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Cox's Orange Pippin' apple (Pyrus domestics Berth.) trees, which were previously heavily cropped compared to previously defruited trees, had smaller flowers, lower initial fruit set, and a shorter effective pollination period (EPP) than defruited trees. The morphology of apple flower styles and stigmas was studied using scanning and transmission electron microscopy. Virgin stigmas from previously unstressed trees had fully expanded papillae, but 7 days after pollination the papillae were collapsed and distinct pollen tubes penetrated the stigmatic surface. Virgin stigmas from flowers of previously heavily cropped trees were smaller than those from previously defruited trees. The virgin stigmas of the flowers of heavily cropped trees showed collapsed papillae, and differences were observed in the intercellular material in the subtending transmitting tissue. These differences may explain the poor fruit setting characteristics of apple flowers on trees that have borne a heavy crop in the previous year. Cultivars with a biennial bearing habit produce few flower buds in cropping years (Luckwill, 1970), and the number of flower buds is decreased as crop load increases (Abbott, 1984). Williams et al. (1980) showed that the earlier fruit were picked in a cropping year, the better the yield in the subsequent year. This was initially attributed to insufficient nutrients remaining for bud development after fruit demands had been satisfied (Childers, 1978), but later defoliation experiments showed inhibition of bud development to be hormonal rather than nutritional (Fulford, 1966a). Chan and Cain (1967) found that parthenocarpic fruit did not inhibit bud development, whereas seeded fruit did, and concluded that the effect of fruit on return bloom was caused by the seeds and was an hormonally regulated process. The main effect of the presence of developing fruit in a cropping year is to lengthen the plastochron, potentially to such an extent that floral primordia fail to develop (Abbott, 1977). The plastochron in developing buds is normally 7 days during the 4 weeks after full bloom, regardless of fruit load; however, after this period, cell division in the cortex of fruitlets is reduced (Denne, 1960), coinciding with a peak in seed-gibberel lin production (Luckwill et al., 1969). Flower quality, that is, the ability of a flower to develop into a fruitlet after pollination with compatible pollen during the flower's receptive period, is linked to ovule maturity and longevity and pollen receptivity at the stigmatic surface. Jackson et al (1981), studying poor fruit set in flowers exposed to high preblossom temperatures, found that misting treatments to cool blossoms delayed flowering, but that at anthesis a greater proportion of ovules were mature in the cooled vs. control flowers. With respect to previous cropping, Williams (1970) showed that the effective pollination period (EPP) was shorter in the off-year than in the cropping year. Schwabe (1978) showed that a large crop load in one season resulted in smaller flower buds in the subsequent off- year. The study presented here sought to determine the-effect of prior fruit load on morphology and function of apple flowers, particularly with respect to stigmatic anatomy.

Effect of Previous Crop Load on Stigmatic Morphology of Apple Flowers