Poinsettia Developmental and Postproduction Responses to Growth Retardants and Irradiance

Abstract

Plants of Euphorbia pulcherrima Wind. `Glory' were grown under 13.4, 8.5, or 4.0 mol·m ⁻² ·day ⁻¹ and sprayed with water (control); 2500 mg·liter ⁻¹ daminozide + 1500 mg·liter ⁻¹ chlormequat chloride (D+C); 62.5 mg·liter ⁻¹ paclobutrazol; or 4, 8, 12 or 16 mg·liter ⁻¹ uniconazole to ascertain plant developmental and pest-production responses to the treatment combinations. Days to anthesis increased as irradiance was decreased. Anthesis was delayed by the D+C treatment, while other growth retardant (GR) treatments had no effect on anthesis. Irradiance did not affect plant height at anthesis, but all GR treatments decreased height over control plants. Bract display and bract canopy display diameters declined as irradiance was decreased. Growth retardants did not affect individual bract display diameters, but all GR treatments except paclobutrazol reduced bract canopy display diameter. Plants grown under lower irradiance had fewer axillary buds develop, fewer bract displays per plant, and fewer cyathia per bract display. Cyathia abscission during a 30 day post-anthesis evaluation was not affected by treatment; however, plant leaf drop was linearly proportional to irradiance. All GR treatments increased leaf drop over controls, and the D+C treated plants had the highest leaf loss. Results indicate the irradiance and GR treatments during production can affect poinsettia crop timing, plant quality at maturity, and subsequent post-production performance.

