Yield and Nutrient Concentrations of Kohlrabi Bulbs and Leaves as Affected by Spring Transplanting Dates - MDPI
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agronomy
Article
Yield and Nutrient Concentrations of Kohlrabi Bulbs and
Leaves as Affected by Spring Transplanting Dates
Alexandra Smychkovich * and Masoud Hashemi
Stockbridge School of Agriculture, University of Massachusetts Amherst, 161 Holdsworth Way,
Amherst, MA 01003, USA; masoud@umass.edu
* Correspondence: asmychko@umass.edu
Abstract: Transplanting kohlrabi (Brassica oleracea var. gongylodes), a cool-season vegetable crop, in
early spring may provide the opportunity for double cropping in short-season regions such as the
Northeastern United States. A two-year field study evaluated the impacts of transplanting dates
on yield and nutrient concentration of kohlrabi. Additionally, this study aimed to quantify the
nutritional value of kohlrabi leaves. The yield of kohlrabi increased by as much as 307 kg ha−1 for
each day transplanting was delayed. Soil temperature increased by 2.8 ◦ C between April 23 and
May 14, resulting in the increased accumulation of Ca, Cu, Mn, and Fe in kohlrabi bulbs and Ca
and Cu in leaves. The nutrient concentration in leaf and bulb were positively correlated, indicating
that the two commodities can be simultaneously harvested for optimum quality. Leaf yield was not
significantly different among transplanting dates. However, the number of leaves and total leaf area
increased with delayed transplanting. Leaf yield and leaf area were not correlated with bulb yield,
suggesting that the reductions in yield and nutrient concentrations were unrelated to photosynthetic
efficiency. Although the earlier transplanting of kohlrabi may have provided opportunities for double
cropping, the yield and nutrient accumulation of kohlrabi transplanted early in the spring were
considerably compromised.
Citation: Smychkovich, A.; Hashemi,
M. Yield and Nutrient Keywords: nutrient accumulation; brassica; transplanting date; kohlrabi; bulb yield; leaf yield
Concentrations of Kohlrabi Bulbs and
Leaves as Affected by Spring
Transplanting Dates. Agronomy 2022,
12, 770. https://doi.org/10.3390/ 1. Introduction
agronomy12040770
Kohlrabi (Brassica oleracea var. gongylodes) is a cool-season, biennial vegetable revered
Academic Editor: Alberto for its mildly sweet flavor and many health benefits. Kohlrabi is known by many names
San Bautista around the world and is cultivated as an annual crop in many parts of Europe, Asia, and
Received: 25 February 2022
North America. Like other cruciferous vegetables, kohlrabi is a potent source of essen-
Accepted: 20 March 2022
tial nutrients and bioactive compounds produced by plant secondary metabolites [1,2].
Published: 23 March 2022
The regular consumption of kohlrabi can aid in reducing the risk of chronic disease, de-
pressive disorders, and many types of cancer [2–5].
Publisher’s Note: MDPI stays neutral
Kohlrabi is primarily cultivated for its round, thickened stem, often referred to as
with regard to jurisdictional claims in
the ‘bulb’, which resembles a turnip at maturity. Cultivars can be white, pale green, or
published maps and institutional affil-
purple and differ significantly in size and storage ability. Fresh market spring varieties
iations.
are typically harvested when bulb diameter reaches 6–7 cm, while fall storage varieties
can reach up to 20 cm in diameter by the time they are harvested and be stored for up to
four months in proper conditions. The bulbs, similar to radish in consistency but slightly
Copyright: © 2022 by the authors.
sweeter in taste, can be cooked, pickled, or consumed raw in salads [1]. The flavorful leaves
Licensee MDPI, Basel, Switzerland. of kohlrabi, comparable to kale in appearance and texture, are also highly nutritious and
This article is an open access article can provide an additional commodity to growers and consumers.
distributed under the terms and Secondary metabolites found in kohlrabi, including anthocyanins, phenolic com-
conditions of the Creative Commons pounds, and glucosinolates contribute to its antioxidant and anti-inflammatory proper-
Attribution (CC BY) license (https:// ties [4,6]. Unlike pale green varieties, purple kohlrabi cultivars contain anthocyanins,
creativecommons.org/licenses/by/ bioactive flavonoids important for human health, as well as higher concentrations of pheno-
4.0/). lic compounds resulting in greater anti-inflammatory and antidiabetic effects [7–9]. While
Agronomy 2022, 12, 770. https://doi.org/10.3390/agronomy12040770 https://www.mdpi.com/journal/agronomyAgronomy 2022, 12, 770 2 of 11
the majority of research has focused on the health benefits of kohlrabi bulbs, the antibacte-
rial and antioxidant potential of kohlrabi leaves have also been documented [10,11]. More
recently, kohlrabi sprouts have been found to contain high concentrations of bioactive
compounds, including glucosinolates and fatty acids [12].
