BREEDING, CULTIVARS, ROOTSTOCKS, & GERMPLASM RESOURCES

HORTSCIENCE 34(1):132–136. 1999. and Elfving, 1981; Takeda, 1993; Vasilakakis

et al., 1979a, 1980), although air temperature,
Flower Bud Initiation in Primocane- daylength, and solar radiation are associated
with early flowering (Privé et al., 1993).
Knowledge of the time of onset of flower
fruiting Blackberry Germplasm bud initiation in primocane-fruiting blackber-
ries may prove useful for the implementation
Jose Lopez-Medina1 and James N. Moore2 of strategies aimed at manipulating fruit pro-
Department of Horticulture, University of Arkansas, Fayetteville, AR 72701 duction. The objective of this research was to
determine the time of floral initiation and
Kyung-S. Kim3 development under field and greenhouse con-
Department of Plant Pathology, University of Arkansas, Fayetteville, ditions in erect PF blackberry selections.
AR 72701
Materials and Methods
Additional index words. Rubus sp., brambles, electron microscopy, light microscopy
Field-grown root cuttings (5–7.5 cm long)
Abstract. Scanning electron microscopy (SEM) and light microscopy (LM) were used to of A-1836 and APF-13 PF blackberries from
study the transition of meristems from vegetative to floral phase in erect primocane- the Arkansas Agricultural Experiment Sta-
fruiting (PF) blackberries [Rubus (Tourn.) L. subgenus Rubus] developed at the Univ. of tion, Fayetteville, were harvested and planted
Arkansas. Dormant root cuttings of A-1836 and APF-13 blackberries were dug from the in metal trays containing Sunshine LC1 mix
field and planted on 28 Dec. 1996 and 1 Mar. 1997 to produce plants for use in a greenhouse (Sungro Horticulture, Bellevue, Wash.) on 28
study. In a field study, terminal buds of field-grown A-1836, APF-13, NC194, and summer- Dec. 1996 or 1 Mar. 1997 under greenhouse
fruiting ‘Arapaho’ were sampled on 21 Mar 1997 (before shoot emergence from soil), and conditions. The soil mix was fortified with
then weekly from 14 to 28 May 1997. Flower bud primordia were first observed at five and Osmocote 14N–4.2P–11.6K (Scotts-Sierra
six nodes of growth in greenhouse-grown A-1836 and APF-13 plants, respectively, 35 to 42 Horticultural Products, Marysville, Ohio) at
days after root cuttings were planted (DAP). Under field conditions, floral primordia were 100 g per 0.06 m3 of compressed potting soil.
not observed until 21 May when A-1836 and APF-13 had at least 20 nodes of growth; The greenhouse was maintained under natural
NC194 did not differentiate floral structures until 10 July. The developmental patterns of daylight and 29 ˚C day/21 ˚C night tempera-
the vegetative apical meristem in the PF selections, both field- and greenhouse-grown tures. Ten terminal buds of each selection
plants, were similar to those of ‘Arapaho’. Opening of the terminal flower of the were randomly sampled when plants from the
inflorescence occurred 32 to 35 days after floral initiation in APF-13, and 8 to 10 days later first planting date reached five and 10 nodes of
on A-1836. Field-grown NC194 bloomed in late August. The first fruits of greenhouse- growth (11 and 28 Feb. 1997, respectively).
grown APF-13 were harvested 120 DAP. These findings demonstrate that PF blackberries From the second planting, five terminal buds
form flower buds after a short period of vegetative growth. of each selection were sampled weekly from
21 Mar. (just before shoot emergence from
Primocane fruiting (PF), a trait that is To date, there are no reports on the process soil) to 15 Apr. 1997 at two, four, six, eight,
present mainly in raspberry [R. idaeus L. of flowering in PF blackberry. As to summer- and 10 nodes of plant growth. In a field study,
(Daubeny, 1996; Keep, 1961; Ourecky, 1976)], cropping blackberries, floral initiation varies five terminal buds each of A-1836, APF-13,
has been introduced and intensified in erect with cultivar and location; some cultivars ini- NC194, and ‘Arapaho’ [a summer-cropping
blackberries at the Arkansas Agricultural Ex- tiate flower buds in autumn, others in midwin- cultivar (Moore and Clark, 1993)], were
periment Station. Blackberry germplasm of ter, and still others in spring (MacDaniels, sampled on 21 Mar. (before emergence from
this type has already been released for breed- 1922; Robertson, 1957; Takeda and soil), and again from 14 to 28 May 1997 when
ing purposes (Ballington and Moore, 1995). Wisniewski, 1989; Takeda et al., 1996; Waldo, the plants reached five, 10, 15, 20, and 25
The intensification of such a trait could result 1933). For instance, two eastern thornless cul- nodes of growth (±1 node at each stage).
in several revolutionary possibilities in black- tivars had different dates of floral initiation in The plant material was fixed in a modified
berry culture, including: 1) “off-season” (fall) West Virginia: October for ‘Black Satin’ and Karnovsky’s fixative (Karnovsky, 1965) con-
production with enhanced market opportuni- late March for ‘Hull Thornless’ (Takeda and taining 2% paraformaldehyde and 2% glut-
ties; 2) two crops per year could be harvested, Wisniewski, 1989). In another study, buds of araldehyde in 0.05 M cacodylate buffer at pH
fall and spring; 3) if only fall fruiting was ‘Chester Thornless’ remained undifferentiated 7.2 and postfixed in 1% osmium tetroxide in
practiced, all canes could be mowed to the until spring in Arkansas, Oregon, and West the same buffer. The specimens were en bloc
ground in winter, avoiding most pruning needs, Virginia, while sepal development in buds of stained overnight in 0.5% uranyl acetate at ≈ 4
overwintering pests, and cold injury to over- ‘Cherokee’ began in October in Arkansas and ˚C and then dehydrated in a graded ethanol
wintering canes; and 4) several crops per year in November in Oregon (Takeda et al., 1996). series. For SEM, buds were critical-point-
might be harvested in tropical and subtropical In thorny blackberry, flower bud differentia- dried in hexamethyldisilazane, sputter-coated
climates (Moore, 1997). tion occurs basipetally within canes and inflo- with gold, and observed on a 30-kV ISI-60
rescences, with formation of the terminal flower scanner (International Scientific Instruments,
in first place (Daubeny, 1996); in eastern thorn- Mountain View, Calif.). For LM, dehydrated
less cultivars, however, once the terminal buds were embedded in Spurr’s medium (Spurr,
Received for publication 16 Mar. 1998. Accepted flower forms, subsequent differentiation oc- 1969). One-micrometer sections were
for publication 22 July 1998. Published with the curs at the base of the inflorescence and pro- microtomed, flattened on glass slides coated
approval of the Director, Arkansas Agricultural ceeds acropetally (Takeda, 1987; Takeda and with Haupt’s adhesive, and stained with 1%
Experiment Station, as manuscript #98010. We thank Wisniewski, 1989). toluidine blue (Harris, 1972) and 1% Azure II
Dr. R.W. McNew for assistance with statistical In summer-cropping red raspberries, the in 1% borax. At each sampling date, the devel-
analysis. The cost of publishing this paper was time of flower bud initiation is believed to be opmental stage of each bud was recorded on a
defrayed in part by the payment of page charges. triggered mainly by shortening daylengths and 1-to-10 scale as described by Takeda and
Under postal regulations, this paper therefore must
be hereby marked advertisement solely to indicate
falling temperatures (Williams, 1959, 1960), Wisniewski (1989), with minor adaptations
this fact. with the age and size of canes, expressed by (Table 1).
1
Former Graduate Assistant. the number of nodes, also playing a role (Wil- All experiments were conducted using com-
2
Distinguished Professor; to whom reprint requests liams and Hudson, 1956). In PF red raspber- pletely randomized designs. At each sam-
should be addressed. ries, on the other hand, floral initiation can pling, each bud was taken from a different
3
University Professor. occur independently of these factors (Lockshin plant, each plant representing a replication.

