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  • Author or Editor: Brian A. Birrenkott x
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Selective flower removal was used in 1987 and 1988 to evaluate intraplant competition or inhibition within flowering uprights of `Searles' cranberry (Vaccinium macrocarpon Ait.). The lowest two flowers were removed from uprights at various stages of plant development in 1987. With one or both of the two earliest, i.e., lowest, flowers developing `into fruit, 25% of the remaining flowers matured into fruit. Removal of the earliest two flowers at preblossom or late blossom resulted in ≈ 46% fruit set for the remaining flowers. Slightly fewer upper flowers set (36%) when the earliest flowers and fruit were removed at early fruit development. In 1988, the lowest two flowers were removed at preblossom and natural insect pollination was supplemented by hand pollination. Hand-pollinated (upper) flowers set 58% when the lowest two flowers were removed, compared to 17% for the unthinned control. Yield and fruit numbers were lowered slightly as a result of flower thinning in both years. A significant amount of variation in fruit production was explained by the number of flowering uprights per unit of production area in both years.

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Pollination and pollen tube growth were evaluated in two years as potential factors suppressing cranberry (Vaccinium macrocarpon Ait.) fruit set. Supplementing insect pollination with hand pollination increased fruit set from ≈30% to ≈38% in both years. The number of flowers per unit area was an important contributor to fruit set variation in one year. Cranberry uprights exhibited a temporal decline in fruit set when flowers were pollinated sequentially; the first flowers to open had a higher probability of fruit development than flowers opening later. Examination of stigmas indicated flowers receiving low amounts of pollen (<10 tetrads), or pollen that fails to germinate, are more likely to abort. An inadequate number of pollen tubes and lack of subsequent fertilization provides a partial explanation of fruit abortion in cranberry. Cranberry fruit set under existing field conditions appears to be limited, in part by insufficient pollination and pollen tube growth, with the latter apparently the result of intraplant competition for resources. Providing supplemental hand pollination increased cranberry yields in both years, 48% over natural insect pollination when the number of flowering uprights per unit area was high (≈3000/m2). A significant amount of yield variation was explained by the number of flowering uprights per unit area in both years.

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Cranberry (Vaccinium macrocarpon Ait. cv. Searles) vegetative tissue was analyzed at various stages of development to determine carbohydrate levels under field and greenhouse conditions and to identify the carbohydrates. Except during dormancy, cranberry uprights in the field had the highest concentration of carbohydrates (soluble and starch) at early blossom, when the lower flowers were at anthesis. As early flowers developed into fruit and upper flowers were at or just beyond anthesis, uprights had lower carbohydrate concentrations. As fruit growth slowed, soluble carbohydrate levels increased and were highest at dormancy. Upright shoot tissue produced the previous year and trailing woody stems followed the same trend as the current season's growth but had consistently lower soluble carbohydrate levels at each growth stage. Starch levels were low in current growth and did not change appreciably with fruit development. Starch was primarily stored and subsequently depleted in the previous season's upright growth and trailing woody stems. Tissue from the greenhouse was generally higher in carbohydrates than was field-grown tissue. Fruit developed from 53% of the flowers under greenhouse conditions, compared to 38% in the field. Insufficient carbohydrate levels may be responsible for the low fruit set observed in the field. Sucrose, glucose, fructose, raffinose, and stachyose were present in cranberry vegetative tissue.

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A leach collection unit (LCU) was assembled to capture all leachate draining from a nursery container. An injection molded 2.8-L nursery container was plastic welded into the lid of a 7.6-L black plastic collection bucket so that the bottom 2.5 cm of the nursery container protruded through the lid. The LCU was designed to track total N release from CRFs without confounding effects of plant uptake or N immobilization. Total N released between any two sampling periods is determined by multiplying the N concentration in a leachate subsample × total leachate volume. The LCU were placed in a container nursery area with overhead irrigation. LCU were thoroughly leached before sampling the leach solution. To study the effects of substrate on N leach rates, Osmocote 18.0N–2.6P–9.9K (8 to 9 months 21 °C) was incorporated at 1.8 kg N/m3 using a locally available, bark-based substrate or medium-grade quartz sand. The experiment was conducted at Scotts Research locations in Apopka, Fla., and Marysville, Ohio. Osmocote incorporated into either a bark-based substrate or sand resulted in similar N release profiles. Although substrate did not affect N leach rate, quartz sand was recommended as the substrate in the leach collection system for polymer-coated CRFs. Quartz sand is chemically and biologically inert, does not immobilize nutrients and has low ion exchange capacity compared to bark-based potting substrates. More than 90% of the total nitrogen applied from Osmocote was recovered from leachate and unreleased N in fertilizer granules. This research has demonstrated the leach collection system as a reliable means to quantify nitrogen release rate of a polymer-coated CRF under nursery conditions. The LCU, when used with a crop plant, allows nutrient budget and nutrient uptake efficiency to be determined for CRFs.

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