability analysis simultaneous to the treatment; pollen was killed by spreading a small amount of the pollen mixture into a 70 % ethanol droplet on a glass microscope slide, which was then heated with a flame. This procedure was repeated twice. Control and treatment pollen samples were placed on glass microscope slides and the reagents MTT or DAB were applied following Dafni (2001)Go. The droplets were mixed and spread out to minimize pollen clumping and to facilitate drying, and the slides were placed on a slide warmer to dry. This procedure was repeated twice. Randomly selected fields of view were examined with a compound microscope at x100 magnification. The total of viable and non-viable pollen grains was counted in each field of view for a total count of at least 300 pollen grains. Non-viable pollen grains, which stay light coloured, were distinguished from viable ones, which turn a dark colour (violet-purple-brown in MTT; brown-purple-red in DAB), indicating the presence of dehydrogenases or peroxidases, respectively (Dafni, 2001Go). Percentage viability was calculated for both reagents.

Stigma receptivity period for hand pollination studies
Stigma receptivity of ‘Summerland McIntosh’ flowers at different stages of development was investigated in 2002 using a method which rated oxygen bubble generation on stigmatic surfaces. Receptive stigmatic surfaces produce peroxidase enzymes (Zeisler, 1938Go; Galen and Kevan, 1980Go; Dafni, 1992Go) which break down hydrogen peroxide, generating oxygen bubbles (Galen and Plowright, 1987Go; Dafni, 1992Go). Flowers at various stages of development were collected in plastic bags and immediately placed into a cooler containing ice packs. A ‘blind’ design was used to evaluate the level of oxygen generation where the evaluator had no knowledge of stage of flowering. It was assumed that the rate of bubble production from the stigmatic surface in hydrogen peroxide was directly related to its level of receptivity. A grading scale of 1–5 was developed to rank receptivity (1 = no bubble production; 5 = very rapid bubble production). The flowers were randomly selected for evaluation. The styles were then removed from each flower just above the point of emergence from the hypanthium and passed to the evaluator who placed the stigmatic ends into a depression slide containing fresh 3 % hydrogen peroxide (after Galen and Kevan, 1980Go). The evaluator graded and recorded the level of bubble production for each flower.

Stigma excision and pollination experiments
A 3-ha orchard at the Agriculture and Agri-Food Canada research farm in Canard, Kings Co., Nova Scotia (45°07'N, 64°28'W) was used for the experiments. The orchard consisted of alternating north-to-south double rows of 18-year-old ‘Summerland McIntosh’ (semi-dwarf ‘MM 106’ root stock, pruned to modified central leader) and 8-year-old ‘Royal Court Cortland’ (semi-dwarf ‘MM 106’ root stock) apple trees, with 6-m spacing between rows, and 4·5-m tree spacing within rows.

A randomized complete block (CB) design was used. The blocks were three (2002) and five (2003) randomly selected ‘Summerland McIntosh’ trees. On each tree, six limbs were selected based on flowering potential. Six levels of pollination treatment (i.e. none, one, two, three, four or five stigmas remain) were randomly assigned to each of the six limbs on each tree. Previously opened, damaged, unhealthy and under-developed flowers were removed. All remaining flowers at, or slightly beyond, the balloon stage had their styles snipped and cauterized (using forceps heated with a mini-torch) approx. 3–5 mm below the stigmatic surface to leave the desired stigma number. The remaining stigmas were then hand pollinated using the eraser end of a pencil coated with the previously collected pollen mixture. This procedure was repeated on two consecutive days to ensure pollination of open flowers. The total number of flowers receiving the pollination treatments remaining on each branch was recorded. Branches were then covered with approx. 45 x 50 cm Delnet® apertured film bags (DelStar Technologies, Inc., Middletown, DE) for 3–4 weeks to minimize insect feeding damage.