132 HORTSCIENCE, VOL. 34(1), FEBRUARY 1999

Table 1. Developmental stages of buds of primocane-fruiting blackberry selections. (Adapted from Takeda fruiting cultivars (Takeda and Wisniewski,
and Wisniewski, 1989). 1989), but occurs much sooner in fall-fruiting
Stage Description genotypes.
1 Buds in vegetative phase with leaf primordia encircling the flat apical meristem. Under greenhouse conditions, the time re-
2 A1 inflorescence apex beginning to develop with a few leaf, phyllome, and bract primordia quired from the date of planting of the root
evident. cuttings to the time of floral initiation ranged
3 Terminal flower of A1 axis is differentiated. The apex is enlarged and sepal primordia is evident. from 32 d in the second planting (1 Mar.) to 45
4 Terminal flower of A1 apex is developed. Sepal primordia enlarge and become three-lobed. d in the first planting (28 Dec.); however, the
Petal primordia start differentiating. number of nodes at which the floral initiation
5 The central receptacle in the terminal flower of the A1 axis is enlarged. Sepals fuse and enclose process took place was about the same in both
central receptacle. planting dates. This difference of 2 weeks
6 Receptacle of terminal flower is elongate dome and petal primordia are enlarged. Sepals and
petals are in alternate arrangements.
might be attributable to more chilling unit
7 Stamen primordia are differentiated on floral cup. (CU) accumulation in plants from the second
8 Gynoecial structures are noticeable at the base of the receptacle. planting date. In ‘Heritage’ red raspberry,
9 Gynoecia cover the central receptacle. Petals expand and enclose stamen and receptacle. Takeda (1993) showed that the time to flower
10 Anthers and filaments are developed. Style and stigma are evident. was negatively correlated with CU accumula-
tion prior to cane emergence. In the present
study, the accumulation of CU may explain
why young plants initiated flower primordia at
The values for the developmental stage of started. Differentiation of floral structures then an early stage of growth (as early as five
sampled buds were analyzed by the nonpara- followed and continued uninterrupted until nodes).
metric Jonckheere-Terpstra (JT) test using the completion (Fig. 1E–H). Although floral dif- Dormant buds of ‘Arapaho’, A-1836, and
frequency procedure of SAS (SAS Institute, ferentiation seemed to be slightly more ad- APF-13 sampled on 28 Feb. 1997 from over-
Cary, N.C.). vanced in A-1836 than in APF-13, bud devel- wintering plants in the field were at an ad-
opmental stages were statistically similar (P ≥ vanced stage of differentiation (data not
Results 0.10) in both PF blackberry selections within shown). The process of floral differentiation,
the same number of nodes (Fig. 2). First bloom however, was still incomplete, since anther
The change from the vegetative to the on terminal buds of the inflorescence occurred and gynoecial structures in terminal flower
reproductive phase was evident in at least four 32 to 35 d after floral primordia initiation in buds on primary inflorescence axes were not
out of 10 five-node plants of A-1836 black- APF-13, and ≈10 d later on A-1836, with totally developed. Presumably, the process of
berry originating from the root cuttings col- harvest of the first APF-13 fruits 120 d after floral differentiation started earlier in the fall,
lected on 28 Dec. 1996, while the meristems of the root cuttings were planted in the green- but probably was arrested by low tempera-
similar APF-13 plants remained vegetative house. tures during winter.
(data not shown). In 10-node plants (28 Feb. In the field study, floral primordia were not In raspberry, flower induction in
1997), however, floral differentiation was al- observed until the plants of A-1836 and APF- primocane-fruiting cultivars occurs indepen-
ready in progress in all sampled buds of both 13 reached at least 20 nodes of growth (21 dently of photoperiod and temperature (Takeda,
blackberry selections, with inception of May). Plants of NC194, however, were still in 1993; Vasilakakis et al., 1979b, 1980; Will-
gynoecial structures at the base of the recep- their vegetative phase at 25 nodes, similar to iams, 1960). This might also be true for the PF
