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EFFECTS OF PGRS ON ROOT GROWTH OF HERBACEOUS PERENNIALS
Joyce G. Latimer and John Freeborn1
ABSTRACT
Plant growth regulators, especially those applied directly to the root system, are
thought to significantly reduce growth of the root systems. Various perennials were
treated with uniconazole (Concise, Fine Americas, Inc.) to evaluate effects on plant
height as well as shoot and root dry weights. Plant height of Lobelia cardinalis was not
affected by a 1 ppm uniconazole liner dip, but shoot dry weight was reduced 52% and
root dry weight 41% at 8 weeks after treatment (WAT) relative to untreated control,
resulting in a root:shoot ratio comparable to the control. A 60 ppm uniconazole spray
application reduced plant height of Stokesia laevis ‘Silver Moon’ over 40% at and
beyond 4 WAT. Shoot dry weight of these treated plants at 8 WAT was 57% less than
that of control plants while root dry weight was reduced 42%. This differential effect on
shoots and roots resulted in a root:shoot ratio that was higher than that of the control
plants. Reductions in root dry weight appear to correspond to reductions in shoot dry
weight but the root:shoot ratio is not generally reduced. Higher root:shoot ratios are
associated with better field establishment of transplants so maintenance of comparable
root:shoot ratios should indicate comparable establishment rates.
INTRODUCTION
Growers frequently express concern about the effect of plant growth retardants
(PGRs) on root growth. We have observed reduced root mass with plants stunted by
excessive rates of PGRs but we have just assumed that root growth is reduced by PGRs in
a manner less significant than shoot growth. These tests evaluate direct effects of
uniconazole (Concise, Fine Americas, Inc.) on root and shoot growth and the root:shoot
dry weight ratio of the treated plants.
MATERIALS AND METHODS
Four crops were evaluated: Helenium 'Coppelia,' Lobelia cardinalis, Phlox
paniculata 'David,' and Stokesia laevis ‘Silver Moon.’ All crops were started as size 72
plugs obtained from Aris Horticultural, Inc. (Leola, PA) and planted into quart pots (1.1
liter) filled with custom medium of three parts Sphagnum peat moss and one part perlite
to which we added the equivalent of six pounds per cubic yard of lime at mixing for pH
adjustment. This medium was designed to facilitate harvesting of roots. All plants were
irrigated as necessary with 200 ppm N using Peter’s 20-10-20.
1
Department of Horticulture, 301 Saunders Hall, Virginia Tech, Blacksburg, VA 24061-0327
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After plant establishment and resumption of growth (about two weeks after
planting), uniconazole (Concise, Fine Americas, Inc., Walnut Creek, CA) was applied as
foliar spray at 60 ppm using a hand-held CO2 pressurized sprayer. Sprays were evenly
applied over a given time period to apply the label recommended volume, two quarts per
100 sq.ft. Drenches were applied by hand to each pot using 2 fl.oz. of a 2 ppm
uniconazole solution per quart pot. Liner dips were performed by soaking the “irrigationready” root ball of the plugs in a 1 ppm uniconazole solution for a period of 2 minutes
(Schnelle and Barrett, 2006). Not all treatments were applied to each crop. Liner dips
were applied only to Helenium and Lobelia (see results).
Each plant species or cultivar was set up as an individual experiment with plants
arranged in a completely randomized design with six single plant replications. Data
collected included plant height (from rim of pot to top of plant, vegetative or flowers,
unless otherwise noted, in cm). At the conclusion of the experiment (eight weeks after
treatment), the roots were washed by hand and bagged separate from the shoots for a final
shoot and root dry weight. When flowers were present at the final termination, they were
dried and weighed separate from the shoot dry weight. Data were subjected to GLM with
LSD mean separation within a crop and study or a paired t-test when only two treatments
were included in the test.
RESULTS AND DISCUSSION
There were no significant effects of the 1 ppm uniconazole liner dips on
Helenium. Therefore these data are not presented.
