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International Journal of Vegetable Science
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/wijv20
Seedling emergence and growth of ‘China
Rose’ radish microgreens in response to radicle
breakage following pre-sowing seed germination
Wallace G. Pill , Carrie J. Murphy , Michael Olszewski & Thomas A. Evans
To cite this article: Wallace G. Pill , Carrie J. Murphy , Michael Olszewski & Thomas A. Evans
(2020) Seedling emergence and growth of ‘China Rose’ radish microgreens in response to radicle
breakage following pre-sowing seed germination, International Journal of Vegetable Science, 26:5,
441-449, DOI: 10.1080/19315260.2019.1642281
To link to this article: https://doi.org/10.1080/19315260.2019.1642281
Published online: 17 Jul 2019.
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Seedling emergence and growth of ‘China Rose’radish
microgreens in response to radicle breakage following
pre-sowing seed germination
Wallace G. Pill
a
, Carrie J. Murphy
b
, Michael Olszewski
c
, and Thomas A. Evans
a
a
Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, USA;
b
New Castle
County Cooperative Extension Service, Newark, Delaware, USA;
c
Department of Horticulture, Temple
University, Ambler, Pennsylvania, USA
ABSTRACT
Germinating seeds in moist, fine-grade vermiculite followed by
broadcasting the mix over the seedbed can reduce greenhouse
production time for microgreens, but seedling radicles could be
broken. The study was undertaken to determine if radicle breakage
affected plant yield of radish (Raphanus sativus L) used for micro-
greens. During pre-sowing handling of germinated seed in moist
vermiculite the percent of broken radicles increased as radicle
length increased, and as mechanical force increased (resulting in
0%, 8%, or 22% with minimal, medium or high mechanical force,
respectively). Imposition of these levels of radicle damage was
achieved by cutting radicles half-way along their length after
germination for 2 days in moist vermiculite (200% water, dry
weight) at which time radicles were about 1.5 cm long. This treat-
ment had no effect on numbers of shoots (as a percent of sown
seed) or number of shoots∙m
−2
at 7 days after sowing at 25% or
100% of a commercial rate (9,266 seed∙m
−2
). Yield as shoot fresh
weight∙m
−2,
or per seedling was unaffected by percent of cut
radicles sown. Removal of up to 12 mm of radicle from the tip
increased numbers of roots per seedling but decreased total root
length and decreased fresh weight per shoot. The higher seeding
rate decreased fresh weight per shoot from raw (42%) or germi-
nated (32%) seed. Sowing germinated seed in moist vermiculite,
rather than raw seed, and sowing seed at the higher rate, increased
shoot fresh weight∙m
−2
and shoot fresh weight per seedling.
Seedlings with broken radicles had lower shoot fresh weights but
smaller shoots were counted in the final yield. Microgreen yield
(shoot fresh weight∙m
−2
) was unaffected after sowing germinated
seed that had been subjected to a high mechanical force (with up
to 22% broken radicles) during broadcasting .
KEYWORDS
Raphanus sativus; pre-
sowing germination; radicle
damage; root architecture;
root injury; root pruning
Greenhouse production of salad crop shoots for harvest and consumption
within 7 to 20 days of seedling emergence (microgreens) is becoming
increasingly popular. In one production technique, seed is broadcast onto
peat-lite contained within trays. Techniques that increase the speed of crop
establishment would permit more efficient use of expensive greenhouse
CONTACT Thomas A. Evans tomevans@udel.edu Department of Plant and Soil Sciences, University of
Delaware, Newark, Delaware 19717, USA
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE
2020, VOL. 26, NO. 5, 441–449
https://doi.org/10.1080/19315260.2019.1642281
© 2019 Taylor & Francis Group, LLC
space. Presowing germination (pregermination) is one way to speed crop
production. The potential for increased percentage, rate, and synchrony of
crop establishment in the field was the impetus for fluid drilling planting
(Pill, 1991). With this technique, germinated seed was mixed in a gel and the
mix extruded onto the seedbed. The gel protected radicles from damage and
kept germinated seed moist.
