Effects of Lime Sulfur and Fish Oil on Pollen Tube Growth, Leaf Photosynthesis and Fruit Set in Apple

Additional index words. fruit abscission, bloom thinning, organic apple production, Malus ×domestica Abstract. The effect of liquid lime sulfur (LS) and fi sh oil (FO) application during bloom on leaf photosynthesis (Pn) and pollen tube growth in apple (Malus ×domestica) fl owers were investigated in order to determine their mode of action as a bloom thinning agent. LS increased the percentage of fl owers with fewer than 10 pollen tubes per fl ower to more than 64% compared to 5% or less in the control. Pollen tubes were completely absent from 27% to 48% of fl owers following LS treatment compared with fewer than 4% of fl owers having no pollen tubes on control trees. These data indicate that 30% to 50% of fl owers that open on the day of LS application are unlikely to set a fruit due to the complete inhibition of embryo fertilization. Increasing the rate of LS from 0.5% to 4% increased the propor- tion of fl owers with limited pollen tube number in a concentration dependent manner. LS suppressed the rate of light saturated Pn; successive LS sprays during the bloom period had an additive effect on suppression of Pn and fruit set. In one study the reduction in Pn was greatest 12 days after application of LS but Pn recovered by about 19 days after initial treatment. In a second study Pn of primary spur leaves had still not recovered when measured 57 days after the fi rst of three applications. FO had no effect on the number of pollen tubes per fl ower, but reduced Pn and fruit set by about 10% and 20% respectively. An increase in the proportion of fl owers with no pollen tubes, and therefore no embryo development, can account, at least in part, for the thinning response following application of LS to apples during bloom. It is likely that suppression of Pn contributes to the thinning response, although the importance of this mechanism will depend on perturbation of the total carbohydrate supply to developing fruit. Liquid lime sulfur (LS, calcium polysulfate) is frequently used as a bloom-thinning agent in organic apple production systems in countries where its use is permitted, and is increasingly being used in non-organic production systems for the same purpose. Emulsifi ed fi sh oil (FO) is frequently combined with LS to provide greater thinning effi cacy (J. McFerson, personal com- munication). At this time the precise mode of action of LS as a bloom thinning agent is not known. Sulfur sprays may reduce germination (MacDaniels and Furr, 1930) and tube growth (He and Wetzstein, 1994) of pollen grains. The apple fl ower typically consists of fi ve carpels, each containing two ovules so that there is the potential to develop ten seeds per fruit (West- wood, 1978). Parthenocarpy is seldom observed in most commercial cultivars, indicating that at least some embryo development is necessary for fruit retention. However, for preharvest drop at least, there does not appear to be any clear relationship between seed number in seeded fruit and their retention (Ward et al., 2001). Since fl owers of 'Summerland McIntosh' exhibit perfect syncarpy, i.e., pollination levels among the fi ve stigmas need not be uniform to obtain full seed development (Sheffi eld et al., 2005) then the potential seed number per fruit will be related more closely to the number of pollen tubes per fl ower than per style, and only the absence of pollen tubes from all styles in a fl ower will result in complete ovule abortion. Sulfur may also suppress photosynthesis (Pn) in apple leaves (Hoffman, 1935; Hyre, 1939), and it has been proposed that postbloom LS applications thin apple fruit by inhibiting Pn and decreasing assimilate supply to developing fruit (Noordijk and Schupp, 2003). Palmer et al. (2003) have recently shown that using LS or sulfur as a fungicide over the whole season can reduce Pn of 'Braeburn' by almost 50% in mid-summer. The current study was undertaken to quantify the effect of LS applications during bloom on pollen tube growth, spur leaf Pn, and fruit set of apple.