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

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.

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