The most pronounced seedling emergence advancement for beet, or chard,
(both Beta vulgaris L.) microgreens was due to germinating seed in fine-
grade moist vermiculite and sowing this mix (Lee et al., 2004). For the faster-
germinating species radish (Raphanus sativus L.), kale [Brassica napus L. var.
pabularia (D.C.) Rchb.] and amaranth (Amaranthus tricolor L.), a two-step
procedure of matric priming (3 d in vermiculite with 50% water), followed by
germination (1 d in vermiculite with 150% water; % dry weight) before
sowing the germinating seed-vermiculite mixture produced the greatest
emergence advancement and seedling shoot fresh weight (Lee and Pill,
2005). During broadcasting of the germinating seed plus vermiculite mixture,
radicle breakage could occur.
The commercially recommended seed sowing rate for microgreen rad-
ish, cv. Red Rose (Johnny’s Selected Seeds, Winslow, MA) is 9,266
seeds∙m
−2
. As seed sowing rates decreased from 100% to 25% of the
commercially recommended rate, fresh weight per shoot increased in
arugula [Eruca vesicaria (L.) Cav. subsp. sativa (Mill.) Threll]; (Murphy
and Pill, 2010) and in beet (Murphy et al., 2010). Shoot fresh weight∙m
−2
for both species increased with increasing seed sowing rate up to the
recommended rate. We know of no reports on microgreen shoot growth
in response to radicle breakage during sowing of germinated seed plus
moist vermiculite mixture. It is hypothesized that if germinated seed with
broken radicles can regenerate roots and support shoot growth, the
resultant plants would contribute to economic yield, especially at lower
seed sowing rates when interplant competition would be lower.
Following root-pruning of 10-d old hydroponically grown seedlings of
lettuce (Lactuca sativa L.) the initial response was an increase in the number
of lateral roots but a decrease in total root length, and the root:shoot dry
weight ratio rapidly increased indicating new root growth was occurring at
the expense of shoot growth (Biddington and Dearman, 1984).
The objective of this study was to establish the extent of radicle breakage
of ‘China Rose’radish in germinated seed in moist vermiculite in response to
various levels of force-induced mechanical damage that might occur during
broadcasting the mix. The effect of levels of radicle breakage when imposed
by cutting the radicles half-way along their length at 2 seed sowing rates after
germination on economic yield was determined.
442 W. G. PILL ET AL.
Materials and methods
In the first experiment, 15 lots of 100 seed (0.92 g) of ‘China Rose’radish (Johnny’s
Selected Seeds) were added to 70 mL of fine-grade horticultural vermiculite (Sta-
Green, Sun-Gro Horticultural Distribution, Agawam, MA) contained within
266 mL transparent plastic cups (Huhtamaki Co., Albertville, AL). After adding
200% water (dry weight of vermiculite; 17 mL), the vermiculite, seed, and water
were thoroughly mixed by gentle stirring. The cups were not covered. After 2 days
at 20°C, the content of each cup was added to a 16.5 × 14.9 cm sealable plastic bag
(Ziploc sandwich bag, Racine, WI). Five replicate bags containing the mix were
subjected to three levels of mechanical force to mimic levels of handling that might
occur during mixing and handling of the mixture before being broadcast onto
a seedbed for microgreen production. The levels of mechanical force were:
“minimal”(baghandledverygently),“medium”(bag held at arm’slengthand
passed quickly through a 1-m arc 5 times), and “high”(bag held at arm’slength
and passed quickly through a 1-m arc 10 times with further rigorous manual
manipulation). It was calculated that each passage generated 0.219 N of centrifugal
force. Contents of bags were emptied, and seed counted and classified as non-
germinated or germinated with radicle lengths of <7.5 mm, 7.5 to 15 mm, or
>15 mm. Numbers of broken radicles within each radicle length category were
counted and expressed as a percent of the total number of radicles.