Full text

HORTSCIENCE 26(12):1501-1503. 1991.
Poinsettia Developmental and
Postproduction Responses to Growth
Retardants and Irradiance
Douglas A. Bailey
l
and William B. Miller
2
Department of Plant Sciences, University of Arizona, Tucson, AZ 85721
Additional index words. Euphorbia pulcherrima, chlormequat chloride, daminozide,
light, uniconazole, paclobutrazol
Abstract. Plants of Euphorbia pulcherrima Wind. ‘Glory’ were grown under total
irradiances of 13.4, 8.5, or 4.0 mol·m-2·day-1 and sprayed with water (control), 2500
mg daminozide/liter + 1500 mg chlormequat chloride/liter (D + C), 62.5 mg paclo-
butrazol/liter, or 4, 8, 12, or 16 mg uniconazole/liter to ascertain plant developmental
and postproduction responses to treatment combinations. Anthesis was delayed for
plants grown under the lowest irradiance. Anthesis was delayed by the D + C treat-
ment, whereas other growth retardant treatments had no effect on anthesis date. Ir-
radiance did not affect plant height at anthesis, but all growth retardant treatments
decreased height over control plants. Inflorescence and bract canopy diameters were
decreased at the lowest irradiance level. Growth retardants did not affect individual
inflorescence diameters, but all, except paclobutrazol and 4 and 8 mg uniconazole/
liter, reduced bract canopy diameter compared with control plants. Plants grown under
the lowest irradiance developed fewer inflorescences per plant and fewer cyathia per
inflorescence. Cyathia abscission during a 30-day postanthesis evaluation increased as
irradiance was decreased; cyathia abscission was unaffected by growth retardant treat-
ment. Leaf abscission after 30 days postanthesis was lowest for plants grown under the
lowest irradiance. At 30 days postanthesis, all growth retardant treatments increased
leaf abscission over controls. Results indicate that irradiance and growth retardant
treatments during production can affect poinsettia crop timing, plant quality at ma-
turity, and subsequent postproduction performance. Chemical names used: 2-chloro-
ethyl-N,N,N-trimethylammonium chloride (chlormequat chloride); butanedioic acid mono
(2,2-dimethyl hydrazide) (daminozide); ß-[(4-chlorophenyl) methyl]- α α -(1,1-dimethyle-
thyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol), (E)-1-(p-chlorophenyl)-4,4-dimethyl-
2-(1,2,4-triazol-1-yl)-l-penten-3-ol (uniconazole, XE-1019).
Crop timing, plant quality at sale, and
postproduction performance of poinsettias
have increased in importance as expanding
markets require longer shipping times and as
the holiday sales season has become ex-
tended. Many production factors including
nutrition (Staby and Kofranek, 1979), tem-
perature (Miller and Heins, 1986), irradiance
(Marousky and Shanks, 1966; Nell and Bar-
rett, 1986; Staby and Kofranek, 1979), plant
spacing (Miller and Heins, 1986), time of
harvest (Marousky and Shanks, 1966), and
duration of postharvest storage (Scott et al.,
1982) have been evaluated as to their effects
on poinsettia plant quality at sale and after
placement into interior environments. One
Received for publication 6 Dec. 1990. Apprecia-
tion is extended to Paul Ecke Poinsettias, Encin-
itas, Calif., for donating the poinsettias, to Valent
Corp., Fresno, Calif., for financial support, and
to Larry J. Kaspersen for technical assistance
throughout this project. The cost of publishing this
paper was defrayed in part by the payment of page
charges. Under postal regulations, this paper
therefore must be hereby marked advertisement
solely to indicate this fact.
‘Current address: Dept. of Horticultural Science,
North Carolina State Univ., Box 7609, Raleigh,
NC 27695-7609.
‘Current address: Dept. of Horticulture, Clemson
Univ., Clemson, SC 29634.
production factor that has not been fully in-
vestigated is the effect of chemical growth
retardants. High doses of chlormequat chlo-
ride can decrease leaf retention after place-
ment in an interior environment (Marousky
and Shanks, 1966); however, little is known
about the effects of recently introduced
chemical growth retardant treatments on
poinsettias. Therefore, the objective of this
-study was to examine the effects of currently
employed (daminozide + chlormequat chlo-
ride and paclobutrazol foliar sprays) and fu-
ture (uniconazole foliar sprays) plant growth
regulator treatments on crop timing, crop
quality at harvest, and postproduction per-
formance in an interior environment.
Rooted cuttings of poinsettia ‘Gutbier V-
14 Glory’ were potted, one per 1.3-liter (15
cm-diameter) plastic pot, on 30 Aug. 1989
and placed at a 38 x 38 cm spacing in a 26/
18C (venting set point/night set point tem-
peratures) double layer polyethylene green-
house covered with 70% transmittance shade
cloth. The root substrate consisted of 1 sandy
loam soil :2 sphagnum peat :2 perlite (by
volume) mixture amended with 890 g treble
superphosphate, 593 g KN03, 1187 g
MgS04·7H20, 1780 g CaS04·2H20, and 74
g Frit Industries Trace Elements no. 555 (Pe-
ters Fertilizer Products, Grace-Sierra, Fo-
gelsville, Pa.) per cubic meter. The plants
were fertilized at each watering with 300 mg
each of N and K/liter supplied from 776 mg
KN0
3
and 550 mg NH
4
NO
4
/liter. Fertilizer
solution was maintained at 6.0 pH by in-
jecting 75% (w/w) technical grade H
3
P0
4
into
the system, which supplied 37 mg P/liter at
every watering. Vegetative growth was
maintained by the use of incandescent light
from 1600 to 2400
HR
daily starting 5 Sept.
1989. Natural short days were started on 4
Oct. 1989 and maintained throughout the re-
mainder of the experiment. The temperature
was monitored constantly throughout exper-
imentation using LI-COR LI-1000-16 Tem-
perature Sensors connected to a LI-COR LI-
1000 Data Logger (LI-COR, Inc., Lincoln,
Neb.). Ambient temperature averaged (±SD)
25 ± 2C day (0601-1800
HR
) and 19 ± 3C
night (1801-0600
HR
) during the course of
the experiment. Photosynthetic photon flux
(PPF) was monitored throughout experimen-
tation using LI-COR LI-190SA Quantum
Sensors connected to a LI-COR LI-1000 Data
Logger; PPF averaged 15.2 ± 4
mol·m-2·day-1 from time of potting to the
start of irradiance treatments, coincident with
pinch date. Plants were pinched, leaving five
nodes per plant on 13 Sept. 1989 and placed
under ambient irradiance, ~70% or ~33%
of ambient irradiance. All growth retardant
applications were made on 27 Sept. 1989 by
spraying 204 ml of solution evenly over 1
m
2
of bench area, and each pot received ~7.1
ml of spray. At this time, plants averaged
21 ± 2 cm and 4 ± 1 cm in diameter and
height, respectively, and newly developed
shoots averaged 5.3 ± 1 cm in length.
Plant height (measured from the top rim
of the pot to the top of the bract canopy
display), number of inflorescences, inflores-
cence diameter of the two most acropetal
inflorescences, diameter of the plant bract
canopy display, number of aborted/senesced
cyathia, number of remaining cyathia, and
days to anthesis from start of short days were
recorded for each for each plant at anthesis.
Anthesis was defined as the time when pol-
len was first shed by cyathia. At anthesis,
each plant was placed into an interior envi-
ronment for postproduction performance
evaluation. The postproduction evaluation
room averaged 20 ± lC day (0801-1600
HR) and 19 ± lC night (1601-0800 HR),
and relative humidity averaged 32% ± 3%.
Lights were kept on from 0800-1600 HR,
and irradiance at plant canopy height aver-
aged 0.27 ± 0.07 mol·m-2·day-1. Upon
placement in the postproduction environ-
ment, cyathia drop was recorded daily for
each plant for 30 days. On day 30, leaf ab-
scission was recorded for each plant.
The experiment was conducted using a
replicated split-plot design, with irradiance
as the main plot (three levels; 13.4 ± 3.9,
8.5 ± 2.5, and 4.0 ± 1.4 mol·m-2·day-1)
and growth regulator as the subplot [seven
treatments; foliar sprays of water (control);
2500 mg daminozide/liter + 1500 mg chlor-
mequat chloride/liter (D + C); 62.5 mg pac-
lobutrazol/liter; or 4, 8, 12, or 16 mg
uniconazole/fiter] with three replications and
three plants per experimental unit. Data were
tested using analysis of variance. Statistical

