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Flowering of Watsonia laccata as influenced by corm storage and forcing temperatures

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The genus Watsonia, belonging to the family Iridaceae, is comprised of about 50 species including W. laccata (Jacquin) Ker Gawler that flowers from September to November following low temperature and winter rainfall. Therefore, we hypothesized that flowering would be favored by forcing at low greenhouse temperatures. Using clonal W. laccata corms, four experiments were designed to investigate the effect of temperatures during corm storage, forcing, and their interaction on growth and flowering. Corm formation is favored by growing plants at 18°–20°/15°–17°C and 21°–23°/18°–20°C, day/night temperatures. Flowering was earliest with corms produced at 24°–26°/18°–20°C and forced at 18°–20/15°–17°C, and was significantly delayed when forced at 27°–29°/24°–26°C. Flowering was, however, favored by 2 or 4weeks of high temperatures (27°–29°/24°–26°C) prior to forcing at low temperatures (18°–20°/15°–17°C). The number of florets was not significantly affected by corm storage, forcing temperatures, or their interaction, although forcing at high temperatures tends to reduce the floret number. Burn symptom at the tips of leaves was frequently observed, and further studies are required to understand the cause of the tip burn and how to correct the symptom.
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Flowering of Watsonia laccata as influenced by corm storage and
forcing temperatures
J.K. Suh
a,
, J.H. Kim
a
, A.K. Lee
a
, M.S. Roh
b
a
Dankook University, College of Bio-Resources Science, Department of Environmental Horticulture, Cheonan, Chungnam 330-714, Republic of Korea
b
US Department of Agriculture, Agricultural Research Service, National Arboretum, Floral and Nursery Plants Research Unit, Beltsville MD 20705, USA
Received 24 March 2010; received in revised form 28 November 2010; accepted 22 December 2010
Abstract
The genus Watsonia, belonging to the family Iridaceae, is comprised of about 50 species including W. laccata (Jacquin) Ker Gawler that
flowers from September to November following low temperature and winter rainfall. Therefore, we hypothesized that flowering would be favored
by forcing at low greenhouse temperatures. Using clonal W. laccata corms, four experiments were designed to investigate the effect of
temperatures during corm storage, forcing, and their interaction on growth and flowering. Corm formation is favored by growing plants at 18°
20°/15°17 °C and 21°23°/18°20 °C, day/night temperatures. Flowering was earliest with corms produced at 24°26°/18°20 °C and forced at
18°20/15°17 °C, and was significantly delayed when forced at 27°29°/24°26 °C. Flowering was, however, favored by 2 or 4 weeks of high
temperatures (27°29°/24°26 °C) prior to forcing at low temperatures (18°20°/15°17 °C). The number of florets was not significantly affected
by corm storage, forcing temperatures, or their interaction, although forcing at high temperatures tends to reduce the floret number. Burn symptom
at the tips of leaves was frequently observed, and further studies are required to understand the cause of the tip burn and how to correct the
symptom.
© 2010 SAAB. Published by Elsevier B.V. All rights reserved.
Keywords: Controlled flowering; Corms; Dormancy; Leaf-tip burn symptoms; New floral crops; Watsonia laccata
1. Introduction
The endemic flora of the Cape area in South Africa includes
germplasm of many geophytes such as Lachenalia (Duncan,
1988), Ornithogalum (Du Plessis and Duncan, 1989), and
Sparaxis and other Iridaceae (Ehrich et al., 2009) which have
the potential to develop into new floral crops (Helme and
Trinder-Smith, 2006). One of these attractive genera, Watsonia,
closely related to Gladiolus, is comprised of about 50 species
(Goldblatt, 1999). Seeds of W. laccata, a species native to an
area with winter-rainfall area, germinated well at temperatures
ranging 10°20 °C, whereas germination was inhibited at
temperatures ranging 25°40 °C. The average maximum and
minimum temperatures at the native sites were, respectively
20.5 °C and 9.9 °C (Ascough et al., 2007a,b). Growth and
flowering of W. tabularis (Eck.) J. W. Mathews & L. Bolus was
influenced by paclobutrazol to produce a compact container
plant (Thompson et al., 2005; Wulster and Ombrello, 2000).