In the second experiment, lots of 50 (0.46 g) and 200 (1.84 g) radish seed
representing 25% and 100% of a commercially recommended seed sowing
rate for radish microgreen production were subjected to the pre-sowing
germination treatment described above and sown in a 12 × 17 cm flat. Just
prior to sowing, germinated seed was subjected to three levels of radicle
breakage determined from results of the first experiment, i.e., 0%, 8%, and
22% for the minimum, medium and high levels of mechanical force, respec-
tively. Radicle damage as a substitute for unintended breakage during sowing
was established by cutting radicles half-way along their lengths with a sharp
knife (4 and 16 radicles for the medium force at the 25% and 100% seeding
rates, respectively; 11 and 44 radicles for the high force at the 25% and 100%
seeding rates, respectively). Radicles were cut according to their distribution
within radicle length categories (Table 1).
Contents of each cup were returned to the cup and the mix broadcast on the
surface of ProMix BX (Premier Brands, New Rochelle, NY) contained within 12 ×
17 × 6 cm plastic flats. Immediately after sowing, seed was covered with a thin
(2 mm average) layer of Redi-Earth (a fine-grade peat-lite; The Scotts Company,
Marysville, OH). Raw seed was sown as the control. Flats comprising a 2 (seed
sowing rate) × 4 (seed treatment; raw seed and three levels of radicle breakage)
factorial were arranged in a randomized complete block design in a greenhouse set
at 23/20°C (day/night) under natural light (4–11 March 2019). Flats were irrigated
daily with water and on alternate days with a 100 mg N∙L
−1
from 21N-2.2P-12.4K
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 443
fertilizer solution. After 1 week, shoots from a flat were cut at the growth medium
surface, immediately weighed, and placed in a plastic bag. Within 4 h, numbers of
shoots within bags were counted. Numbers of shoots and shoot fresh weight∙m
−2
and fresh weight per shoot were calculated.
In the third experiment, radish seed was sown at about a 1 cm spacing on
a double-thickness of 36 × 23 cm germination paper (Germination Blotter
No 385; Seedburo, Chicago, IL) moistened to saturation with deionized
water. The blotters were contained within 36 × 23 × 5 cm plastic growth
boxes with tightly fitting transparent plastic domes (Stewart Plastics,
Croydon, UK). After 2 d at constant 20°C in darkness, radicles were 1.0 to
1.5 cm long. Radicles were cut at 0, 4, 8, or 12 mm from the tip using a razor
blade. Immediately after tip removal, and before further treatment, pruned
seedlings were placed on the surface of a moist paper towel to prevent
desiccation. The 4-mm cut was about half-way along the root hair region,
and the 8-mm cut removed most of the root hair region. These two cuts
represented the majority of radicle breakage occurring in the previous
experiment. Pruned seedlings were used for measuring root morphology
and growth, and for determining seedling emergence and growth.
To observe root regeneration responses to radicle pruning, the slant test
(Smith et al., 1973) was used. Germination papers were cut into 9.5 ×
14.5 cm rectangles and a pencil line drawn 1.5 cm down from the edge
across the longer dimension to provide a guide for placing seedlings in
a straight line. The papers, soaked in half-strength Hoagland solution
(Hoagland and Arnon, 1950), were placed on 2 mm thick Plexiglass
support plates (slants). Fifteen seedlings from each treatment were spaced
equally along the pencil line and support plates (held at 20° from vertical)
were placed randomly in 36 × 23 × 5 cm plastic growth boxes. Half-
strength Hoagland solution was maintained at a 2.5 cm depth in the
bottom of each box so that germination papers would remain moist by
capillary action. Tightly fitting transparent dome lids maintained 100%
relative humidity in the boxes. Each treatment of five seedlings (a slant)
Table 1. Radicle length categories and percentage broken radicles within each length category
in response to post-germination pre-sowing mechanical force.
