Flower emasculation is widely used in breeding programs for hybridization of deciduous fruit trees (Layne, 1983). This technique is also used in cytological and genetic studies (Okie and Hancock, 2008) and in different field experiments in which controlled pollination is required to avoid the interference of undesired pollen (Hedhly et al., 2009).
For fruit breeding purposes, flower emasculation is used to carry out controlled pollinations when the female parent is self-fruitful to avoid self-pollination and ensure crossing between cultivars. Emasculation consists of removing, with fingernails or other tools, the petals, sepals, and stamens before anther dehiscence (Layne, 1983). The technique makes flowers unattractive to pollinator insects (Free, 1964) and hand-pollinations can be done without the interference of undesired self- or cross-pollen. The late balloon stage 1 d before anthesis is considered most favorable for emasculation in stone fruit breeding (Bailey and Hough, 1975). Other pollination techniques can be used to control crosses in Prunus. For example, trees used as male and female can be enclosed in portable screenhouses with honeybee (Apis mellifera) hives in them or single female trees can be enclosed with bouquets or potted trees with honeybee hives. However, a common problem for these techniques is synchronizing blooms of pollenizers with the female tree (Okie and Weinberger, 1996). Thus, flower emasculation is particularly used in Prunus breeding, including japanese plum (Okie and Hancock, 2008; Okie and Weinberger, 1996; Weinberger, 1975).
Although many japanese plum cultivars are either mutations or chance seedlings, others are the result of planned hybridizations (Boonprakob et al., 2001; Byrne, 1989; Okie and Ramming, 1999; Okie and Weinberger, 1996; Weinberger, 1975). However, many crosses made by emasculation have resulted in very low fruit set or not fruit set at all (Okie and Hancock, 2008; Okie and Weinberger, 1996). A negative effect of flower emasculation on fruit set has been reported in other Prunus species, including almond [P. dulcis (Kester et al., 1994)], sour cherry [P. cerasus (Brown et al., 1996)], and sweet cherry [P. avium (Hedhly et al., 2009)] and points to flower emasculation as one of the underlying causes.
Factors that have been suggested as the causes for low or lack of fruit set in emasculated flowers of Prunus species include adverse weather conditions during and after the blooming period (Okie and Weinberger, 1996), inadequate developmental stage of the bud when the emasculation is carried out (Hjeltnes and Stanys, 1998), loss of the nutritive and protective functions of the perianth (Badr and Crane, 1965), possible damage to the pistil (Okie and Hancock, 2008; Okie and Weinberger, 1996), or acceleration of ovule degeneration (Hedhly et al., 2009). However, the causes leading to the lack of fruit set specifically in some japanese plum crosses are not clear.
In this work, the influence of flower emasculation on fruit set was evaluated in four japanese plum cultivars by comparing cross-pollinations performed with and without emasculation in orchard conditions. To ascertain which factors in the reproductive process could be related to the lack of fruit set, microscopic observations of pollen tubes and ovules were related with the behavior of flowers and developing fruit in the tree. The results revealed a high incidence of ovule degeneration caused by emasculation.
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