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J. AMER.SOC.HORT.SCI. 138(3):167–172. 2013.
A Moderate to High Red to Far-red Light Ratio
from Light-emitting Diodes Controls Flowering
of Short-day Plants
Daedre S. Craig
1
and Erik S. Runkle
2
Department of Horticulture, 1066 Bogue Street, Michigan State University, East Lansing, MI 48824
ADDITIONAL INDEX WORDS.LEDs, long days, phytochrome, protected cultivation
ABSTRACT. In protected cultivation of short-day (SD) plants, flowering can be inhibited by lighting from incandescent
(INC) lamps during the night. INC lamps are being phased out of production and replaced by light-emitting diodes
(LEDs), but an effective spectrum to control flowering has not been thoroughly examined. We quantified how the red
[R (600 to 700 nm)] to far red [FR (700 to 800 nm)] ratio (R:FR) of photoperiodic lighting from LEDs influenced
flowering and extension growth of SD plants. Chrysanthemum (Chrysanthemum ·morifolium), dahlia (Dahlia
hortensis), and african marigold (Tagetes erecta) were grown at 20 8C under a 9-hour day with or without a 4-hour
night interruption (NI) treatment by INC lamps or LEDs with seven different R:FR ranging from all R to all FR.
Flowering in the most sensitive species, chrysanthemum, was not inhibited by an R:FR of 0.28 or lower, whereas an
R:FR of 0.66 or above reduced flowering percentage. Flowering in dahlia was incomplete under the FR-only NI and
under SDs, but time to flower was similar under the remaining NI treatments. The least sensitive species, african
marigold, flowered under all treatments, but flowering was most rapid under the FR-only NI and under SDs. For all
species, stem length increased quadratically as the R:FR of the NI increased, reaching a maximum at R:FR of ’
’0.66.
We conclude that in these SD plants, a moderate to high R:FR (0.66 or greater) is most effective at interrupting the
long night, blue light is not needed to interrupt the night, and FR light alone does not regulate flowering.
Many plants exhibit a photoperiodic flowering response,
including a broad range of field and ornamental crops (Erwin
and Warner, 2002; Mattson and Erwin, 2005; Runkle and Heins,
2003). This photoperiodic response is determined primarily by
the duration of the dark period, also known as the critical night
length (Thomas and Vince-Prue, 1997). Plants have been classi-
fied into photoperiodic response groups depending on how the
critical night length influences flowering. Short-day plants
(SDPs) flower most rapidly when uninterrupted dark periods
are longer than some genotype-specific critical night length
(Vince, 1969). Within the SDP response category, plants can be
further classified based on whether SDs are required for flower-
ing (a qualitative response) or hasten it (a quantitative response).
Photoperiodic (low-intensity) lighting is used by commercial
crop producers to alter the natural photoperiod (e.g., to extend
the natural daylength or to interrupt the dark period) to mani-
pulate flowering.
The spectral quality of photoperiodic lighting can influence
flowering responses. Light quality is perceived by three identi-
fied families of plant photoreceptors: cryptochromes, ultraviolet
receptors, and phytochromes (Kami et al., 2010). Cryptochromes
have been identified in many plant species and mediate a variety
of light responses, including playing a role in flowering time
regulation in arabidopsis [Arabidopsis thaliana (Cashmore et al.,
1999; Mockler et al., 2003)]. The phytochrome photoreceptors
mediate extension growth and flowering in photoperiodic plants
(Smith, 1994). Five types of phytochrome have been identified in
arabidopsis and designated A to E (Kami et al., 2010). Studies
with phytochrome mutants of arabidopsis have shown that phyA
and phyB play dominant roles mediating flowering and stem
extension, respectively, in response to light quality (Franklin
and Quail, 2010). Phytochrome exists in a R (600 to 700 nm;
peak absorption at 660 nm) and a FR (700 to 800 nm; peak
absorption at 730 nm) absorbing form, P
R
and P
FR
,respectively
(Hayward, 1984; Sager et al., 1988). The R:FR incident on the
plant influences the phytochrome photoequilibria (P
FR
/P
R+FR
)
within the plant. On absorbing R light, P
R
converts mainly to the
P
FR
form. The P
FR
form largely converts back to the P
R
form on
absorbing FR light or during a natural, gradual conversion during
the dark period (Thomas and Vince-Prue, 1997). Although the
total pool of phytochrome in the plant is relatively constant, be-
cause natural light environments are ever-changing, the relative
amounts of P
FR
and P
R
, and thus the overall P
FR
/P
R+FR
,also
fluctuate throughout the day.