However, cultural information related to corm production,
growth, and flowering of Watsonia, is not available.
Recently, information on seed germination and in vitro
propagation of four winter-rainfall Watsonia Mill species that
included W. laccata (Ascough et al., 2007a,b), and leaf cutting
propagation of Lachenalia aloides (L. F.) Engl. Pearsoniiand
Ornithogalum dubium Houtt. hybrid that are endemic to South
Africa was reported (Roh and Lawson, 1992). Controlled
flowering as influenced by temperatures during bulb storage
and greenhouse forcing of Lachenalia and Ornithogalum and
four Iridaceae that include Freesia and Sparaxis (Ehrich et al.,
2009) was investigated. Both storage of bulbs and forcing in the
greenhouse require low temperatures around 10°12.5 °C to
A
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Corresponding author. Tel.: +82 41 5503642; fax: +82 41 5633643.
E-mail address: Suh6971@dankook.ac.kr (J.K. Suh).
0254-6299/$ - see front matter © 2010 SAAB. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.sajb.2010.12.007
produce quality plants with optimum flower bud development
and inflorescence elongation (Roh, 2004; Roh et al., 1998; Roh
and Hong, 2007). A protocol for year-round forcing of
Lachenalia hybrid was also developed (Roh et al., 1995).
Flowering time of W. laccata in nature is from September to
November following low temperature and the winter-rainfall
season. Forcing Ixia hybrids at 18 °C day/10 °C night in
conjunction with paclobutrazol treatment produced attractive
plants (Wulster and Ombrello, 2000). Flowering of Freezia laxa
(Thunb.) Goldblatt & J.C. Manning and other South African
Iridaceae was mainly controlled by temperature at 13 °C at
night with day temperature at 17 °C or above, and dormancy
could be maintained and flowering was delayed at temperature
above 20 °C (Ehrich et al., 2009). Therefore, we hypothesized
that flowering could be affected by temperatures ranging from
10° to 20 °C. However, no information on how temperatures
affect these developments is in Watsonia available at present.
Using corms of clonal W. laccata population, several experi-
ments were planned to understand the effect of forcing
temperature on growth, flowering, and corm production, of
temperature during corm enlargement before harvest and during
forcing, of forcing temperature and the duration, of different
duration of forcing temperature, and of bulb storage after
harvest and forcing temperature.
2. Materials and methods
2.1. Source of plant material and general culture
Watsonia laccata corms collected from Bredasdorp, Cape
Province, South Africa, were received from Missouri Botanical
Garden (Goldblatt 4855; St. Louis, MO) in 1987, and multiplied
at the US Dept. of Agriculture, Agricultural Research Service,
Floral and Nursery Plants Research Unit, Beltsville, MD, USA
until 1996. During the corm multiplication years, plants
showing color break symptoms similar to the tulip color breaks
at anthesis were removed. During the multiplication period for
corms, greenhouse temperatures were maintained at 21 °C
during the day (08:00 HR16:00 HR) and 15.6 °C (21°/
15.6 °C) during the night, although day temperatures during
summer exceeded 28°32 °C. Three corms were potted per
15 cm pot in a growing medium (soil:perlite:peat moss, 1:1:1 by
volume) and 0.8 grams of a slow release fertilizer 14N-6P-8.1K
was applied at planting and supplemented with 200 ppm N from
a 20N-8.6P-11.7K water soluble fertilizer once a month. In all
experiments, data was subjected to analysis of variance, and
means were compared by Duncan's Multiple Range Test using
SAS Software (Statistical Analysis System, 2002). The number
of days to leaf emergence and flowering was counted from the
potting day.