In northern climates, kohlrabi can be planted in the spring as a fresh market crop
or in late summer for fall harvest and winter storage. Seedlings are typically started in
the greenhouse and transplanted into the field after 4–5 weeks [13]. The impacts of plant
spacing, variety selection, and fertilizer application rates on growth, yield, and quality
parameters of kohlrabi have been extensively studied [14–18]. However, the optimum time
for transplanting kohlrabi, specifically in the Northern United States, has not been reported.
It is well-established that the date of planting can significantly impact crop yield [19–24].
The growing season in the Northeastern United States is relatively short; therefore, iden-
tifying cold-tolerant crops for early spring planting can extend the growing season and
provide an opportunity for double cropping. Although kohlrabi growing guidelines were
recently added to the New England Vegetable Management Guide (the primary vegetable
crop reference in the Northeast), the optimum time for spring transplanting has not been
specified. Thus, it is necessary to evaluate the impacts of the spring transplanting date
on the yield and quality of kohlrabi in order to optimize land use efficiency and crop pro-
ductivity. Kohlrabi is a cool-season vegetable, and we hypothesized that the early spring
planting of kohlrabi may result in earlier harvest, thus extending the growing season by
several weeks.
The goals of this study were to (1) evaluate the impacts of spring transplanting dates
on kohlrabi yield and nutrient accumulation in its bulbs, and (2) assess the nutritional value
of kohlrabi leaves as an additional commodity by comparing their nutrient content to kale
(Brassica oleracea var. sabellica), a popular fresh leafy vegetable.
2. Materials and Methods
2.1. Experimental Site and Weather Conditions
A two-year field experiment (2020–2021) was conducted at the University of Mas-
sachusetts Crop, Animal, Research, and Education Center, located in South Deerfield, MA,
USA (42◦ N, 73◦ W). The soil at this location is characterized as a coarse–silty, mixed,
non-acid, mesic Typic Udifluvent (Hadley series). In both years, a composite soil sample
was taken at a depth of 15 cm from the field site to ensure that P, K, Ca, and Mg levels were
in the optimum range for kohlrabi production. Relevant weather conditions, including
annual precipitation and growing degree days during the experiment period and the norm
for the experimental site, are presented in the results section.
2.2. Field Experiment Design and Implementation
Four spring dates of transplanting (DOT) were evaluated in this experiment: 23/4,
30/4, 07/5, and 14/5. Experimental plots were laid out in a randomized complete block
design with four replications. An early white variety of kohlrabi, Beas, was seeded into
plastic trays and transplanted after four weeks in the greenhouse into heavyweight, organic,
certified paper mulch (Figure 1). Plants were spaced 15 cm apart, with 30 cm spacing be-
tween the rows. All plots were irrigated throughout the season using drip irrigation. Rows
of kohlrabi were covered with heavyweight, transparent row cover at the time of planting
to prevent insect damage. 180 kg ha−1 of N fertilizer was applied in the form of urea
ammonium nitrate (UAN, 32%) as a split application, with all plots receiving 112 kg ha−1
at the time of planting and the remainder receiving 112 kg ha−1 two weeks later.Agronomy2022,
Agronomy 12,x770
2022,12, FOR PEER REVIEW 3 of 113 of 11
Figure1.1.Kohlrabi
Figure Kohlrabi (var.
(var. Beas)
Beas) transplanted
transplanted into paper
into paper mulchmulch forcontrol.
for weed weed Kohlrabi
control. Kohlrabi was har-
was harvested
vested50%
when when 50% of
of bulbs bulbs 6.4
reached reached
cm. 6.4 cm.
2.3.
2.3. Data
DataCollection
Collection
Kohlrabi plants were harvested when 50% of the bulbs in each DOT treatment reached
Kohlrabi plants were harvested when 50% of the bulbs in each DOT treatment
6.4 cm in diameter. Yield was determined by measuring the fresh leaf and bulb weight
reached 6.4 cm in diameter. Yield was determined by measuring the fresh leaf and bulb
(kg) of ten randomly chosen plants per plot immediately after harvest. Two plants from
weight
each plot(kg)
wereof randomly
ten randomly chosen
selected plants per
for further plot immediately
analysis. Leaves were after harvest.
removed fromTwo plants
bulbs,
fromfresh
and eachweight
plot were randomly
of bulbs selected
and leaves werefor further determined.
separately analysis. Leaves were
The leaf removed
area (cm2 ) offrom
bulbs, and fresh weight of bulbs and leaves were separately determined.
