In a field study, `Gulfcoast' southern highbush blueberry plants were subjected to irrigation [7.5 L (low) or 30 L per week (high)], mulching (none or 15 cm depth), row height (level or raised 15 cm), and soil-incorporated peatmoss (none or 15 L in each planting hole) treatments, in a factorial arrangement, at establishment. Plants were grown on a well-drained fine sandy loam soil that contained <1.0% organic matter. Plant volume and fruit yield were greater with mulching, high irrigation, incorporated peatmoss, and level beds. Plants grown with the combination of mulching, level beds, incorporated peatmoss, and high irrigation levels yielded 2.4 kg per plant or approximately eight times as much as plants grown without mulch, with raised beds, without peatmoss, and with the low rate of irrigation. Of the four establishment practices evaluated, mulching had the greatest influence on plant growth and fruiting.
Yuehe Huang and Gregory A. Lang
To study the effects of pollen sources on ovule and berry development in southern highbush blueberries (Vaccinium corymbosum hybrids), 5-year-old `Sharpblue' plants were moved into a greenhouse for self- and cross-pollination experiments. Cross-pollination with `Gulfcoast' and `O'Neal' as pollen sources increased fruit weight by 58.2% and 54.9%, respectively, compared to self-pollination. Cross-pollination did not affect the number of total and small ovules significantly but did double the number of fully developed ovules and increase the average ovule size by 14%. The increase in number and size of fully developed ovules correlated with the significant difference in berry fresh weight between self- and cross-pollination. Cross- and self-pollination showed good correlations between fruit fresh weight and number or cross-sectional area of fully developed ovules. There was a poor correlation between fruit fresh weight and the number or cross-sectional area of partially developed ovules. This study provides further evidence that berry size in southern highbush is influenced strongly by the development of fully developed ovules.
B.E. Maust, J.G. Williamson, and R.L. Darnell
Floral budbreak and fruit set in many southern highbush blueberry (SHB) cultivars (hybrids of Vaccinium corymbosum L. with other species of Vaccinium) begin prior to vegetative budbreak. Experiments were conducted with two SHB cultivars, `Misty' and `Sharpblue', to test the hypothesis that initial flower bud density (flower buds/m cane length) affects vegetative budbreak and shoot development, which in turn affect fruit development. Flower bud density of field-grown plants was adjusted in two nonconsecutive years by removing none, one-third, or two-thirds of the flower buds during dormancy. Vegetative budbreak, new shoot dry weight, leaf area, and leaf area: fruit ratios decreased with increasing flower bud density in both cultivars. Average fruit fresh weight and fruit soluble solids decreased in both cultivars, and fruit ripening was delayed in `Misty' as leaf area: fruit ratios decreased. This study indicates that because of the inverse relationship between flower bud density and canopy establishment, decreasing the density of flower buds in SHB will increase fruit size and quality and hasten ripening.
P. Manjula Carter, John R. Clark, and R. Keith Striegler
Southern highbush blueberry, a hybrid of northern highbush (Vaccinium corymbosum) and southern-adapted Vaccinium species, has the potential to meet the need for an early-ripening blueberry in the southern U.S. southern highbush cultivars can ripen up to one month earlier than the earliest rabbiteye (Vaccinium ashei) cultivars currently grown in the southern U.S. However, chilling requirement and cold-hardiness are cultivar-dependent for southern highbush and cultivar testing has been necessary to determine the cultivars best adapted to specific hardiness zones. In a 4-year study at Hope, Ark. (hardiness zone 7b), several southern highbush cultivars were evaluated for productivity, fruit quality and reliability of cropping. Yields were based on 1089 plants/acre (2690 plants/ha) for southern highbush cultivars and 605 plants/acre (1494 plants/ha) for rabbiteye cultivars. `Ozarkblue' and `Legacy' showed the most adaptability at this location, yielding on average 11,013 lb/acre (12,309 kg·ha-1) and 10,328 lb/acre (11,543 kg·ha-1) respectively, compared to 4882 lb/acre (5456 kg·ha-1) for `Premier' (rabbiteye) over 4 years. `Ozarkblue' and `Legacy' also rated well for plant vigor and fruit quality. We would recommend `Ozarkblue' and `Legacy' for commercial planting in southwest Arkansas and believe these cultivars have production potential for other areas of the southern U.S. that have similar hardiness zones and soil type to southwest Arkansas.