Percentage fruit set was determined for each of the branches on all trees by counting the developing fruit remaining 4 weeks after petal fall. Percentage fruit set data were subject to arcsine transformation (Zar, 1999Go) for normalization and to achieve homoscedascicity of variance prior to balanced analysis of variance (ANOVA) (Minitab, 2000Go). At harvest in mid-September, all fruits were collected and placed in cold storage at approx. 4 °C until fruit quality was determined. Fruits were cut in half just above the equatorial axis, and the number of plump seeds per carpel and their distribution within the fruit was determined. The number of seeds per fruit was analysed with ANOVA (generalized linear model procedures) (Minitab, 2000Go). The number of seeds produced per pollination treatment was also compared with those expected using chi-square analysis for contingency tables. Expected values were determined by first multiplying the maximum number of seeds expected per fruit for each of the k pollination treatments (i.e. two seeds per remaining stigma) by the number of fruit which developed for each treatment (i), to obtain the value Si. These values were then subject to the following transformation:

where T is the total seeds produced in all the fruit for all treatments. This procedure was done to balance the observed and expected totals in the contingency table for analysis while maintaining the relative proportions of expected seeds for each treatment. All tests were conducted at the 5 % level of probability.

Gynoecium structure and the pollen-tube pathway
Twenty flowers were collected and fixed in 3 : 1 ethanol : acetic acid for 20–24 h (Kearns and Inouye, 1993Go). Flowers were then rinsed in 50 % ethanol for 0·5 h, and then stored in 70 % ethanol at 4 °C. Flowers were dissected longitudinally to determine floral form, specifically the length of various sections of the gynoecium, and transversally to determine the location and structure of the pollen transmitting tissue within the styles. Measurements were made using a binocular microscope with an ocular micrometer.


Results

 

Pollen viability and stigma receptivity
The viability of the pollen mixture used in 2003 was determined to be high using both the DAB (92·2 % ± 4·0 s.d.) and MTT (94·0 % ± 7·5 s.d.) techniques. It was assumed that the pollen mixture used in 2002 had similarly high viability. Freshly opened flowers had the largest proportion of high stigma receptivity as measured by oxygen generating activity, and had the highest mean receptivity grade (Fig. 2). Peroxidase activity was measured in all stages of flower development.

Fruit set
Significant differences were observed among the pollination treatments for percentage fruit set in 2002 (F = 22·43, d.f. = 5, P Go). The zero-stigma pollination treatment had almost no fruit set and differed from the remaining treatments, and the one-stigma pollination treatment differed significantly from only the two-stigma pollination treatment (Fig. 3). In 2003, the same trend was observed with significant differences observed among the pollination treatments (F = 12·15, d.f. = 5, P Fig. 3). No differences were detected among the remaining treatments (Tukey's HSD test, P = 0·05).

Seed production and distribution
In 2002, significant differences in mean seeds per fruit were found among the pollination treatments (F = 2·86, d.f. = 4, P = 0·026), but not the trees (F = 1·71, d.f. = 2, P = 0·185). The data from the trees were subsequently pooled and analysed with Tukey's HSD test (P = 0·05). Significant differences were observed between the one-stigma and three-stigma pollination treatments, and between the one-stigma and five-stigma pollination treatments, but not among the other treatments (Fig. 4). In 2003, the same trend was observed with significant differences found among the pollination treatments (F = 5·15, d.f. = 4, P = 0·001), but not among the trees (F = 0·38, d.f. = 3, P = 0·77). The 1-stigma pollination treatment differed from all other treatments (Tukey's HSD test, P = 0·05) (Fig. 4).

In both years, the mean number of seeds per apple exceeded those expected from an imperfectly syncarpous arrangement in the one-, two- and three-stigma pollination treatments, but was less than expected for the five-stigma treatment (Fig. 4). Similarly, the total seeds produced was also significantly higher than expected for imperfect syncarpy for the one-, two- and three-stigma pollination treatments, but less for the five-stigma pollination treatment (2002: {chi}2 = 202·3, d.f. = 4; 2003: {chi}2 = 121·5, d.f. = 4; data pooled across trees in each year) (Fig. 5). The percentage of fruits with seed-bearing carpels from the different pollination treatments also did not correspond to those expected from an imperfectly syncarpous arrangement, as the majority of fruit from all treatments had five carpels bearing seeds (Table 1). No mature fruits had more than two seedless carpels.  