In photoperiodic crops, the P
FR
/P
R+FR
, through different
types of phytochromes, influences flowering. P
FR
is the active
form of phytochrome, which translocates to the nucleus on receiv-
ing light signals and activates downstream pathways (Franklin
and Quail, 2010). Under a long, uninterrupted night, the P
FR
form of phytochrome slowly converts to the P
R
form, leaving
insufficient P
FR
to inhibit flowering. However, if R light is
provided during the long night, P
R
is converted to P
FR
(creating
agreaterP
FR
/P
R+FR
), which inhibits flowering in SDPs. The
P
FR
/P
R+FR
also influences extension growth, especially in
shade-avoiding plants.
Incandescent lamps are commonly used as photoperiodic
lighting to control development of crops, because they emit an
effective spectrum and are inexpensive. However, INC lamps are
very energy-inefficient and are being phased out of production
in many parts of the world (Waide, 2010). LEDs are an attractive
Received for publication 12 Dec. 2012. Accepted for publication 20 Mar. 2012.
We gratefully acknowledge funding by the USDA National Institute of Food
and Agriculture’s Specialty Crop Research Initiative (Grant 2010-51181-
21369), Michigan’s plant agriculture initiative at Michigan State University
(Project GREEEN), and horticulture companies providing support for Michigan
State University floriculture research. We also thank Mike Olrich for his
greenhouse technical assistance and Catherine Whitman for her input in the
preparation of the manuscript.
1
Former Graduate Student.
2
Corresponding author. E-mail: runkleer@msu.edu.
J. AMER.SOC.HORT.SCI. 138(3):167–172. 2013. 167
technology for NI lighting of photoperiodic crops. Compared
with conventional lamps, LEDs have many desirable charac-
teristics including a very long operating life, narrow bandwidth
capability, full instantaneous irradiance when powered, and
continually improving electrical efficiencies (Bourget, 2008;
Morrow, 2008). Furthermore, LEDs allow researchers to analyze
the effects of specific wavebands without extraneous light. Many
of the original studies on photoperiodic light quality were limited
by the lighting technology of the time. The use of photoselective
filters and tinted lamps may have introduced confounding vari-
ables into these early experiments such as differences in photon
flux between treatments and/or inclusion of potentially con-
founding, extraneous wavelengths (Borthwick et al., 1952; Cathey
and Borthwick, 1957; Downs, 1956).
The objectives of the present study were to use LEDs to
quantify the impact of the R:FR of NI lighting on flowering of
SD ornamental crops and to compare plant responses with those
under traditional INC lamps. To our knowledge, this is the first
study that has identified how R:FR ratios control the flowering
response of SDPs without the confounding effects of other light
wavebands.
Materials and Methods
PLANT MATERIAL AND CULTURE.On 8 Feb. 2011, 7-to 10-d-old
seedlings ofafrican marigold ‘American Antigua Yellow’ grown
in 288-cell (6 mL) plug trays and rooted cuttings of chrysanthe-
mum ‘Adiva Purple’ and dahlia ‘Dahlinova Figaro Mix’ grown
in 36-cell (32 mL) liner trays were received from a commercial
greenhouse (C. Raker & Sons, Litchfield, MI). The young plants
were subsequently grown under non-inductive long days [natural
daylength extended from 0600 to 2200 HR by high-pressure
sodium (HPS) lamps] in a research greenhouse at 20 C until
transfer to the NI treatments.