2.2. Effect of forcing temperature on growth, flowering, and
corm production (Expt. 1)
Corms were planted, one corm (810 cm in circumference)
per 10 cm pot, on Sept. 20, 1994. Temperature treatments
started on Sept. 22. Greenhouse temperatures in air-conditioned
greenhouses were maintained at 18°20°/15°17 °C, 21°23°/
18°20 °C, 24°26°/21°23 °C, and 27°29°/226 °C. Date
of flowering was recorded at anthesis of the first florets, scape
plus inflorescence length was measured, and the number of
florets was counted. At anthesis, the presence of a second and
third inflorescence from each corm was recorded. On Apr. 3, all
plants were moved to a greenhouse at 21°/15.6 °C and watered
once a week until leaves were dried. Harvested corms were
stored at a constant 20 °C until grading and used in other
experiments. There were 24 plants planted singly in 10-cm pot
per treatment, each plant being treated as an experimental unit in
a completely randomized design.
2.3. Effect of temperature during corm enlargement before
harvest and during forcing (Expt. 2)
Corms grown in greenhouses as described in Expt. 1 were
harvested following the schedule indicated (Table 1), and stored
at 20 °C until planting on Sept. 27, 1995. The size of corms was
79 cm in circumference and the leaf length varied from b1cm
(corms grown at 30°32°/27°29 °C) to 23 cm (corms grown
at 21°23°/18°20 °C and 24°26°/2°23 °C). The depth of
corm planting was about 23 cm. Following planting, pots were
placed in air conditioned greenhouses maintained at 18°20°/
15°17 °C, 21°23°/18°20 °C, 24°26°/21°23 °C, and
27°29°/24°26 °C. Experimental design was a 4 (tempera-
tures during corm enlargement) × 4 (temperatures during
forcing) factorial design with 24 plants per treatment. Date of
leaf emergence was recorded, and at anthesis, length of leaves,
scapes, and inflorescences, the number of florets, and the length
of leaf tip burn showing severe symptoms were recorded and tip
of leaves expressing burn symptoms were collected. When
leaves were longer than 3 cm, the date of leaf emergence was
counted from the potting date.
2.4. Effect of different duration of temperature treatment on
growth and flowering (Expt. 3)
Corms (56 cm in circumference) enlarged in a greenhouse
maintained at 21°23°/18°20 °C were used. Corms were
potted on Sept. 28, 1995, and received constant 18°20°/15°
17 °C or constant 27°29°/24°26 °C for 12 weeks. Some pots
received 2, 4, 6, 8, and 10 weeks at 18°20°/15°17 °C and
then were moved to a greenhouse maintained at 27°29°/24°
26 °C for 10, 8, 6, 4, and 2 weeks, respectively, for a total of
12 weeks. Following temperature treatments, all corms were
forced in a greenhouse maintained at 21°23°/18°20 °C.
Table 1
Forcing temperature and subsequent handling of plants for harvesting corms in
1994 (Expt. 1).
Forcing temperature
(°C)
Beginning of
drying plants
Last watering Corm harvest
2123/1820C Apr. 3 May 2 May 16
2426/2123C Apr. 3 May 16 May 30
2729/2426C Apr. 3 May 31 June 13
3032/2729C Apr. 23 July 14 July 14
632 J.K. Suh et al. / South African Journal of Botany 77 (2011) 631637
There were 18 plants per treatment. Date of flowering, the
length of leaf, scape, and inflorescence, and total number of
florets were recorded.
Three to 5 plants grown in a 18°20°/15°17 °C greenhouse
were sampled to observe the development of inflorescences and
development of the new bulb unit under a stereoscope and a
scanning electronic scope (SEM) as described (Roh et al., 1998)
on Nov. 8, 17, and 24 and on Dec. 20.
2.5. Effect of different forcing temperature (Expt. 4)
Bulbs grown and harvested as described in Expt. 3 were
used, and after potting on Oct. 6, they were placed in a
greenhouse maintained at 18°20°/15°17 °C (low) or at 27°
29°/24°26 °C (high). Treatment details are shown in Table 5.