Table 1.
Effects of irradiance and selected growth retardant treatments on ‘V-14 Glory’ poinsettias at anthesis and 30 days after anthesis.
z
analyses were conducted using PROC GLM
in SAS 6.03 (SAS Institute, Inc. Cary, N.C.).
Anthesis was delayed 6 days for plants
grown under 4 mol·m
-2
·day
-l
compared with
plants grown under 13.4 or 8.5
mol·m-2·day-l (Table 1). The D + C treat-
ment delayed anthesis an average of 3 days
compared with water-sprayed controls; no
other growth retardant treatment signifi-
cantly affected date of anthesis. Irradiance
appeared to be the more important factor in
crop timing, with lower light delaying an-
thesis. There was no irradiance × growth
retardant interaction for anthesis or any of
the other data measured.
Irradiance did not affect plant height at
anthesis, but all growth retardant treatments
resulted in shorter plants than the controls
(Table 1). Plant height did not increase at
lower irradiances, unlike noted by Staby and
Kofranek (1979). Because our plants were
spaced at 38
x
38 cm, excessive elongation
due to overcrowding and limited irradiance
appeared to be absent. However, the
inflorescence count was reduced under the
lowest irradiance, thus limiting marketable
value of the plants. The number of inflores-
cences per plant and diameter of the top (most
acropetal) inflorescence were unaffected by
growth retardant treatments. Diameter of the
top inflorescence decreased as irradiance was
decreased. The second-most acropetal
inflorescence responded identically to irra-
diance and growth retardants (data not shown).
Bract canopy diameter was less for plants
grown under 4 mol·m-2·day-1 than under
high irradiances and was significantly de-
creased by all growth retardant treatments
except paclobutrazol at 62.5 mg·liter
-l
and
uniconazole at 4 and 8 mg·liter-l. The
D + C and the 16 mg uniconazole/liter treat-
ments resulted in equally small bract canopy
diameters. Inflorescence diameter and plant
bract canopy diameter reductions are unde-
sirable, resulting in less visual impact per
plant and, subsequently, lower plant quality.
The number of aborted or abscised cyathia
at anthesis was unaffected by treatments and
averaged 0.15 ± 0.7 for all plants (data not
shown). The number of cyathia developed
by the top inflorescence (recorded at an-
thesis; equals number of intact cyathia +
number of aborted cyathia) was decreased by
the lowest irradiance level, but was unaf-
fected by growth retardant treatments (Table
1). Although the degree of premature cyathia
abscission was not increased by lower light
levels as previously reported (Miller and
Heins, 1986), cyathia production was re-
duced with reduced irradiance. Fewer cy-
athia, whether a result of abscission or from
decreased cyathia development, is undesir-
able and can result in significant reductions
in crop quality and market value (Miller and
Heins, 1986).
Cyathia abscission during the 30-day post-
anthesis evaluation was increased by the two
lowest irradiance levels during production but
was not significantly affected by growth re-
tardant treatments. The increase in cyathia
abscission in combination with fewer cyathia
developed on plants grown in low irradiance
resulted in a lower quality product for the
end consumer. However, plants grown under
the lowest irradiance had the least postan-
thesis leaf abscission, similar to results of
previous studies (Miller and Heins, 1986;
Nell and Barrett, 1986). Nell and Barrett
(1986) showed increased leaf retention of
poinsettias by reducing irradiance for only
the last 3 weeks of production without in-
creasing cyathia abscission, as we observed,
on plants grown under lower irradiance
throughout the entire production period.
Therefore, the critical period to maintain high
light levels for cyathia development and sub-
sequent retention appears to be from time of
pinch to 3 weeks preanthesis. During the last
3 weeks of production, a reduction in irra-
diance may be desirable for postproduction
leaf retention.
Growth retardant treatments increased leaf
abscission under interior conditions com-
pared with controls. Abscission was highest
on plants treated with D + C but similar to
that sustained with paclobutrazol or unicon-
azole at 4 mg·liter
-1
. Marousky and Shanks
(1966) also reported that chlormequat chlo-
ride detrimentally affected postanthesis leaf
retention. Although 62.5 mg paclobutrazol/
liter increased leaf abscission over controls,
uniconazole-treated plants (4 to 12 mg·liter
-l
)
had significantly less leaf loss than plants
sprayed with D + C. If chemical height con-
trol is needed, paclobutrazol or uniconazole
may be more desirable than D + C with
respect to postanthesis leaf retention.
It is not known if reducing irradiance dur-
ing the last 3 weeks of production would
help alleviate increased leaf drop caused by
growth retardant treatments. Nell and Barrett
(1986) drenched all plants with chlormequat
chloride in their irradiance acclimation ex-
periment but had no nondrenched plants as
controls. Future studies are needed to ex-
amine the interaction of growth retardant
treatments with irradiance acclimation treat-
ments for postanthesis quality maintenance.
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environmental factors and plant maturity on bract
and leaf abscission in Euphorbia pulcherrima
Wind. Proc. Amer. Soc. Hort. Sci. 88:662-
670.
Miller, S.H. and R.D. Heins. 1986. Factors in-

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