Gregory A. Lang and E. James Parrie
Pollen from six southern highbush blueberry cultivars derived from Vaccinium corymbosum L. and one or more other species (V. darrowi Camp, V. ashei Reade, and V. angustifolium Aiton) was incubated on nutrient agar to determine tetrad viability, pollen tube growth rates, and incidence of multiple pollen tube germinations. `Avonblue' pollen had a significantly lower tetrad germination percentage than `Georgiagem', `Flordablue', `Sharpblue', `Gulfcoast', or `O'Neal', all of which had >90% viable tetrads. The in vitro growth rate of `O'Neal' pollen tubes was significantly higher than the growth rates of `Sharpblue' and `Georgiagem pollen tubes. Of those tetrads that were viable, more than two pollen tubes germinated from 83% and 91% of the `Gulfcoast' and `Sharpblue' tetrads, respectively, while only 11% of the `Flordablue' tetrads produced more than two pollen tubes. The total number of pollen tubes germinated per 100 tetrads ranged from 157 (`Flordablue') to 324 (`Sharpblue'), resulting in actual pollen grain viabilities ranging from 39% to 81%. Genetic differences in pollen vigor, as indicated by pollen viability, pollen tube growth rates, and multiple pollen tube germinations, may influence blueberry growers' success in optimizing the beneficial effects of cross-pollination on fruit development.
P.A.W. Swain and R.L. Darnell
`Sharpblue' southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) was grown in either a dormant or nondormant production system to determine the effect of production system on source limitations to fruit and vegetative growth. Source limited stages were evaluated in the two production systems by reducing reproductive sink load during either the fruit cell division or fruit cell enlargement stage. Source limitation during cell division was evaluated by removing 80% of the flower buds in late fall, since the majority of cell division in blueberry ovaries occurs before bloom. Source limitation during cell enlargement was evaluated by removing 80% of the fruit after fruit set the following spring. In the dormant production (DP) system, mean fruit dry weight (DW) was greatest in the flower bud removal treatment and least in the control (nonthinned) treatment, suggesting that cell number, rather than size, is more important in determining blueberry fruit weight in the DP system. Fruit in the dormant flower bud removal treatment may have approached maximum cell number and therefore fruit size; this was supported by the observation that significant depletion of root carbohydrate concentration did not occur in this treatment, as it did in the control treatment. Mean fruit DW in the nondormant production (NDP) system was greatest in the fruit removal treatment compared with the other two treatments, suggesting that cell enlargement played a larger role in determining fruit size in this production system. However, the effect of the flower bud removal treatment (and therefore the effect of cell division) on fruit DW in the NDP system was apparently masked by continued flower bud initiation in this system after flower bud removal in late fall. Continued floral initiation was apparently an alternative sink to increasing cell division in previously formed flower buds. In both systems, fruit removal increased vegetative growth compared with the control and flower bud removal treatments. Thus, both systems exhibited source limitations to fruit and vegetative growth, although the timing and extent of the limitation to fruit growth differed between the production systems.
Glenn C. Wright, Kim D. Patten, and Malcolm C. Drew
`Tifblue' and `Brightwell' rabbiteye blueberry (Vaccinium ashei Reade) and `Sharpblue' southern highbush blueberry (primarily V. corymbosum) were treated with 0, 25, and 100 mm Na+ as Na2SO4 or NaCl, and 0, 1, 3, and 10 mm supplemental Ca2+ in sand culture in the greenhouse. For rabbiteye plants salinized with Na2SO4, leaf Na+ concentrations increased 54-fold and the percentage of total plant Na+ found in the leaves increased from 9% to 63% with increasing external Na+. Calcium supplementation reduced the Na+ concentrations in leaves by up to 20%. Leaf Ca2+ concentrations increased with Ca2+ supplementation, but accounted for a decreasing percentage of the total Ca2+ found in the plant, since root Ca2+ concentrations were much higher. Root Na+ concentrations increased with increasing Na+ treatments to a smaller extent than in the leaves and were also reduced by Ca2+ supplements. Potassium concentrations in leaves and roots decreased with increasing Na+ treatment levels, particularly in roots, where K+ concentration was about half at 100 mm Na+ (as Na2SO4.) Leaf Na+ concentrations were up to two times greater when Na was supplied as NaCl compared to Na2SO4. For plants salinized with NaCl, leaf Na+ levels increased to 1.1% and did not decrease when supplemental Ca2+ was applied. Leaf Cl- concentrations also increased greatly with NaCl, reaching >1.0% (dry weight basis.). Root Cl- concentrations also increased with increasing salinity and were not affected by Ca2+ supplements. Ca2+ supplementation led only to a greater Ca2+ concentration in leaves and roots, but this did not alter Na+ concentrations. Nutrient concentrations in `Sharpblue' leaves, stems, and roots were greater than those of the rabbiteye cultivars, but were influenced by salinity and Ca2+ in essentially the same way. Excess Na+, Cl-, or both, together with lowered K+, were likely the cause of extensive leaf necrosis and may be indicative of a lack of a mechanism to control Na+ influx into blueberry leaves.