On Jan. 8, 1996, all plants were moved to a 21°/18 °C
greenhouse until data collection at anthesis, as described in
Expt. 3.
3. Results
3.1. Effect of forcing temperature on growth, flowering, and
corm production
Plants forced at 18°20°/15°17 °C were the earliest
flowering in 107 days from all plants forced (Table 2). As
forcing temperatures were increased, flowering was signifi-
cantly delayed, particularly when forced at 24°26°/21°23 °C.
No plants forced at 24°26°/21°23 °C flowered during
forcing, but flowered in 259 days only after they were moved
to a 21°/15.6 °C greenhouse on Apr. 3. Plants forced at 21°
23°/18°20 °C produced the longest leaves (66.0 cm). The
number of florets was significantly higher when forced at 21°
23°/18°20 °C (21 florets) and 24°26°/21°23 °C (20 florets)
as compared with the number of florets when plants were forced
at either lower or higher temperatures. Plants that were forced at
18°20°/15°17 °C produced the second inflorescence from all
plants and third inflorescences from 15 plants out of 24 plants.
However, plants forced at 24°26°/21°23 °C or higher did not
produce any second or third inflorescences.
The general appearance of plants on April 3 is shown in
Fig. 1. Formation of corms was accelerated when forcing
temperatures were lower than 24°26°/21°23 °C and when
harvested on July 12. Poor corm formation could be due to a
short period between flowering and corm harvest. Three to eight
corms were produced per corm when forced at 18°20°/15°
17 °C (data not presented, Fig. 1).
3.2. Effect of temperature during corm enlargement before
harvest and during forcing
Leaf emergence was in 32 days when corms were enlarged at
21°23°/18°20 °C and forced at 18°20°/15°17 °C and
emergence was significantly earlier than when corms were
formed at 24°26°/21°23 °C which took 6 to 9 days,
depending on the forcing temperatures (Table 3). Flowering
was the earliest when corms were produced at 21°23°/18°
Table 2
Growth and flowering response of W. laccata as influenced by forcing temperatures.
Forcing Temperature (°C) Days to flower Flowering percentage Inflore-scence length (cm) No. of florets Presence of second,
third inflorescence
1820/1517 107 d
z)
24/24 54 b 18 b 24/24, 15/24
2123/1820 121 c 24/24 66 a 21 a 17/24, 5/24
2426/2123 185 b 13/24 56 b 20 a 0/24, 0/24
2729/2426 259 a 20/24 49 c 17 b 0/24, 0/25
Level of significance 11.0 5.8 2.6
z)
Mean separation within columns by DMRT at 5% level. Means with the same letter in the same column are not significantly different.
Fig. 1. Appearance of plants as influenced by forcing temperatures (A). Burn
symptom is observed at the tip of the leaves. Corm development is shown at the
time of drying of plants (B). Plants were grown in air-conditioned greenhouses
maintained at (1) 18°20°/15°17 °C, (2) 21°23°/18°20 °C, (3) 24°26°/
21°23 °C, and (4) 27°29°/24°26 °C.
633J.K. Suh et al. / South African Journal of Botany 77 (2011) 631637
20 °C and were forced at 18°20°/15°17 °C or at 21°23°/
18°20 °C, which took 144 and 152 days, respectively.
Flowering was significantly delayed when corms were forced
at 27°29°/24°26 °C. Flowering was significantly delayed as
corm production and forcing temperatures were increased.
There was a 30-day difference in flowering between corms
produced at 21°23°/18°20 °C and forced at 18°20°/15°
17 °C vs. corms produced at 30°32°/27°29 °C forced at 27°
29°/24°26 °C.
When corms were produced at 30°32°/27°29 °C, average
leaf length decreased to 34 cm from 36 cm, but length was not
significantly affected by forcing temperatures (Table 3). The
effect of temperatures during forcing on the scape length was
profound as compared with those during corm production.
