Flower Bud Initiation in Primocane-fruiting Blackberry Germplasm

Abstract

Scanning electron microscopy (SEM) and light microscopy (LM) were used to study the transition of meristems from vegetative to floral phase in erect primocane-fruiting (PF) blackberries [Rubus (Tourn.) L. subgenus Rubus] developed at the Univ. of Arkansas. Dormant root cuttings of A-1836 and APF-13 blackberries were dug from the field and planted on 28 Dec. 1996 and 1 Mar. 1997 to produce plants for use in a greenhouse study. In a field study, terminal buds of field-grown A-1836, APF-13, NC194, and summer-fruiting `Arapaho' were sampled on 21 Mar 1997 (before shoot emergence from soil), and then weekly from 14 to 28 May 1997. Flower bud primordia were first observed at five and six nodes of growth in greenhouse-grown A-1836 and APF-13 plants, respectively, 35 to 42 days after root cuttings were planted (DAP). Under field conditions, floral primordia were not observed until 21 May when A-1836 and APF-13 had at least 20 nodes of growth; NC194 did not differentiate floral structures until 10 July. The developmental patterns of the vegetative apical meristem in the PF selections, both field- and greenhouse-grown plants, were similar to those of `Arapaho'. Opening of the terminal flower of the inflorescence occurred 32 to 35 days after floral initiation in APF-13, and 8 to 10 days later on A-1836. Field-grown NC194 bloomed in late August. The first fruits of greenhouse-grown APF-13 were harvested 120 DAP. These findings demonstrate that PF blackberries form flower buds after a short period of vegetative growth.