African marigold and dahlia were transferred to NI treat-
ments on 14 Feb. and chrysanthemum on 25 Feb. On transfer,
10 plants per treatment of each species were
transplanted into 10-cm (430 mL) round
plastic pots containing a commercial peat–
perlite medium (Suremix; Michigan Grower
Products, Galesburg, MI). All species were
thinned to one plant per pot on the day of
transplant. The experiment was repeated in
the spring with the same treatments and
greenhouse environment as previously de-
scribed. Dahlia ‘Dahlinova Figaro Mix’ was
replaced by dahlia ‘Carolina Burgundy’,
which were propagated by stem cuttings
harvested from plants received from C. Raker
&Sonson21Apr.Chrysanthemumsfromthe
first replicate of the experiment were grown
as stock plants under long days (LDs), and
cuttings were subsequently harvested and
rooted for the second replicate. Chrysanthe-
mum and dahlia shoot-tip cuttings (two or
three nodes) were rooted in 51-cell liner trays
filled with 50% Sure-mix and 50% screened
coarse perlite (Therm-O-Rock East, New
Eagle, PA). Cuttings were rooted under LD
in a propagation greenhouse as described by
Lopez and Runkle (2008). For the second
replicate, african marigold was received and
placed in NI treatments on 26 May and chrysanthemum and
dahlia were transferred on 7 July.
LED LAMPS AND NI TREATMENTS.Opaque black cloth en-
closed all greenhouse benches from 1700 to 0800 HR, creating
a 9-h SD. One bench was designated the SD control bench.
Above the remaining benches, NI lighting was delivered from
2230 to 0230 HR by either 40-W INC lamps or customized LED
fixtures containing three R and/or FR LED diodes per lamp
developed by CCS Inc. (Kyoto, Japan). Lamps were paired to
produce a total of six diodes and thus, seven R:FR ratios were
created (Fig. 1). The R and FR LEDs had peak wavelengths of
660 nm and 735 nm, respectively, which correspond with peaks
of phytochrome absorption (Sager et al., 1988). Because the
photon flux from the R LEDs was approximately twice that
from the FR LEDs, all R diodes were filtered with two layers of
aluminum mesh.
Light spectra under each treatment were measured by two
portable spectroradiometers [LI-1800 (LI-COR, Lincoln, NE)
and PS-200 (StellarNet, Tampa, FL)]. Spectral measurements
were taken at regular intervals across the bench area of each
treatment. Mean photon flux from 600 to 800 nm was 1.3 to
1.6 mmolm
–2
s
–1
for all NI treatments, and plants were positioned
on benches only where the photon flux was 0.7 mmolm
–2
s
–1
or
greater. The R:FR was measured and described using 100- or
10-nm-wide wavebands and the phytochrome photoequilibria
(P
FR
/P
R+FR
) was calculated for each treatment following Sager
et al. (1988) (Fig. 1).
GREENHOUSE ENVIRONMENT.The experiment was conducted
in a glass-glazed, environmentally controlled greenhouse at a
constant temperature set point of 20 C. In late April, whitewash
was applied externally to the greenhouse glazing to reduce light
transmission by 30% to 40% and, thus, decrease radiant heating.
All treatments received supplemental lighting from 0800 to
1600 HR provided by HPS lamps delivering a photosynthetic
photon flux (PPF)of60to90mmolm
–2
s
–1
at plant height. The
HPS lamps were operated by an environmental control computer
Fig. 1. Light quality emitted from incandescent and light-emitting diodes (LEDs) between 600 and 800
nm. The number of red (R) and far-red (FR) diodes per lamp pair is indicated for each LED treatment.
R to far-red FR ratios and estimated phytochrome photoequilibria (P
FR
/P
R+FR
) values (Sager et al.,
1988) for the night interruption treatments are given in the inset table. R:FR
wide
equals 600 to 700:
700 to 800 nm; R:FR
narrow
equals 655 to 665: 725 to 735 nm.
168 J. AMER.SOC.HORT.SCI. 138(3):167–172. 2013.
andwereswitchedonwhentheambientPPF outside the
greenhouse was less than 185 mmolm
–2
s
–1
, and switched off
when ambient PPF was greater than 370 mmolm
–2
s
–1
.Line
quantum sensors (Apogee Instruments, Logan, UT) were posi-
tioned at plant height throughout the greenhouse. The sensors
measured PPF every 10 s, and hourly averages were recorded
by a data logger (CR10; Campbell Scientific, Logan, UT). The
mean photosynthetic daily light integrals were 15.2 and
14.5 molm
–2
d
–1
for the first and second experiment replica-
tions, respectively.
