Pollination deficits and subsequent low fruit set remain among the most important horticultural challenges to sustained yields of highbush blueberry in northwestern Washington (Washington Blueberry Commission, 2017). Washington State is a part of the Pacific Northwest (PNW), which is a major blueberry production region, contributing significantly to global production of both conventional and organic highbush blueberry. In 2016, Washington State was the nation’s leading producer of blueberries with over 54.4 million kg of berries harvested from 5423 ha and a production value of about $94 million (USDA, 2016). Blueberry growers in the PNW frequently apply Ca and B foliarly as annual applications for field maintenance and promotion of fruit set and crop yields. This practice is often done regardless of plant nutrient status and despite a lack of evidence supporting their regular use for this specific purpose. The goal of this project was to evaluate the impact of foliar applications of Ca and B and assess whether these practices should be recommended to promote fruit set and subsequent yields in highbush blueberry grown in the PNW.
Pollination and ovule fertilization in highbush blueberry normally occurs over a 5- to 12-d period on a per flower basis (Dogterom et al., 2000). Pollination may be constrained in this short time-period because of marginal weather conditions that reduce honeybee (Apis mellifera L.) activity (i.e., air temperature <12 °C, wind speed >20 kph) and nutrient deficiencies (Tuell and Isaacs, 2010). Reproductive tissues (i.e., pollen, ovules, and developing embryos) are sensitive to nutrient concentrations in the flower and stem tissues (May and Killingbeck, 1992). Increased availability of nutrients involved in pollen tube elongation, plasma-membrane functioning, and other reproductive processes critical for fertilization and berry development, specifically Ca and B, could potentially increase fruit set under conditions that are otherwise limiting (Kumar et al., 2016; Muengkaew et al., 2017).
Blueberry plants are calcifuges, requiring low soil pH and having generally efficient uptake of Ca relative to other temperate fruit crops (Taiz and Zeiger, 2002). Calcium movement and distribution within the blueberry plant varies by cultivar (Strik and Vance, 2015). Despite sufficient leaf Ca concentrations (0.41% to 0.8% Ca; Hart et al., 2006), limited movement of Ca from leaf and stem tissues into developing reproductive tissues may lead to deficiencies, reducing fruit quality and, in some cultivars (e.g., ‘Draper’), causing poor fruit development and premature fruit drop (Gerbrandt, 2015). Calcium mobility is often limited due to relatively low rates of fruit transpiration (2.5–3.0 mmol CO2/kg/h) compared with leaves, thereby limiting the movement of dissolved Ca in the xylem to fruit (Angeletti et al., 2010).
Calcium (Ca) is involved in fruit abscission and pollen tube growth (Brewbaker and Kwack, 1963; Malho and Trewavas, 1996). Calcium deficiency can reduce the strength of the middle lamella and consequently make cells more prone to shearing, leading to fruit abscission (Glenn et al., 1988; Gough and Litke, 1980). Low Ca concentrations in the pollen of primrose (Primula officinalis L.) and butcher’s broom (Ruscus aculeatus L.) have been correlated with reduced rates of pollen germination, with some dependence of germination on Ca levels found within the stigma (Bednarska, 1991; Brewbaker and Kwack, 1963; Taylor and Hepler, 1997). Calcium concentrations in the stigma and style may similarly impact the effectiveness of pollen germination and tube elongation, as suggested in studies in tobacco (Nicotinia tabacum L.) and Japanese pear (Pyrus pyrifolia L.) (Ge et al., 2009; Hiratsuka et al., 2002). To improve pollen viability and fruit set, Ca is often applied alone or mixed with other foliar-applied agrochemicals during bloom and throughout the growing season in highbush blueberry grown in the PNW. Growers also apply foliar Ca because preharvest applications may increase fruit firmness, an important attribute of quality for fresh fruit (Angeletti et al., 2010). Calcium is rarely soil applied in blueberry, in part because it has been shown to inconsistently affect leaf and fruit Ca levels and has had no effect on berry yield, size, or firmness based on studies conducted in Michigan (Hanson and Berkheimer, 2004). However, there is risk when applying commercial Ca-containing foliar fertilizers, specifically for formulations that contain calcium chloride. High rates of calcium chloride can cause leaf phytotoxicity because of blueberry’s sensitivity to the chloride ion, especially in young blueberry plants (Retamales and Arredondo, 1995).
Boron (B) is an important micronutrient for plant growth. In blueberries, B may be especially important for pollen germination and cell elongation (Retamales and Hancock, 2012). Boron is often deficient in blueberry production in western Washington and Oregon, and this is exacerbated by dry weather and heavy crop loads (Marshner, 1995). Boron in the soil reaches plant roots largely through mass flow; deficiency is common in soils with low organic matter, sandy/coarse texture, or both (Goldberg, 1997). The difference between sufficiency and toxicity of B is marginal in plants, including blueberry. Typically, B is not applied at concentrations higher than 500 ppm in blueberry because of the risk of phytotoxicity (Hart et al., 2006). Although B is readily absorbed and mobile within the xylem of plants, making foliar applications during the dynamic growth phase, occurring during floral development, pollen germination, fertilization, and early berry development, may increase absorption and movement to targeted tissues (Brown and Hu, 1996; Lord and Russell, 2002). This intense period of development is metabolically demanding, and plant nutrient levels at this stage may have far-reaching consequences on fruit set, berry weight, yield, and fruit composition at harvest to various degrees across cultivars. Annual applications of B are recommended in blueberry based on tissue nutrient analyses, but the justification and impact of these applications on promoting fruit set is largely unknown in the PNW region (Hart et al., 2006).
The application of Ca and B as foliar nutrients in early spring may improve pollen health and ovule fertilization, as well as fruit set, berry development, and subsequent berry yields. It is hypothesized that application of these nutrients during the dynamic growth stages of flower and fruit development, when these nutrients may be limiting, will be most effective at enhancing these variables associated with yield. Therefore, a trial was conducted with the objective to investigate whether foliar Ca and B applied from early pink bud to petal fall would increase fruit set, yield, and attributes of berry quality (size and firmness) in highbush blueberry grown in northwest Washington.
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