Percent of radicles in each length category
a
Mechanical force <7.5 mm 7.5–10.0 mm >10.0 mm
Percent injured roots within a force level
(across radicle length categories)
Minimal 46 ± 6
b
(0) 39 ± 7 (0) 6 ± 1 (0) 0
Medium 44 ± 7 (2) 43 ± 7 (12) 8 ± 4 (23) 8
High 47 ± 5 (8) 41 ± 4 (34) 6 ± 3 (71) 22
a
Non-germinated seed were 8%, 4%, and 6% in response to the minimal, medium, and high mechanical
forces, respectively; values.
In parentheses are percent radicle breakage in each radicle length category.
b
Mean±S.D. (n = 5).
444 W. G. PILL ET AL.
was replicated 4 times in boxes arranged in a randomized complete block
design. The boxes were held at constant 21°C in low light (10 µmole of
photosynthetically active radiation ∙m
−2
∙s
−1
). After 5 d, numbers of visible
roots on each germinated seedling were counted and root lengths
measured.
The fourth experiment determined seedling emergence and shoot growth
responses to radicle tip pruning. Seed was germinated, and seedlings pruned
as described above. Fifty pruned seedlings for each of 0, 4, 8 or 12 mm root
tip removal treatments were transferred to 70 mL of fine-grade vermiculite
moistened uniformly with 200% (dry weight) of deionized water. The seed-
lings were gently stirred into the moistened vermiculite. After 2 d at 20°C,
the vermiculite-germinated seedling mix was sown on the surface of ProMix
BX peat-lite contained within 12 × 17 × 6 cm plastic flats. This seed sowing
rate of 2,451 seed∙m
−2
was 25% the commercial rate. All seed were covered
with a 2-mm layer of Redi-Earth peat-lite. The five seed treatments of each
species were replicated 4 times with flats arranged in randomized complete
blocks in a greenhouse maintained at 23/20°C (day/night) under natural light
(March).
All plants received 100 mg N∙L
−1
from 21N-2.2P-12.4K as a solution on
alternate days by overhead application to the foliage and growing medium.
At 7 d after planting (when the first true leaf laminas from non-pruned
germinated seed were about 2 cm long), shoots of all seedlings were cut at the
growth medium surface, counted, and fresh weights determined. Shoot fresh
weights∙m
−2
and per seedling were calculated.
Except for experiment 1, where means were expressed with plus or minus
1 standard deviation (n = 5), data for the other experiments were subjected to
ANOVA using Proc GLM of SAS (ver. 9.2, SAS Institute, Cary, NC). Single
degree of freedom trend analyses were included in the ANOVA for experi-
ments 3 and 4. Means of significant main effects were separated by Least
Significant Difference (LSD).
Results and discussion
For experiment 1, after 2 d in moist vermiculite at 20°C, 94% of the seed had
germinated with almost half having radicles <7.5 mm long, a smaller min-
ority having radicles 7.5 to 10 mm long, and few having radicles >10 mm
long when averaged across mechanical force levels (Table 1). While no
radicles were broken with minimal mechanical force, the percent of broken
roots within each radicle length category increased as the mechanical force
increased from medium to high. As radicle length increased the percent of
broken radicles increased in radicles >10 mm long which were subjected to
high mechanical force.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 445
There does not appear to be another report quantifying the extent of
radicle breakage of germinated seed resulting from mechanical force during
pre-sowing handling. In addition to radicle breakage, mechanical force may
have caused other, non-observable damage to germinated seedlings.
For experiment 2, average emergence (number of shoots as a percent of
seed sown) was unaffected by seed sowing rate or mechanical force (Table 2).
Numbers of shoots∙m
−2
were affected only by seed sowing rate. Seedlings
with damaged radicles were able to emerge and be counted at shoot harvest.