Air temperature was measured on each greenhouse bench by
an aspirated thermocouple [36-gauge (0.127-mm diameter)
type E] every 10 s, and hourly averages were recorded by a data
logger. The actual mean daily temperature was 19.9 and 21.9 C
for the first and second experiments, respectively. When the
nighttime air temperature at bench level was less than 18.9 C,
a 1500-W electric heater, controlled by a data logger, provided
supplemental heat during the night. Plants were irrigated as
necessary with reverse-osmosis water supplemented with a
water-soluble fertilizer providing (milligrams per liter) 125
nitrogen, 12 phosphorus, 100 po-
tassium, 65 calcium, 12 magnesium,
1.0 iron and copper, 0.5 manganese
and zinc, 0.3 boron, and 0.1 molyb-
denum (MSU RO Water Special;
GreenCare Fertilizers, Chicago, IL).
DATA COLLECTION AND ANALYSIS.
Plant height (from media surface to
shoot tip)was measured on the day of
transplant, and nodes were counted
on each plant. The date of first
flowering was recorded; plants were
considered flowering when at least
50% of the ray flowers of an inflo-
rescence were reflexed. At flowering,
the total number of inflorescences
and plantheight and number of nodes
below the first flower (replicate 2
only) were recorded. Plants that did
not have an open flower within 150%
of average flowering time were con-
sidered non-flowering. Time from
transplant to first flower as well as
node number increase were calcu-
lated for each plant. Data were ana-
lyzed with SAS (Version 9.1; SAS
Institute, Cary, NC) and data were
pooled between replications if sta-
tistical interactions between main
effects and replication were not
significant (P$0.05). Regression
analysis was performed with SAS
to relate the data parameters to the
estimated P
FR
/P
R+FR
of the plants
in the NI treatments.
Results
All chrysanthemum plants flow-
ered under the FR-only NI treatment
and under SDs in both replicates
(Fig. 2A). Among the other treatments,
flowering percentage generally decreased with increasing R:FR.
For plants that did flower under an LED NI with a R:FR
wide
0.66
or greater (P
FR
/P
R+FR
0.63 or greater), flowering was delayed by
42 d compared with plants under SDs or FR-only NIs (Fig. 2B).
Similarly, under the INC NI, flowering was delayed by 30 d
compared with under SDs or FR-only NIs. Inflorescence number
was greatest (163 or greater) under a moderate R:FR
wide
and 43
under the FR-only NIs or SDs [Fig. 2C (note that inflorescence
number was divided by 10 in the figure)]. Extension growth of
plants was greater in the second experimental replicate but trends
were similar (Fig. 2D; Table 1). Height increased quadratically
as the R:FR increased to a maximum at R:FR
wide
0.66 (P
FR
/
P
R+FR
0.63 or greater). Plants grown under the FR-only NIs were
4.3 and 7.8 cm shorter than plants under INC NIs in replicates 1
and 2, respectively. Under SDs, extension growth was 8.2 cm
less in replicate 1 and 14.9 cm less in replicate 2 compared with
plants under the INC NIs.
Flowering of dahlia ‘Figaro Mix’ was incomplete under the
FR-only NI and SD treatments (40% and 50%, respectively),
which was surprising because dahlia is considered an SD plant
Fig. 2. (A–L) Influence of the estimated P
FR
/P
R+FR
of night interruption lighting on flowering characteristics and
extension growth of the short-day (SD) plants chrysanthemum ‘Adiva Purple’, dahlia ‘Figaro Mix’ (solid
symbols; replicate 1), dahlia ‘Carolina Burgundy’ (open symbols; replicate 2), and african marigold ‘American
Antigua Yellow’. Single open data symbols indicate pooled data. With the exception of flowering percentage,
associated correlation coefficients (R
2
) are presented. Multiple plots indicate replicate 1 data (solid symbols) and
replicate 2 data (open symbols) with associated R
1
2
and R
2
2
values, respectively. Dotted circle symbols indicate
the incandescent control treatment. Square data symbols indicate the SD control treatment. Data for
chrysanthemum inflorescence number has been divided by 10. NS, *, **, *** indicate nonsignificant or
significant at P#0.05, 0.01, and 0.001, respectively. See Table 1 for regression equations. P
FR
/P
R+FR
=
estimated phytochrome photoequilibria values (Sager et al., 1988).
