Chemical thinning is one of the most effective measures to improve apple (Malus domestica Borkh.) fruit size, color and quality at harvest, increase return bloom the next year, reduce biennial bearing, and increase the profitability of commercial apple orchards. Blossom thinning is superior to postbloom thinning because it can be done earlier, thus having a greater effect on fruit size and return flowering (Greene, 2002). Blossom thinners usually reduce fruit set by preventing pollination and fertilization through damaging the anthers, stigma, and style of flowers or inhibiting pollen tube growth in the style of flowers and are applied when some, but not all, flowers are fertilized (Fallahi and Willemsen, 2002; Greene, 2002; McArtney et al., 2006; Williams and Edgerton, 1981). However, blossom thinning results are extremely variable and it is difficult to predict final fruit set because we do not know how environmental factors, especially temperature, affect pollen germination, pollen tube growth, fertilization and fruit set, and optimum timing for application (Fallahi and Willemsen, 2002; Greene, 2002).
Several reports have documented that temperature is the most important factor affecting pollen germination and pollen tube growth rate in pear (Mellenthin et al., 1972; Vasilakakis and Porlingis, 1985), sour cherry (Cerovic and Ruzic, 1992), avocado (Sedgley and Annells, 1981), and almond (Socias i Company et al., 1976). In apple, Williams (1970) also studied the influence of temperature on pollen tube growth and ovule longevity, but used only detached flowers of cultivars that are no longer of economic importance to the U.S. apple industry. It has been found that pollen tubes grew more rapidly to the base of styles in “attached” flowers than flowers that were detached and placed on artificial media (Yoder and Combs, unpublished data). Furthermore, temperatures studied by Williams (1970) were lower than typically experienced during the bloom period (7 to 15 °C). Better understanding of pollen tube growth after pollination under a wide range of temperatures will facilitate better-informed decisions relative to apple crop load management. The purposes of this investigation were to 1) study the effect of temperature on apple pollen germination and pollen tube growth in styles; and 2) evaluate effects of liquid lime sulfur (LLS) + fish oil (FO) at different times after pollination on apple pollen germination, pollen tube growth, and fruit set in the field.
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