Shoot fresh weight∙m
−2
and shoot fresh weight per seedling were unaffected
by percent of broken radicles at sowing. In this study, it was impossible to
monitor shoot growth of individual seedlings arising from seed with broken
radicles. It is possible that shoots from germinated seed with broken radicles
may have been smaller, but this would be impossible to determine since
shoot fresh weight per seedling was determined by dividing the total shoot
fresh weight by the number of counted shoots. Future research could track
fresh shoot fresh weights of individual plants with broken roots.
Presowing germination compared to sowing raw seed increased shoot
fresh weigh∙m
−2
less at the 2,450 seed∙m
−2
sowing rate than at the 9,800
seed∙m
−2
(Table 3). The 25% seed sowing rate resulted in greater fresh weight
per shoot than the 100% seed sowing rate (Table 3), a response that could be
attributed to the lower interplant competition. Pre-sowing seed germination
increased fresh weight per shoot less at the 25 than at the100% seed sowing
rates, compared to sowing a raw seed (Table 3). Pre-sowing seed germination
caused increased shoot growth in beet and chard compared with raw seed
(Lee et al., 2004) demonstrating that pre-sowing germination reported here
provided the same benefit in those species. A further increase in shoot
growth could be achieved by subjecting seed to a 2-step procedure of matric
Table 2. Analysis of variance for seedling emergence and shoot fresh weights at 7 d after
planting pre-sowing germinated ‘China Rose’radish seed at 2 seed sowing rates in response to
the percent of seedlings with the distal one-half of the radicle removed by cutting.
Source of variation
Number of shoots (% of
sown seed)
Number
shoots∙m
−2
Shoot fresh
weight (g∙m
−2
)
Shoot fresh weight
(mg/seedling)
Seed sowing rate (R) ns
a
*** *** ***
Seed condition (C)
b
ns ns *** ***
Radicle breakage (B)
a
ns ns ns ns
R×C ns ns ns ns
B×C ns ns ns ns
R×B ns ns ns ns
R×C×B ns ns ns ns
ns, **, *** not significant or significant at P≤.01 or P≤.001, respectively, ANOVA.
a
Radicle breakage represents the 0%, 8%, or 22% of seedlings with broken radicles resulting from minimal,
medium, or high.
Mechanical force before sowing.
b
Seed condition refers to whether seed was raw or germinated.
446 W. G. PILL ET AL.
priming in the vermiculite (3 days at 50% water) followed by 1 day with an
additional 100% water added to the vermiculite during which germination
occurred (Lee and Pill, 2005). The effect of matric priming on subsequent
possible radicle breakage following seed germination has not been deter-
mined. Lower shoot fresh weight per seedling, but higher shoot fresh
weight∙m
−2
, with the higher seed than lower seed sowing rate (Table 3) has
been reported in arugula (Murphy and Pill, 2010) and beet (Murphy et al.,
2010). This shows that the response demonstrated in this paper has occurred
in studies with other plant species.
For experiment 3, seedlings with intact radicles had fewer roots than those
with the distal 4, 8 or 12 mm of the radicles removed (Figure 1(a)). More
roots per seedling with radicle pruning resulted in less total root length
per seedling than occurred in intact seedlings Figure 1(b). The results agree
with Biddington and Dearman (1984) who found an increase in lateral roots,
but a decrease in total root length, in response to pruning seedlings of lettuce.
The root hair zone occurred 4 to 7 mm proximal to the root tip. Radicles
broke in the distal 20% to 50% of their lengths. Since radicles were cut at
halfway along their lengths, most radicles had at least most of the root hair
zones removed. Since no complete radicles were in the vermiculite following
mechanical force, it can be concluded that radicles did not break at the seed
coat.