Scape length was increased significantly as forcing tempera-
tures increased from 18°20°/15°17 °C to 27°29°/24°
26 °C. There was no interaction between temperatures during
corm production and forcing for scape length. The inflorescence
length varied from 16.8 cm (21°23°/18°20 °C corm produc-
tion and 21°23/18°20 °C forcing temperature) to 24.1 cm
(30°32°/27°29 °C corm production and 24°26°/21°23 °C
forcing temperature). The incidence of leaf tip burning was
affected significantly by forcing temperatures and also by
interaction with corm production temperatures, showing longer
tip burn at higher forcing temperatures. The number of florets
was significantly affected by both corm production and forcing
temperatures, ranging from 19 to 25.
3.3. Effect of different duration of temperature treatment on
growth and flowering
Flowering was earliest at 127 days when plants received
10 weeks at 18°20°/15°17 °C followed by 2 weeks at 27°
29°/24°26 °C. As the duration at 18°20°/15°17 °C in-
creased, flowering was accelerated (Table 4). Less than
134 days were required to flower when forced at18°20°/
15°17 °C for 8 weeks or longer durations. The length of leaf,
scape, and inflorescence was significantly shorter when forcing
Table 3
Effect of temperature during corm formation before harvest and during forcing on growth and flowering of W. laccata.
Temperature (°C) No. of days to Length (cm) of No. of Floret
corm multiplication greenhouse forcing leaf emergence flower leaf Scape inflore-scence leaf burn
2123/1820 1820/1517 32 144 40 28 16 0.03 23
2123/1820 43 152 45 25 17 0.11 22
2426/2123 43 170 47 31 22 0.19 25
2729/2426 32 184 43 32 21 0.13 21
2426/2123 1820/1517 6 147 40 30 18 0.14 23
2123/1820 9 161 40 29 18 0.05 23
2426/2123 8 175 40 29 22 0.03 24
2729/2426 6 186 40 32 20 0.31 23
2729/2426 1820/1517 12 151 38 29 19 0.04 24
2123/1820 12 153 38 28 20 0.04 23
2426/2123 22 183 35 27 20 0.13 21
2729/2426 20 189 33 32 18 0.45 19
3032/2729 1820/1517 21 137 35 30 21 0.01 24
2123/1820 21 157 36 28 24 0.01 24
2426/2123 19 173 34 30 24 0.04 23
2729/2426 18 184 34 34 20 0.88 23
Least significant difference 3.8 12 3.7 4.4 4.2 0.31 2.7
Corm multiplication (CM) ***
z)
*** *** * *** NS **
Greenhouse forcing (GF) * *** *** *** *** *** ***
CM × GF *** NS *** NS ** *** ***
z)
NS, *, **, ***: Non-significant or significant at p 0.05, 0.01 or 0.001, respectively.
Table 4
Effect of different durations of temperature treatment on growth and flowering of W. laccata.
No. of weeks at (°C) No. of days to flowering Length (cm) No. of florets
1820/1517 2729/2426 Leaf scape Inflore-scence
0 12 175 a 33.5 b
z)
33.8 a 14.7 b 22.3 a
2 10 177 a 38.0 a 34.3 a 18.2 a 20.6 ab
4 8 167 b 36.9 a 34.9 a 18.0 a 19.6 b
6 6 143 c 39.1 a 22.5 d 14.1 b 22.1 a
8 4 134 d 34.5 b 26.1 c 15.2 b 22.0 a
10 2 127 e 30.6 c 30.7 b 17. 0 a 21.9 a
12 0 134 d 38.6 a 36.0 a 18.4 a 19.9 b
Least significant difference 4.60 3.09 4.46 2.63 2.32
z)
Mean separation within columns by DMRT at 5% level. Means with the same letter in the same column are not significantly different.