Gynoecium structure
The external fused portion of the styles, as measured from the hypanthium to the area of stylar separation, of Summerland McIntosh flowers is approx. 3 mm in length (Fig. 6). Internal microscopic examination indicated that the compitum is much smaller, and is confined between the upper part of the ovaries and the lower part of the connate styles (Fig. 6).


Discussion

 

The gynoecia of most taxa within the Maloideae are typically described as syncarpous, but Rohrer et al. (1991Go, 1994Go) indicate that much variation exists in the level of connation among the carpels, including at least three genera which are strictly apocarpous. Connation among carpels in the Maloideae is at two levels (Rohrer et al., 1994Go). At the level of the ovaries, connation is normally congenital (Endress, 1994Go). Most maloid genera have carpels that are fully fused to each other at this level, but variable degrees of fusion exist in some genera, including Pyrus (Rohrer et al., 1994Go), of which some species have been described as apocarpous (Sterling, 1965bGo). In the Maloideae, connation at the level of the ovaries is external (Rohrer et al., 1994Go) which probably limits the degree of inter-carpel communication in some genera, leading to the assumption of imperfect syncarpy in Malus (Carr and Carr, 1961Go; Cresti et al., 1980Go; Anvari and Stösser, 1981Go; Pratt, 1988Go; Weberling, 1989Go), a condition which does exist in other genera (Gorchov and Estabrook, 1987Go; Gorchov, 1988Go). However, small openings may be present at the centre of the core (Rohrer et al., 1994Go), which may allow some degree of inter-carpel communication. The second level of connation among carpels occurs in the styles (Rohrer et al., 1994Go), which develop from the apical portions of the carpel primordial (Evans, 1999Go). Some genera (e.g. Pyrus) develop styles which appear completely separate throughout their length (Rohrer et al., 1994Go; Aldasoro et al., 1998Go). Both congenital and post-genital fusion can lead to the formation of a compitum within connate areas of the carpels, including the styles (P. K. Endress, pers. comm.). However, Endress and Igersheim (2000)Go indicate that the degree of post-genital fusion is probably dependent on its time of commencement, being more intensive in forms which fuse earlier in development.

Several studies have presented microscopic examination of carpel structure (Sterling, 1964Go, 1965aGoGocGo, 1966Go; Rohrer et al., 1991Go, 1994Go; Evans, 1999Go) and, at least partially, pollen tube pathways in the Maloideae (Stott, 1972Go; Cresti et al., 1980Go; Gorchov, 1988Go; Embree and Foster, 1999Go; Kaufmane and Rumpunen, 2002Go; Broothaerts et al., 2004Go). However, no studies have traced the transmitting tissue within each carpel in its entirety, from stigma to micropyle. For instance, the microscopic/histochemical study by Cresti et al. (1980)Go of Starkrimson apple clearly shows five separate areas of transmitting tissue within a transverse section of the gynoecium just below the point of stylar union (indicated here in Fig. 6), supporting their claim of imperfect syncarpy. Unfortunately the authors did not continue to examine tissue below the point of stylar union, as it is below this level that the majority of perfectly syncarpous gynoecia have a compitum (Endress, 1994Go). Endress (1994)Go reports that in flowers with free stigmatic lobes (as in Malus), only 0·5 mm of joined transmitting tissue is required to evenly distribute pollen tubes among carpels when not all stigmas are pollinated.