The 1 ppm uniconazole liner dip did not affect plant height of Lobelia cardinalis
plants at eight weeks after treatment (Table 1). Although there were no obvious growth
reductions as a result of the uniconazole treatment, shoot dry weight was significantly
reduced 52% relative to the controls. Root dry weight was reduced 41% by the PGR
treatment (NS). There was no difference in the root:shoot dry weight ratio of the plants
treated with the liner dip vs. the controls.
Table 1. Effect of 1 ppm uniconazole liner dip on growth and shoot and root dry weights
of Lobelia cardinalis plants at eight weeks after treatment.
Treatment
Control
Rate
(ppm)
0
Height
(cm)
3.4
Shoot dry wt
(mg)
3.70
Root dry wt
(mg)
9.16
Root:shoot
ratio
2.41
Liner dip
1
3.7
1.76
5.42
2.73
0.670
0.027
0.215
0.556
t-test
Although plant height of Phlox paniculata 'David' was not affected by treatment,
shoot dry weight was significantly reduced 25% by the 60 ppm spray treatments of
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uniconazole and less by the 2 ppm drench treatment (Table 2). With less reduction (not
significant) in root dry weight, this resulted in a tendency toward higher root:shoot ratios.
If we consider the shoot dry weight including the flower dry weight, the root:shoot ratio
of treated plants still tends to be higher than that of untreated plants (control 0.889 vs 60
ppm spray 1.16, p=0.13). Multiple samples over time would help to more specifically
determine PGR effects on the roots and ratios.
Table 2. Effect of a 2 ppm drench or 60 ppm foliar spray of uniconazole on growth and
shoot and root dry weights of Phlox ‘David’ plants at eight weeks after treatment.
Rate
(ppm)
0
Height
(cm)
16.2
Shoot dry wt
(mg)
3.62 a
Root dry wt
(mg)
5.68
Root:shoot
ratio
1.59
Drench*
2
17.6
3.36 ab
6.20
1.87
Spray
60
14.5
2.70 b
5.32
2.05
0.177
0.048
0.687
0.371
---
0.74
---
---
Treatment
Control
F-test
lsd
*Drench applied at 2 fl.oz. per pot.
Although Stokesia laevis ‘Silver Moon’ showed no response to the 2 ppm drench
application of uniconazole, the 60 ppm spray application reduced plant height over 40%
at eight weeks after treatment (Table 3). Shoot dry weight of these treated plants was
57% less than that of control plants while root dry weight was reduced 42%. This
differential effect on shoots and roots resulted in a root:shoot ratio that was higher than
that of the control plants.
Table 3. Effect of a 2 ppm drench or 60 ppm foliar spray of uniconazole on growth and
shoot and root dry weights of Stokesia ‘Silver Moon’ plants at eight weeks after
treatment.
Rate
(ppm)
0
Height
(cm)
8.8 a
Shoot dry wt
(mg)
5.54 a
Root dry wt
(mg)
9.14
Root:shoot
ratio
1.64 b
Drench
2
8.3 a
4.46 a
6.74
1.56 b
Spray
60
5.2 b
2.36 b
5.26
2.23 a
0.022
0.006
0.057
0.049
2.6
1.73
---
0.57
Treatment
Control
F-test
lsd
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Reductions in shoot dry wt appear to correspond to reductions in root dry weight
but the root:shoot ratio is not generally reduced. Even the excessive growth reduction of
Stokesia in response to the 60 ppm uniconazole spray did not reduce the root:shoot ratio.
Higher root:shoot ratios are associated with better field establishment of transplants
(Leskovar and Stofella, 1995; Liu and Latimer, 1995), so maintenance of comparable
root:shoot ratios should indicate comparable establishment rates. However, the smaller
size and decreased transpiration of PGR-treated plants also enhance plant establishment
in landscape conditions. We are currently evaluating PGR effects on root growth over
time after treatment.
REFERENCES CITED
Leskovar, D.I. and P.J. Stoffella. 1995. Vegetable seedling root system: morphology,
development, and importance. HortScience 30:1153-1159.
Liu, A. and J.G. Latimer. 1995. Root cell volume in the planter flat affects watermelon
seedling development and fruit yield. HortScience 30:242-246.
Schnelle, R. and J. Barrett. 2006. Reducing PGR liner dip variables. Greenhouse Product
News 16(7):70-72.
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