For experiment 4, shoot fresh weight per seedling (Figure 1(c)) and shoot
fresh weight∙m
−2
(Figure 1(d)) at 12 days after sowing germinated seed with
intact radicles were higher than those from germinated seed with pruned
radicles. The reduction in shoot fresh weight from radicle pruning was
pronounced when the distal 12 mm of the radicle was excised. The decrease
in shoot growth may reflect an altered source–sink relationship because of
radicle pruning. Biddington and Dearman (1984) noted a rapid increase in
root:shoot ratio in response to root pruning of lettuce which indicated root
growth was occurring at the expense of shoot growth. The results in radish
are consistent with those in lettuce.
Table 3. Numbers of shoots produced, shoot fresh weights, and shoot fresh weight/seedling at 7
d after planting pre-sowing germinated ‘China Rose’radish seed in response to seed sowing rate
and whether raw or germinated seed were sown.
Source
Number of shoots
(shoots∙m
−2
)
Shoot fresh weight
(g∙m
−2
)
Shoot fresh weight
(mg∙seedling
−1
)
Seed sowing rate:
2340 seed∙m
−2
(0.25)
9266 seed∙m
−2
(1.00)
2340 b
a
9266 a
754 b
1977 a
323 a
213 b
Seed condition:
Raw
Germinated
5825 a
5795 a
1122 b
1447 a
227 b
282 a
a
Means within a column and factor followed by the same letter are not significantly different by LSD
0.05
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 447
Figure 1. Response to removal of 0, 4, 8 or 12 mm of the radicle prior to planting for: (a) number
of roots per seedling, excludes seedlings with no roots, values >1 represent regenerated roots,
and (b) total root length at 5 days after planting; (c) shoot fresh weight per seedling and (d)
shoot fresh weight∙m
2
at (c) and (d) at 12 days after planting. Values within each sub-figure
having the same letter are not significantly different at P ≤.001. During ANOVA, it was
determined that in (a) and (b) values fit a quadratic distribution and in (c) and (d) values fit
a linear distribution.
448 W. G. PILL ET AL.
Microgreen yield (shoot fresh weight∙m
−2
) was unaffected after sowing
germinated seed that had been subjected to a high mechanical force (with up
to 22% broken radicles) during broadcasting. Even with extensive radicle
breakage during pre-sowing germination in moist vermiculite, there was no
economic yield reduction in radish microgreens.
References
Biddington, N.L., and A.S. Dearman. 1984. Shoot and root growth of lettuce seedlings
following root pruning. Ann. Bot. 53:663–668. doi: 10.1093/oxfordjournals.aob.a086731.
Hoagland, D.R., and D.I. Arnon. 1950. The water culture method for growing plants without
soil. Calif. Agr. Expt. Sta. Circ. No. 347. University of California Press, Berkley.
Lee, J.S., and W.G. Pill. 2005. Advancing greenhouse establishment of radish, kale and
amaranth microgreens through seed treatments. J. Korean Soc. Hort. Sci. 46:363–368.
Lee, J.S., W.G. Pill, B.B. Cobb, and M. Olszewski. 2004. Seed treatments to advance green-
house establishment of beet and chard microgreens. J. Hort. Sci. Biotechnol. 79:565–570.
doi: 10.1080/14620316.2004.11511806.
Murphy, C.J., K.F. Llort, and W.G. Pill. 2010. Factors affecting the growth of microgreen table
beet. Int. J. Veg. Sci. 16:253–266. doi: 10.1080/19315261003648241.
Murphy, C.J., and W.G. Pill. 2010. Cultural practices to speed the growth of microgreen
arugula (roquette; Eruca vesicaria subsp. sativa). J. Hortic. Sci. Biotechnol. 85:171–176. doi:
10.1080/14620316.2010.11512650.
Pill, W.G. 1991. Advances in fluid drilling. Hort Technol. 1:59–65. doi: 10.21273/
HORTTECH.1.1.59.
Smith, O.E., N.C. Welch, and T.M. Little. 1973. Studies on lettuce seed qualityI. Effect of seed
size and weight on vigor. J. Am. Soc. Hortic. Sci. 98:529–533.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 449