Glenn C. Wright, Kim D. Patten, and Malcolm C. Drew
`Tifblue' and `Brightwell' rabbiteye blueberries (Vaccinium ashei Reade.) were subjected to 0, 25, or 100 mM Na+, as Na2SO4 or NaCl, and 0, 1, 3, or 10 mM supplemental Ca2+, primarily as CaSO4, in an irrigated sand culture in the greenhouse. Additionally, the effect of NaCl on `Sharpblue' southern highbush blueberry (primarily V. corymbosum L.) was examined. For unsalinized plants, fastest growth occurred in plants not receiving supplemental Ca2+. Root and shoot growth were depressed as salinity increased in plants lacking additional Ca2+. With 100 mM Na+ as Na2SO4. `Tifblue' root and shoot dry weight increases were only 37% and 25%, respectively, of the increase of unsalinized controls, while with 100 mM Na+ as NaCl, the corresponding shoot and root dry weight increases were only 38% and 43%, respectively. `Brightwell' plants reacted similarly to `Tifblue' in salinity treatments with Na2SO4 and NaCl, but `Sharpblue' plants were more severely affected by 100 mM NaCl than were the rabbiteye cultivars. In no case did addition of Ca2+ have any ameliorative effect on either the dry weight of roots of plants exposed to 25 or 100 mM NaCl or on the shoot growth of plants exposed to NaCl. The inability of Ca2+ to counter Cl- entry or toxicity may account for the lack of amelioration. In contrast, additional Ca2+ did improve shoot growth of plants exposed to Na2SO4. For `Tifblue' plants supplied with 25 mM Na+ as Na2SO4, growth increased by almost 25% in the presence of 10 mM Ca2+, while for `Tifblue' plants treated with 100 mM Na+ as Na2SO4, growth was more than three times greater in plants supplied with 1 mM Ca than in those not given any Ca2+. Growth increase was primarily due to increased leaf area and number. Low (1 mM) concentrations of Ca2+ were more effective in ameliorating the effects of 100 mM Na+ as Na2SO4 than were 3- and 10-mM Ca2+ supplements, possibly because higher Ca2+ additions lead to metabolic damage in these calcifuge Vaccinium species.
B.E. Maust, J.G. Williamson, and R.L. Darnell
Vegetative budbreak, leaf area development, and fruit size in southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrids) decrease as flower bud density increases. The effect on fruit size has been attributed to both insufficient carbohydrate reserves and reductions in current photoassimilates caused by decreased vegetative growth. Experiments were conducted with two southern highbush blueberry cultivars, `Misty' and `Sharpblue', to test the hypothesis that increased carbohydrate reserve concentrations can overcome the detrimental effects of high flower bud density by increasing vegetative budbreak, shoot development, and whole-canopy net CO2 exchange rate (NCER), which in turn will increase fruit size. Fully foliated plants were placed in greenhouses with either ambient (AMB) CO2 levels (≈360 μmol·mol-1) or enriched (ENR) CO2 levels (≈700 μmol·mol-1) for 38 d during fall. Plants were then moved outdoors, hand defoliated, and flower bud density (flower buds/cm cane length) adjusted to range from 0.07 to 0.31. Root starch and whole plant carbohydrate concentrations increased in ENR compared with AMB plants of both cultivars. Vegetative budbreak (number per centimeter cane length), leaf area, and whole-canopy NCER decreased as flower bud density increased in AMB and ENR plants of both cultivars; however, ENR `Sharpblue' plants had significantly greater vegetative growth and wholecanopy NCER at a given flower bud density compared with AMB `Sharpblue'. Concomitant with this was an increase in fruit fresh weight in ENR compared to AMB `Sharpblue'. This was not the case with `Misty', where vegetative development and fruit size were similar in ENR and AMB plants. Thus, the hypothesis that increased carbohydrate reserves will increase vegetative development and subsequent fruit size may be true only in certain cultivars of southern highbush blueberry. Alternatively, the increased carbohydrate reserve concentrations in ENR compared with AMB `Misty' plants may have been insufficient to affect subsequent vegetative or reproductive development.
Jeffrey G. Williamson and E.P. Miller
In 1998, representative canes of mature, field-grown, `Misty' and `Sharpblue' southern highbush blueberry were hand-defoliated on 4 Sept., 2 Oct., 6 Nov., 7 Dec., or not defoliated. The experiment was repeated in 1999. Randomized complete-block designs with 11 (1998) or 10 (1999) replications were used. The early defoliation treatments (4 Sept. and 2 Oct.) resulted in reduced flower bud number per unit length of cane for `Misty', but not for `Sharpblue', when compared with later defoliation treatments or controls. A similar response to early defoliation was found both years for both cultivars. The later defoliation treatments (6 Nov. and 7 Dec.) had no significant effect on flower bud number compared to controls. Early defoliation had a negative effect on flower bud development for both cultivars. Flower buds that developed on canes defoliated on 4 Sept. or on 2 Oct. had smaller diameters than flower buds on canes defoliated on 6 Nov., 7 Dec., or on non-defoliated canes. Fruit fresh weight per unit cane length was less for the September and October defoliation treatments than for the December defoliation treatment or controls. These results support the need for summer pruning and a effective summer spray program to control leaf spot diseases that often result in early fall defoliation of southern highbush blueberries grown in the southeastern United States.