tacle in at least 10% of A-1836 buds. Although those of summer-cropping ‘Arapaho’ (Fig. 3). blackberries used in this research. These plants
flower buds were initiated slightly earlier in The first evidence of floral initiation in NC194 formed flower buds and bloomed while the
A-1836 than in APF-13 (P = 0.08), anthesis of occurred on 10 July in plants ranging from 35 temperature remained relatively constant (29
the terminal flower at the A1 inflorescence axis to 40 nodes of growth. First bloom on the ˚C day/21 ˚C night), and the natural daylength
(Takeda, 1987) occurred more rapidly in APF- terminal flowers of the inflorescence occurred was gradually increasing rather than decreas-
13, since this selection started to bloom ≈10 d on 22 June and 30 June in APF-13 and A-1836, ing throughout the period of study (early Feb.
earlier than did A-1836. On average, plants of respectively. In NC194, only a few primocanes to 15 Apr.). However, a discrepancy existed
both PF blackberry selections reached five showed open flowers at their tips by late Au- between greenhouse- and field-grown plants
and 10 nodes of growth 45 and 62 d after the gust; the remaining primocanes continued to as to cane length at which the onset of flower
root cuttings had been planted, respectively. be vegetative until the end of the season, bud initiation occurred (five nodes vs. 20
Histological examinations of terminal buds similar to ‘Arapaho.’ nodes, respectively). The temperatures pre-
from plants of A-1836 and APF-13 sampled vailing in the field were milder (20.8 and 9.7
just before emergence from the soil revealed Discussion ˚C average maximum and minimum, respec-
the typical features of a vegetative meristem as tively, from 1 Apr. to 28 May) than those in the
described by Bernier et al. (1981): a central Buds of fall-fruiting and summer-fruiting greenhouse. Lockshin and Elfving (1981) re-
zone, composed of the corpus and the centrally types of blackberry were similar in their veg- ported that higher temperatures (29 ˚C day/24
located cells of a three-layered tunica; a pe- etative stage. After a transitional stage, a whorl ˚C night) induced flowering of ‘Heritage’ red
ripheral zone, from which leaf primordia origi- of sepal primordia arose at the outer edge of raspberry 2 weeks earlier than did lower tem-
nated; and the pith-rib meristem (Fig. 1A). the terminal apex of the A1 inflorescence axis peratures (25.5 ˚C day/20 ˚C night); however,
Observations by SEM of those buds revealed and began to elongate (Fig. 1E). Soon after the canes in both temperature regimes flowered at
whorls of leaf primordia tightly encircling the sepals differentiated and became three-lobed, 24 to 25 nodes of growth. In the same rasp-
somewhat flat apical meristem (Fig. 1B). These a whorl of petal primordia in alternate arrange- berry cultivar, exposure to 25 ˚C quickly
features were similar in greenhouse- and field- ment with the sepals developed (Fig. 1F). stopped cane elongation and resulted in flow-
grown PF blackberry plants, and field-grown Sepals began to enlarge and enclosed the ter- ering and fruiting, while canes exposed to 16
‘Arapaho’ plants. When shoots reached four minal apex (Fig. 1G), which by then had ˚C were extremely long (Ourecky, 1976).
nodes, the buds remained vegetative in the two become the central receptacle (Fig. 1H). By Another factor that might be closely related
PF blackberry selections, but slight “doming” the time the terminal apex of the A1 inflores- with the discrepancies mentioned above is the
was evident on the apical meristem of the cence axis was enclosed by the sepals, some environment of the root system. Factors that
terminal flower (Fig. 1C). At six nodes, there degree of differentiation in the stamen and are associated with the slowing or suppression
was a marked “doming” and broadening of the gynoecial structures was evident, and sub- of primocane growth usually hasten the devel-
apical meristem, with bracts being formed and tending buds began differentiating acropetally opment of floral primordia in summer-fruiting
whorls of leaves loosely surrounding the mer- (Fig. 1G). This sequence of floral develop- raspberries (Crandall and Chamberlain, 1972).
istem (Fig. 1D). Thus, floral initiation was ment is similar to that reported for summer- This might also be applicable to PF blackber-