J. AMER.SOC.HORT.SCI. 138(3):167–172. 2013. 169
(Fig.2E).However,the‘Figaro Mix’ plants that flowered
under the FR-only NI and SD treatments did so slightly earlier
than those under the other LD treatments: flowering was
hastened by 11 and 19 d under FR-only NI and SD, respec-
tively, compared with plants in R:FR
wide
treatments 0.28 or
greater (P
FR
/P
R+FR
0.46 or greater) (Fig. 2F). Inflorescence
number was variable and statistically similar under all
treatments (Fig. 2G). Extension growth exhibited a quadratic
trend and was greatest under moderate R:FR
wide
treatments
(Fig.2H).Heightincreaseof plants grown under FR NIs and
SDs was 5.2 and 10.4 cm less, respectively, than that of plants
grown under the INC NIs.
Flowering of dahlia ‘Carolina Burgundy’ was incomplete
under FR-only NI and SD treatments, whereas nearly all plants
flowered under the other treatments (Fig. 2E). Time to flower
was similar under all NI treatments but 11 d earlier under SDs
(Fig. 2F). Node number at flowering was variable and averaged
from 13 to 18 in all treatments (data not shown). There was a
small, positive correlation between inflorescence number and
the R:FR
wide
of the NI (Fig. 2G). Extension growth of ‘Carolina
Burgundy’ exhibited a quadratic trend and was greatest under
intermediate LED R:FR
wide
values (Fig. 2H).
All african marigold plants flowered under all treatments
(Fig. 2I), but plants in both replications flowered 10 to 20 d
earlier under SDs or the FR-only NI treatment compared with
the other treatments (Fig. 2J). Time to flower under the re-
maining LED treatments (R:FR
wide
0.28 or greater) and under
INC lamps was similar. However, plants under SDs or the FR-
only NI treatment developed five or six nodes from transplant to
flowering, whereas those under the other NI treatments devel-
oped 11 to 13 nodes (data not shown). There was a small nega-
tive correlation between inflorescence number and the R:FR
wide
of the NI in the second experimental replicate (Fig. 2K). Ex-
tension growth of plants grown under the FR-only NI treatment
or under SDs was 3.9 to 5.8 cm less than that of plants under the
other NI treatments (Fig. 2L).
Discussion
In several classic photoperiod studies, flowering of cockle-
bur [Xanthium strumarium (Borthwick et al., 1952; Downs,
1956)], chrysanthemum (Cathey and Borthwick, 1957), and
soybean [Glycine max (Downs,
1956)] could be inhibited by an R
night break, which promotes for-
mation of P
FR
and thus increases
the P
FR
/P
R+FR
. A subsequent FR ex-
posure, however, could reverse the
flowering inhibition imposed by R
light, showing that the inhibition of
flowering in SDPs depends on R
light and the resulting formation
of the P
FR
form of phytochrome
(Thomas and Vince-Prue, 1997).
Although it is well established that
R light is most effective at inhibiting
flowering in SDPs, some plants are
more sensitive than others (Cathey
and Borthwick, 1957; Downs, 1956).
In addition, these classic R:FR stud-
ies used broad-spectrum lamps with
or without photoselective filters,
which could have introduced confounding wavelengths such as
blue light into these experiments.
Like in previous studies (Borthwick et al., 1952; Cathey and
Borthwick, 1957; Downs, 1956), R light was as effective as
INC for flower inhibition among the SDP species we studied.
LED treatments with an R:FR
wide
of 0.66 or greater and the INC
lamps (R:FR
wide
= 0.59) inhibited flowering the most. Therefore,
LEDs with a moderate-to-high R:FR are a viable replacement
for INC lamps to inhibit flowering of SDPs. In addition, be-
cause the LED treatments did not emit blue light, and flower-
ing was similar to that under INC lamps (which emit a small
amount of blue), blue light is apparently not needed to regulate
flowering of these SDPs tested. A variety of crop character-
istics (e.g., internode length, branching, and bud number) can
be influenced using LEDs with different R:FR. However, in
terms of flower inhibition and height control, the NI treatments
that primarily emitted R light were most effective for the SDP
species studied.