The present findings support the notion that not all stigmas have to be pollinated to obtain uniform pollen tube distribution and full fertilization in Summerland McIntosh. Seed yield in the present study evidences the presence of a compitum, hence perfect syncarpy, in the apple flower which allowed pollen tubes growing down individual styles to cross into any of the five carpels. Seed number was higher than expected from an imperfectly syncarpic gynoecium when four or fewer stigmas were pollinated (Figs 4 and 5). It is believed that neither pollen viability nor incompatibility contributed to the lower than expected seed yield observed for the five-stigma pollination treatment in both years. This was probably a result of unrealized seed development in some fruit, as ovules do not always develop into seeds in the Maloideae (Rohrer et al., 1991Go). Further evidence of perfect syncarpy was provided by the even distribution of seeds among the carpels within fruits from all treatments receiving pollination (Table 1). High levels of fertilization and seed development among the carpels resulted in >40 % fruit set in both years (Fig. 3) when at least one stigma was pollinated. Only flowers which were not pollinated failed to set fruit.

Other studies have indicated that several apple cultivars may have gynoecia that are perfectly syncarpic (Beaumont, 1927Go; Visser and Verhaegh, 1987Go, and references therein). Perfect syncarpy is considered a derived condition within the angiosperms (Endress, 2001Go), and several evolutionary advantages over apocarpous and imperfectly syncarpous gynoecia have been recognized (Williams et al., 1993Go; Endress, 1982Go, 1994Go). Among these are greater seed set through more regular pollen tube distribution, economy in flower construction, and increased gametophyte selection among pollen grains in a unified transmitting tract (Endress, 1982Go, 1994Go).

Additional advantages may be gained with apically subdivided stigmatic surfaces for pollen capture (Howpage et al., 1998Go). The flowers of Malus and most other Maloideae attract a wide range of pollinators, and are not specialized for a single group (Cambell et al., 1991Go). For Malus, which has at least 44 bee visitors in Nova Scotia (Sheffield et al., 2003Go), pollination efficacy among apoidean visitors varies considerably (Boyle and Philogène, 1983Go; Boyle-Makowski, 1987Go; Free, 1993Go, and references therein; Goodell and Thompson, 1997Go; Vicens and Bosch, 2000Go). However, subdivided stigmatic surfaces promote pollen capture from a variety of positions, and accommodate differing foraging behaviours, albeit deposition may be uneven among stigmas. Perfect syncarpy via the compitum allows pollen tubes to be evenly delivered to ovules, despite unequal deposition during pollination. As a result, total pollen deposition by different bee visitors of Malus may not be as indicative of potential fruit quality as previously thought, and smaller or less effective floral visitors may be contributing significantly to fruit production if at least one stigmatic surface is adequately pollinated. More important factors in determining seed set and fruit production in some cultivars of apple are pollen viability, pollen compatibility and pollen dispersal. The importance of these factors, particularly with respect to orchard design, was recently investigated by Kron et al. (2001aGo, b)Go.

Visser and Verhaegh (1987)Go indicate that imperfect syncarpy may occur in some apple cultivars. Horticultural practices and breeding programmes have developed over 2000 apple cultivars worldwide (Morgan and Richards, 2002Go), many of which show variability in fruit form (Rohrer et al., 1991Go; Morgan and Richards, 2002Go). Differences in floral form have also been reported among many cultivars (Stott, 1972Go; Ferree et al., 2001Go). For instance, Stott (1972)Go reports differences in the proportion of stylar fusion among many apple cultivars. Some of the differences in floral form among cultivars can be great enough to cause variation in pollinator floral handling behaviour (Schneider et al., 2002Go), in some instances to a level that pollinator effectiveness (i.e. stigma contact) declines, as reported with the ‘sideworking’ behaviour of honey bees on ‘Delicious’ apples (Robinson, 1979Go; Degrandi-Hoffman et al., 1985Go). It is possible that intensive cultivar development has created or caused the loss of the compitum in some apple cultivars, either through variable degrees of carpel separation at the level of the ovaries or through reduced stylar fusion. Gynoecial arrangements in the non-cultivated forms of Malus (55 species reported in Phipps et al., 1990Go), including the ancestor of the domesticated forms, M. sieversii (Ledeb.) Roem. (Morgan and Richards, 2002Go), are presently unknown. Clearly, gynoecial structure in apple cultivars warrants further study as the presence or absence of a compitum directly influences fruit quality when pollination is incomplete.