HORTSCIENCE, VOL. 34(1), FEBRUARY 1999 133

 BREEDING, CULTIVARS, ROOTSTOCKS, & GERMPLASM RESOURCES

Fig. 1. Flower bud initiation and differentiation in erect primocane-fruiting blackberry. (A) Longitudinal section of the terminal bud sampled just before emergence
from the soil. All features correspond to the vegetative stage. c = Corpus, t = tunica, l = leaf primordium, p = pith-rib meristem, i = primordial internode, v =
vascular trace-procambium. (B) Terminal bud as in (A) showing leaf primordia developing and encircling the apical meristem (m). (C) Terminal bud at four
nodes of growth. (D) Terminal bud at five nodes of growth. The apical meristem has broadened and become the primary inflorescence axis (A1). b = Bract, L
= leaf. (E) Development of sepal primordia (s) in terminal flower of the A1 axis. (F) Primary inflorescence axis showing inception of petal primordia (arrows)
in alternate arrangement with the sepals. a = Axillary buds. (G) Primary axis showing differentiation of axillary buds. Sepals have fused and enclosed the central
receptacle of the terminal flower. (H) Terminal flower (“floral cup”) showing advanced carpel development in central receptacle (r). Sepals were removed to
facilitate view of petals (p), gynoecial (g), and stamen structures (at arrows). Solid bar in all micrographs = 0.1 mm.
Fig. 1 continued on next page

134 HORTSCIENCE, VOL. 34(1), FEBRUARY 1999

Fig. 1. Continued. With an understanding of floral initiation 1989), a combination of N fertilizer and high
and differentiation, cultural practices may be temperature (Lockshin and Elfving, 1981),
ries since root system in the field was not used to promote or suppress flowering in PF row covers (Pritts et al., 1992), and cold tem-
restricted, in contrast with plants in the green- blackberry. For example, the use of plant perature pretreatments (Takeda, 1993;
house, which grew on metal trays with a layer growth regulators (Braun and Garth, 1984, Vasilakakis et al., 1980) performed on fall-
of soil no deeper than 6 cm. 1986; Crandall and Garth, 1981; Goulart, fruiting red raspberry could be adopted to

HORTSCIENCE, VOL. 34(1), FEBRUARY 1999 135

 BREEDING, CULTIVARS, ROOTSTOCKS, & GERMPLASM RESOURCES

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136 HORTSCIENCE, VOL. 34(1), FEBRUARY 1999