Short-day plants differ in their sensitivity to the R:FR and
duration of NI lighting. Only 1 min of 11 mmolm
–2
s
–1
light
from an INC lamp during a long night was needed to inhibit
flowering of cocklebur and soybean (Downs, 1956), whereas
several hours of light at the same irradiance, for multiple cycles,
was needed to inhibit flowering of chrysanthemum (Cathey and
Borthwick, 1957). Chrysanthemum appears to be particularly
sensitive to the light quality of the NI. Flowering can be inhi-
bited by several hours of NI from a fluorescent (FL) or INC lamp
or by 1 min of low-intensity FL light (Cathey and Borthwick,
1957). However, 1 min of high-intensity INC light was not
sufficient to inhibit flowering. The R:FR of INC light is much
lower than that of FL light. Therefore, a brief INC NI converts
less phytochrome to the P
FR
form than would a brief FL NI.
Theoretically, R light is most effective at inhibiting flowering of
SDPs because the high R:FR of FL light is sufficient to convert
enough phytochrome into the P
FR
form to inhibit flowering, even
at low intensity and short duration.
In our study, we also observed variations in sensitivity to the
light quality of the NI. In agreement with Cathey and Borthwick
(1957), chrysanthemum was highly sensitive to the R:FR of the
NI, at least compared with the other species tested. Flowering
of chrysanthemum was inhibited more by NI treatments with
higher R:FR compared with those with lower R:FR. Because
Table 1. Parameters of regression analysis relating days to flower, inflorescence number, and increase
in height to the estimated phytochrome photoequilibrium of plants in the night interruption
lighting treatments.
Species Parameter Replicate Regression equation R
2
Chrysanthemum
‘Adiva Purple’
Days to flower Pooled y = 25.3 + 144.8x – 94.5x
2
0.85***
Inflorescence no. Pooled y = –78.8 + 817.8x – 608.4x
2
0.48***
Height increase 1 y = 7.9 + 61.5x – 49.5x
2
0.57***
Height increase 2 y = 3.6 + 40.5x – 29.9x
2
0.29***
Dahlia ‘Figaro Mix’ Days to flower 1 y = 36.8 + 59.8 – 43.5x
2
0.14***
Height increase 1 y = 2.9 + 48.0x – 42.5x
2
0.19***
Dahlia ‘Carolina
Burgundy’
Inflorescence no. 2 y = 7.5 + 5.0x + 5.6x
2
0.12*
Height increase 2 y = 4.6 + 50.1x – 40.7x
2
0.25***
African marigold
‘American
Antigua Yellow’
Days to flower 1 y = 32.8 + 53.0x – 35.2x
2
0.63***
Days to flower 2 y = 35.0 + 47.3 x – 31.9x
2
0.58***
Inflorescence no. 2 y = 12.0 – 3.1x 0.07*
Height increase 1 y = 3.7 + 26.5x – 21.8x
2
0.46***
Height increase 2 y = 6.4 + 50.2x – 43.6x
2
0.42***
*, *** indicate significant at P#0.05 and 0.001, respectively.
170 J. AMER.SOC.HORT.SCI. 138(3):167–172. 2013.
chrysanthemum is an obligate SDP, one might expect a more
dramatic response to the R:FR than in dahlia or african
marigold (two facultative SDPs). Surprisingly, flowering per-
centage of chrysanthemum grown under the INC NI treatment
was 100 in experimental replicate 1 and 0 in replicate 2. Plants
in the first replicate were received from a commercial grower
and some may have been exposed to inductive photoperiods
before arrival; alternately, we may have had a burned out INC
bulb that went unnoticed for a time sufficient to induce them.
Within our populations of dahlia plants, sensitivity to NI light
quality was variable. Flowering percentage was lowest under
the FR-only NI and SD treatments. Among the remaining
treatments, the effect on flowering time was similar regardless
of the R:FR of the NI. Although these results were unexpected,
a variety of photoperiodic responses have been observed in
dahlia and our SD conditions may not have been optimal for the
cultivars we used. When ‘Royal Dahlietta Yellow’ were grown
under photoperiods ranging from 10 to 24 h, the optimal
photoperiod for flowering was 12 to 14 h (Brøndum and Heins,
1993). Flowering percentage was reduced and flowers devel-
oped abnormally in two cultivars of dahlia grown under 8-h
photoperiods compared with plants grown under a 4-h NI or
16-h photoperiod (Durso and De Hertogh, 1977). Some varieties
require SD for flower induction but LD for optimal flower bud
development (Legnani and Miller, 2001). African marigold ex-
hibited a weakly facultative SD flowering response and was
the least photoperiodic species in our study because all plants
flowered in all treatments, and flowering was delayed similarly
under all NI treatments with an R:FR
wide
0.28 or greater.
