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Late spring frosts are a major concern to blueberry growers in the southeastern United States. Cold hardiness of flower buds (stages 4 to 6) was evaluated in three southern highbush blueberry cultivars (`Cooper', `O'Neal', and `Gulfcoast'). Differential thermal analysis (DTA) and tissue browning tests revealed that the critical temperature and ovary damage occurred at –11C in `Cooper', –12C in `O'Neal', and –13C in `Gulfcoast'.

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Seven highbush blueberry (Vaccinium corymbosum L.) cultivars were evaluated for their photosynthetic heat stability. Ail showed significant reductions in CO2 assimilation rates (A) as leaf temperatures were raised from 20 to 30C, although `Blue-crop', `Jersey', `Elliot', and `Rubel' (22% to - 27%) were significantly less affected than Spartan', `Bluejay', and `Patriot' (-41% to -51%). To determine whether temperature adaptations of highbush types can be broadened through hybridization with native, heat-tolerant species, `Bluecrop' was crossed with the V. darrowi Camp. selection Florida 4B, and F2, BC1, and BC2, populations were generated. This approach showed promise as genotypes were identified in all the derivative populations that were more heat tolerant than `Bluecrop' and had a high A.

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Pollen deposition on the stigmatic surface of blueberry pistils was studied with regard to maximum pollen load and stigmatic fluid production (stigma receptivity). Three hybrid southern highbush cultivars (Vaccinium corymbosum L. with V. darrowi Camp, V. ashei Reade, and/or V. angustfolium Aiton), two northern highbush cultivars (V. corymbosum), and one hybrid half-high cultivar (V. corymbosum with V. angustifolium) were selfand cross-pollinated with counted pollen tetrads until saturation of the stigmatic surface occurred. Stigmatic saturation generally required 200 to 300 tetrads and was characterized by the cessation of stigmatic fluid production and the inability to absorb further tetrads. The loss of stigmatic receptivity was irreversible. Cross-pollination resulted in cessation of stigmatic fluid production at lower levels of tetrad deposition than did self-pollination, suggesting a potential pollen-stigma recognition phenomenon. Northern highbush, half-high, and southern highbush cultivars required 7% to 10%, 12% to 17%, and 14% to 21%, respectively, more self-pollen to develop the stigmatic saturation condition. The potential relation of the pollenstigma phenomenon to self-incompatibility mechanisms is discussed.

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Eighty-seven highbush blueberry and species-introgressed blueberry cultivars were evaluated for fruit firmness in the 1998-2000 growing seasons with a FirmTech 1 automated firmness tester. Significant differences were observed among cultivars. An average firmness of 136.1 g·mm-1 of deflection (g·mm-1 dfl) was observed across all studied cultivars, and a range of 80.4 g·mm-1 dfl (`Herbert') to 189.0 g·mm-1 dfl (`Pearl River'). Species ancestry was not consistently related to firmness; however, cultivars with higher firmness values often possessed a higher percentage of Vaccinium darrowi Camp and V. ashei Reade ancestry. Conversely, cultivars with softer than average fruit often possessed a higher percentage of lowbush (V. angustifolium Ait.) ancestry. This information may help to identify sources of breeding material for increased firmness in highbush blueberry hybrids.

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Pollen germination differed significantly between and within 3 interspecific hybrid populations and their parents. Pollen germination of populations of ‘Sharpblue’ (Vaccinium X sp.) × V. myrsinites Lamark (38%) and ‘Sharpblue’ × V. darrowi Camp (33%) was lower than mid-parent means of 45% and 37%, respectively. ‘Avonblue’ × V. myrsinites averaged higher in pollen germination than the mid-parent, 42% vs. 32%. Pollen germination of ‘Sharpblue’ × V. myrsinites hybrids varied most in pollen germination, ‘Avonblue’ × V. myrsinites hybrids least, and ‘Sharpblue’ × V. darrowi hybrids intermediate.

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