634 J.K. Suh et al. / South African Journal of Botany 77 (2011) 631637
at 18°20°/15°17° for 6 to 8 weeks than when forcing for
other durations. Although the number of florets was affected by
treatments, the significant difference was only 2.7 florets (a
range of 22.3 florets to 19.6 florets).
No evident scape elongation was observed 41 days after
potting when 5 plants grown at 18°20°/15°17 °C were
observed under a dissecting stereoscope. An inflorescence with
25 florets was observed 50 days after potting, and 46 layers
of florets were evident with shoot apices of the daughter corms
visible. Scape and inflorescence was about 1.8 and 0.9 cm long,
respectively, with clear enlargement of daughter corms visible
83 days after potting. Formation of terminal florets with bracts,
and tepal was evident in a scanning electron microscope
photograph (Fig. 2).
3.4. Effect of different 2-week of temperature during forcing
When forced at 18°20°/15°17 °C (low; L) or at 27°29°/
24°26 °C (high; H) for 12 weeks, flowering was the earliest
when forced at low temperature, which took 134 days, as
compared with forcing at high temperature, which took
187 days (Table 5). Although flowering was not accelerated
by a 2 or 4 weeks of low temperature given at the beginning of
the forcing period (184 or 164 days to flower, respectively),
flowering was accelerated by a 2 or 4 weeks of high temperature
treatment (142 or 150 days to flower, respectively). As the
durations of low or high temperature treatment increased,
flowering was accelerated or delayed, respectively. Accelera-
tion of flowering by low temperature treatment was not
observed when low temperature treatment was given for
8 weeks or longer. Flowering acceleration by low temperatures
given prior to a high temperature treatment period was more
significant than by low temperatures following a high
temperature treatment period. However, extended high temper-
ature treatments for 6 weeks or longer prior to low temperature
treatments did not accelerate flowering, but rather delayed
flowering.
Leaf length was increased to 37 or 38 cm, as the durations of
high temperature exceeded 10 or 8 weeks following 2 or
4 weeks of low temperature, respectively. In general, the scape
was longer when forced at high temperature, but was the
shortest (16 cm) when forced at 6 weeks each of low followed
by high temperature. Following 12 weeks of low temperature
treatment, scape length became longer (35 cm), but did not
differ from those that received only 2 weeks of low temperature
at the beginning of forcing (37 cm). No significant differences
were found in inflorescence length that ranged from 14 to 20 cm
when plants were forced initially at low temperatures regardless
of the duration. The number of florets was significantly
increased as the duration of low temperature treatment was
increased from 2 weeks (18 florets) to 12 weeks (24 florets).
Forcing at high temperatures longer than 10 weeks reduced the
number of florets.
4. Discussion
Very limited information on the requirement of temperature
during various growth and developmental stages of Watsonia is
Fig. 2. Development of the inflorescence showing flower buds (FB 1; terminal,
FB2, and FB3), bracts (B1, B2, and B3) sustaining flower buds, and tepal (*, **)
photographed in December 20.
Table 5
Treatment details and growth and flowering responses of W. laccata.
Temperature treatment
z)
(2-wk interval) No. of days to flower Length (cm) of No. of florets
02244668810 1012 leaf scape Inflore-scence
LHHHH H 184a
y)
37ab 37 a 18 bcf 18 d
L L H H H H 164 c 38 a 24 d 20 abdef 21 c
LLLHH H 148ef 35bc16e14f 21c
LLLLH H 138gh 33c22d15ef 21c
LLLLL H 140gh 33c29c16def 22bc
LLLLL L 134h 30d35ab17def 24a
HLLLL L 142fg 31d33ab17cde 23abc
H H L L L L 150 e 33 c 32 bc 20 abc 24 ab
HHHLL L 156d 33c35ab20ab 23abc
HHHHL L 163c 33c31bc22a 21abc
HHHHH L 174b 34c35ab17cde 21c
HHHHH H 187a 35bc35ab16def 18d
Least significant difference 8.0 2.96 5.01 3.20 2.63
z)
Low temperature: (L: 18°20°/15°17 °C) or high temperature: (H: 27°29°/24°26 °C).
y)
Mean separation within columns by DMRT at 5% level. Means with the same letters in each column are not significantly different.
