We investigated the response of staminate and pistillate floral components of Prime-Jan™ and Prime-Jim™ primocane-fruiting blackberry (Rubus L. subgenus Rubus Watson) to three different growth chamber temperature regimes, 35.0/23.9 °C (HT), 29.4/18.3 °C (MT), and 23.9/12.8 °C (LT). Temperature was negatively related to flower size and morphological abnormalities in floral structures were evident in 41% and 98% of the MT- and HT-grown plants, respectively. The viability (stainability) of pollen from LT- and MT-grown Prime-Jan™ flowers exceeded 70%; that of Prime-Jim™ pollen was significantly reduced (<40%) by the MT regime. Pollen in-vitro germinability was negatively influenced by temperature but was unaffected by cultivar. LT-grown pollen held at 23.9 °C retained 63% of its original germinability over a 32-hour period; the germinability of LT-grown pollen held at 35.0 °C was decreased by 97% from its original level after 16 hours. Virtually all flowers cultured under HT conditions were male-sterile, exhibiting structural and/or sporogenous class abnormalities including petaloidy, malformation of tapetal cells, and microspores or failure of dehiscence. The duration of stigma receptivity, pistil density, and drupelet set were also negatively influenced by increasing temperature; values for these parameters of floral competency among control plants were reduced by 51%, 39%, and 76%, respectively, in flowers cultured under HT conditions. Herein, flowering and fruiting parameters and presumably the yield potential of Prime-Jan™ and Prime-Jim™ were adversely affected by increased temperature. However, assessment of their adaptative response to heat stress under field conditions awaits experimentation.
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