Interestingly, flowering percentage and time to flower for
each species were similar under SDs and the FR NI, indicating
that the FR-only NI was largely ineffective and perceived as an
SD. Because R light is most effective at inhibiting flowering
of SDPs, we postulated that as the proportion of R light rela-
tive to FR light increased (as the R:FR increased), inhibition
of flowering in SDPs would progressively increase. Indeed,
thehigherR:FRNItreatmentsweremoreeffectiveandthose
without R light were relatively ineffective. Therefore, it appears
that some threshold amount of R light (or some threshold R:FR
value) is required for SDPs to perceive an NI. The threshold
R:FR
wide
for delaying flowering was 0.66 or greater (P
FR
/P
R+FR
0.63 or greater) for chrysanthemum and african marigold, but
one was not identified for dahlia.
Regardless of photoperiodic classification, most plants ex-
hibit some degree of shade-avoidance response. Natural
daylight has an R:FR of 1.15, and when plants detect a
reduced R:FR (resulting from mutual shading, canopy cover,
photoselective filters, etc.), extension growth increases in an
effort to better harvest photosynthetic light (Smith, 1982).
Alternatively, stem extension can be inhibited by growing plants
under an increased R:FR, especially in shade-avoiding plants.
For example, chrysanthemum grown under an FR-absorbing
photoselective filter (R:FR = 2.2) were 20% shorter than plants
grown under a neutral filter (Li et al., 2000). Yamada et al. (2008)
used FR FL lamps (R:FR = 0.01), INC lamps (R:FR = 0.65), and
FL lamps (R:FR = 5.00) as NI treatments on lisianthus (Eustoma
grandiflorum) ‘Niel Peach Neo’, an LD plant. Lamps with an
R:FR of 0.01 and 0.65 increased internode length by 26% and
23%, respectively, compared with plants grown without an NI.
In contrast, plants grown with FL NI had 14% shorter in-
ternodes than plants grown without an NI. Internode length of
the LDPs petunia (Petunia ·hybrida)‘WavePurpleClassic’
and black-eyed susan (Rudbeckia hirta) ‘Becky Cinnamon
Bicolor’ was significantly shorter when a 4-h NI was provided
by compact FL lamps (R:FR = 8.5) than by INC lamps (R:FR =
0.6) (Runkle et al., 2012).
In our study, plant height of chrysanthemum and dahlia
‘Figaro’ under an NI with a high proportion of R light (R:FR
wide
2.38 or greater) was shorter than when grown under a moderate
R:FR
wide
(0.66 and 1.07). Surprisingly, plants grown under the
FR-only NI were generally shorter than plants in the other NI
treatments. We anticipated that plants grown under the FR-only
NI (R:FR
wide
= 0.05) would exhibit a shade-avoidance response
and thus have greater stem elongation. However, because plants
did not perceive an FR NI as an LD, flowering occurred earlier
in development, so there was less time for stems to elongate
before flowering. For example, marigold grown under an FR NI
flowered with six fewer nodes than plants in the other NI
treatments, so their overall height at first flowering was actually
less.
Commercial growers have traditionally used INC lamps to
provide photoperiod lighting because they are effective and
inexpensive to install. However, INC lamps convert less than
10% of the energy consumed into visible light (Thimijan and
Heins, 1983; Waide, 2010). With the phaseout of INC lamps,
greenhouse growers will need other sources of light to control
flowering of photoperiodic crops. As we have shown, LED tech-
nology provides an alternative to INC lamps for photoperiodic
lighting. In addition to the improvements in lamp lifespan and
energy efficiency, the narrow waveband nature of LEDs can
be used to create lamps that are tailored to ornamental crop pro-
duction needs. In SDPs, LEDs with a moderate to high R:FR
are effective at preventing premature flowering and, thus, are a
viable replacement for INC lamps.
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