635J.K. Suh et al. / South African Journal of Botany 77 (2011) 631637
available: however, the use of Paclobutrazol inhibits growth of
W. tabularis (Thompson et al., 2005) and smoke treatment
promotes flowering of W. borbonica (Pourret) Goldblatt (Light
et al., 2007). However, to our knowledge, flowering of
Watsonia, occurring typically from August to November in
nature has not been studied with regard to an optimum
temperature requirement during bulb storage and forcing.
Temperatures around Cape Town, South Africa average around
12 °C with 105 mm rainfall in June and July, and W. laccata
flowers naturally following this period, and therefore is
considered a species occurring in the winter-rainfall regions.
4.1. Early flowering by forcing at 18°20°/15°17 °C
Plants forced at 18°20°/15°17 °C flowered earliest in all
experiments. This is close to the maximum temperature 20.5 °C
(Ascough et al., 2007a) at the native site. It is not known
whether flowering can be accelerated if corms are stored at low
temperatures and forced at temperatures lower than 18°20°/
15°17 °C. However, Watsonia is considered a cool season
crop for early forcing. Flowering of Lachenalia aloides Engl.
Pearsoniiis also accelerated by storing at low temperature and
forcing at high temperature (Roh et al., 1995). Temperatures
higher than 21°23°/18°20 °C during the entire forcing period
should be avoided to assure 100% flowering (Expt. 1) and to
have the second and third inflorescence formed. This clearly
suggests that flower formation is favored at low temperatures
and flowering is delayed at temperatures above 20 °C when
dormant corms are supplied from South Africa to Germany are
forced (Ehrich et al., 2009) and in vitro induced corms of W.
vanderspuyiae when maintained in greenhouse maintained at
temperatures of 16°24 °C (Ascough et al., 2008).
Flowering time of W. laccata is also affected by the
environment during corm production in the greenhouse
conditions which may be useful information to select corm
production sites other than the Cape area with similar
environments for in the field. It seems that producing corms
at temperatures higher than 24°26°/21°23 °C may delay leaf
emergence, but not flowering after leaf emergence. Therefore,
corms can be produced at high temperatures and forced at low
temperature (18°20°/15°17 °C). It is, therefore, that flower-
ing of W. laccata is favored at low temperatures and may not
require cold treatment before planting corms as summarized in
Freesia (Rees, 1992). Since temperatures lower than 18°20°/
15°17 °C, i.e., at 10° to 13 °C, were not tested, the critical
limit of low temperatures favoring the floral initiation and
development in W. laccata is unknown.
4.2. Further flowering acceleration by forcing for 2 or 4 weeks
of 27°29°/24°26 °C followed by forcing at 20°/15°17 °C
For early flowering, plants could be forced longer than
8 weeks at 18°20°/15°17 °C. During this period, flower bud
initiation, development, and anthesis is completed, but it is not
known how long it takes to initiate flower buds. Flower bud
initiation may be favored by 2 or 4 weeks of high temperatures
(27°29°/24°26 °C) prior to forcing at low temperatures (18°
20°/15°17 °C). Storing corms at high temperatures (27°29°/
24°26 °C) for 2 or 4 weeks may not induce dormancy of fresh-
harvested corms and it is most likely that Watsonia corms do not
require dormancy breaking temperature at low temperature.
More frequent sampling and observation using SEM will
determine when florets are initiated. Longer than 6 weeks of
either low or high temperatures upon potting affects flowering
time significantly. Therefore, to accelerate or delay flowering,
plants should be forced either at low or high temperatures,
respectively, for 8 weeks.
The number of florets was not significantly affected by corm
storage, forcing temperatures, or their interaction, although
forcing at high temperatures tend to reduce the number.