ACKNOWLEDGEMENTS 

This study comprises part of the PhD research undertaken by C.S.S. which was funded in part by the Agri-Focus 2000 Technology Development Program (Nova Scotia Department of Agriculture and Fisheries) and Agri-Futures-Nova Scotia Association (Agriculture and Agri-Food Canada) through the Nova Scotia Fruit Growers' Association. This paper is contribution number 2288 of the Atlantic Food and Horticulture Research Centre. We thank Susan Rigby, for experimental assistance during this project and for her illustration, and the following for field assistance: Kim Jansen, Meg Hainstock, Michelle Larson, Stephanie Moreau, Derek Maske and Darrin Moran. For reviewing the manuscript, we thank Dr Klaus Jensen, Atlantic Food and Horticulture Research Centre–Agriculture and Agri-Food Canada, Kentville, Nova Scotia, and two anonymous reviewers. We also thank Dr Peter K. Endress, Institute of Systematic Botany, University of Zurich, Switzerland for clarification of floral morphology and terminology, and Dr Rodger C. Evans, Acadia University Biology Department, Wolfville, Nova Scotia for his comments and for providing a valuable piece of literature.


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Figures

 

FIG. 1. A solitary bee, Andrena sp. (Hymenoptera: Andrenidae) visiting a Summerland McIntosh apple (Malus x domestica) blossom. Note the central stigmas covered with pollen.

perfect%20f01.gif

 

 


FIG. 2. Proportion of stigma receptivity grades in Summerland McIntosh apple (Malus x domestica) flowers at various developmental stages and their respective mean grade (± s.e.).

perfect%20f02.gif

 

 

 


FIG. 3. Mean percentage fruit set (± s.e.) for Summerland McIntosh apple (Malus x domestica) for each pollination treatment in 2002 and 2003. Bars sharing letters within a year are not significantly different (Tukey's HSD test, P = 0·05); analysis based on arcsine-transformed data.

perfect%20f03.gif

 

 


FIG. 4. Mean number of seeds per fruit (± s.e.) and expected seeds per fruit for Summerland McIntosh apple (Malus x domestica) for each pollination treatment in 2002 and 2003. Bars sharing letters within a year are not significantly different (Tukey's HSD test, P = 0·05).

perfect%20f04.gif

 

 

 


FIG. 5. Total seeds and expected seeds in Summerland McIntosh apple (Malus x domestica) for each pollination treatment in 2002 (data pooled for three trees) and 2003 (data pooled for five trees). Significant differences were found between the number of seeds observed and expected (2002: {chi}2 = 202·3, d.f. = 4; 2003: {chi}2 = 121·5, d.f. = 4).

perfect%20f05.gif

 

 

 


FIG. 6. The gynoecium of Summerland McIntosh apple (Malus x domestica) displaying perfect syncarpy. ‘A’ indicates the most basal transverse section examined by Cresti et al. (1980).

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Table

TABLE 1. The distribution of seed-bearing carpels for each pollination treatment in 2002 and 2003

Year No. of stigmas pollinated No. of fruit % Fruit with 0, 1, 2, 3, 4 or 5 seed-bearing carpels



0
1
2
3
4
5
2002 0 0
  1 17 35·3 23·5 41·2
  2 29 10·3 17·2 72·5
  3 36 13·9 86·1
  4 30 3·4 13·3 83·3
  5 38 26·3 73·7
2003 0 0
  1 17 41·2 23·5 35·3
  2 18 5·6 94·4
  3 12 16·7 83·3
  4 18 11·1 88·9
  5 13 23·1 76·9

Values indicate percentages of fruit with seed bearing carpels from each treatment; highest percentage values within a row are in italic; bold in two diagonal rows indicates the distribution of seeds in carpels expected under imperfect syncarpy.



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