Although the number of florets when forced for 12 weeks at
low and high temperatures was affected significantly by
treatments, the difference which was only 2.7 florets (22.3
florets by forcing at high temperature vs. 19.6 florets by forcing
at low temperature) may not be significant horticulturally. It is
evident that, in a similar repeated experiment, the number of
florets is increased by forcing at low temperature (18 vs. 24
florets and also 23 vs. 18 florets).
The reduction may result from a premature termination of
floral initiation, or from a malformation such as floret abortion
which was also reported in Iridaceae (Ehrich et al., 2009.
Inflorescence blast in Lachenalia aloides occurs when bulbs
stored at 10°15 °C are forced at 26°/24 °C (Roh et al., 1995;
Roh et al., 1998). We conclude that the initiation and
development of W. laccata florets is not influenced by corm
storage and forcing temperatures. It is not known whether
flower malformations such as abortion or blast could be induced
by low irradiance as reported with four Iridaceae plants that
include Freesia laxa under the European conditions (Ehrich et
al., 2009).
4.3. Leaf tip burn incidence
The incidence of tip burn occurring on leaves is a concern if
W. laccata is to be considered for pot plant production even
though the symptom is less than 0.5 cm. Forcing at different
temperatures did not affect symptom incidence. It is not known
whether there are other Watsonia species that do not show tip
burn symptoms nor is the cause of the symptom known.
Nutrient analysis of growing medium and tissues during leaf
development should be performed to determine if symptom
expression is related to a deficiency or toxicity of nutrient
elements. Preferential accumulation of boron and manganese at
the tips of old Curcuma hybrid, Chiangmai Pridewas reported
(Roh and Lawson, 2009).
5. Conclusion
Flowering of W. laccata is accelerated and promoted by
forcing at low temperatures around 18°20°/15°17 °C for
8 weeks followed by 2 to 4 weeks of 27°29°/24°26 °C. Corm
formation is favored at low temperatures, resulting from an
early initiation of daughter corms following flowering. No
malformation of florets was found when plants were forced at
636 J.K. Suh et al. / South African Journal of Botany 77 (2011) 631637
high temperatures, and the length of scape and inflorescence
was not affected significantly by forcing temperature. Further
studies are required to understand the cause of the tip burn
incidence and how to correct tip burn if it is related to the
nutritional status of the leaves.
Acknowledgements
We thank Dr. Peter Goldblatt, Missouri Botanical Gardens,
St. Louis, MO, USA who provided corms used in these
experiments. Critical review of this manuscript and revising the
manuscript to improve the readability by G. Wulster, R.
Anderson, and L. Stephens, and R. Lawson and comments from
anonymous reviewer is greatly appreciated.
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... Therefore, LTB in Ornithogalum is a unique physio logical expression resulting from high B concentra tion at the leaf tips and developing regardless of the leaf age; these symptoms were observed in O. thyr-soides and O. arabicum (Roh, personal observation). The incidence of LTB in Watsonia laccata (Jacquin) Ker Aawler is considered a similar disorder as in O. dubium and adversely affects potted plant produc tion in W. laccata (Suh et al., 2011). Leaf tip scorch observed in O. dubium (De Hertogh and Gallitano, 1997), upper leaf necrosis observed in Oriental hybrid lilies (Chang et al., 2008), and burn appearing at certain developmental stages before the visible bud stages in Lilium 'Pirate' (Berghoef, 1986) ...
... Regardless of the temperature treatments during the preplanting bulb (STA) and postplanting forcing (STB and STC) periods, the LTB symptoms were not entirely eliminated in O. dubium, which was in agree ment with the results for W. laccata (Suh et al., 2011). In our experiment, the number of leaves showing LTB was 1.1 out of 5.1 leaves which was the fewest when received 10°C/STA, followed by forcing at 15/12°C/STB and 21/18 o C/STC (Table 1). ...
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