each plant was measured using a LI-3100C Area Meter (LI-COR, Lincoln, NE, USA). Total The leaf area
(cm 2) of each plant was measured using a LI-3100C Area Meter (LI-COR, Lincoln, NE,
leaf area was calculated by adding the areas of all leaves per plant. Leaf and bulb samples
USA).
were Total
dried in leaf areaair
a forced wasoven at 109 ◦ Cby
calculated adding
until the areas ofa all
they maintained leavesweight.
constant per plant. Leaf and
Moisture
bulb samples
content of leaveswere
anddried
bulbsinwasa forced air oven
calculated at 109 °C the
by subtracting until
drythey maintained
weight a constant
value from the
weight.
fresh Moisture content of leaves and bulbs was calculated by subtracting the dry weight
weight.
value from the fresh weight.
2.4. Nutrient Analysis
The nutrient
2.4. Nutrient content of kohlrabi bulbs was determined using a dry ashing procedure.
Analysis
Dried bulb and leaf tissue samples were ground in a stainless steel container using a
The nutrient content of kohlrabi bulbs was determined using a dry ashing procedure.
Vitamix 5200 high-power blender to pass through a 20-mesh sieve (Dual Manufacturing
Dried bulb and leaf tissue samples were ground in a stainless steel container using a Vit-
Company, Inc., Franklin Park, IL, USA) and homogenized. An amount of 0.2 g of the
amix 5200samples
powdered high-power blender to
were weighed pass
into through
porcelain a 20-mesh
crucibles sieve into
and placed (Dual Manufacturing
a combustion
Company,
oven Inc., Franklin
at a temperature Park,
of 500 ◦ CIL,
forUSA) and Afterwards,
6 hours. homogenized. theAn amountwere
crucibles of 0.2 g of thetopow-
allowed
cool to room temperature and 15 ml of 10% HCl was added to each sample. The resultingoven
dered samples were weighed into porcelain crucibles and placed into a combustion
at a temperature
mixture of 500
was filtered °C forWhatman
through 6 hours. #2
Afterwards, theFinally,
filter paper. crucibles
thewere allowed
Cu, Mn, to cool
Fe, Ca, K, to
room
and Mgtemperature andof
concentrations 15each
ml ofsample
10% HClwerewas added tousing
quantified each microwave
sample. Theplasma-atomic
resulting mixture
was filtered
emission through (4210
spectroscopy Whatman
MP-AES,#2 filter paper.
Agilent Finally, the
Technologies, Cu,Clara,
Santa Mn, CA,
Fe, USA).
Ca, K, and Mg
concentrations of each sample were quantified using microwave plasma-atomic emission
2.5. Statistical Analysis
spectroscopy (4210 MP-AES, Agilent Technologies, Santa Clara, CA, USA).
Statistical analyses were performed using the GLM and CORR procedures in SAS,
version 9.4 (SAS
2.5. Statistical Institute, Cary, NC, USA). The two experimental years were combined
Analysis
for analysis, resulting in eight total replications. Effects that were significant at the p < 0.05
Statistical analyses were performed using the GLM and CORR procedures in SAS,
level were fitted to regression curves.
version 9.4 (SAS Institute, Cary, NC, USA). The two experimental years were combined
for analysis, resulting in eight total replications. Effects that were significant at the p < 0.05
level were fitted to regression curves.Agronomy 2022, 12, 770 4 of 11
3. Results
3.1. Statistical Analysis
The analysis of variance results for bulb yield and nutrient content as affected by the
date of transplanting are shown in Table 1. The analysis of variance results for leaf yield,
total leaf area, leaf number, and leaf nutrient content as affected by the date of transplanting
are shown in Table 2.
Table 1. Analysis of variance results for yield and nutrient content of kohlrabi bulbs, as affected by
date of transplanting.
Yield Nutrient
Cu Mn Fe Ca K Mg
DOTAgronomy 2022, 12, 770 5 of 11
12, x FOR PEER REVIEW Table 4. Time of seeding, transplanting, and harvesting of kohlrabi during the two5 growing
of 11 seasons.
2020 2021
Table 4. Time of seeding,
Seeded transplanting,
Transplanted Harvestedand harvesting
DTH of kohlrabi
Seededduring the two growing
Transplanted seasons. DTH
Harvested
DOT 1 2020
25 March 23 April 22 June 60 2021
25 March 23 April 21 June 59
Seeded DOT 2 4 April Harvested
Transplanted 30 April DTH 22 June Seeded 53 4 April
Transplanted 30 April
Harvested 21 June
DTH 52
5 March DOT23
3 April14 April 22 June
7 May 60 1 July 25 March55 2314April
April 7 May
21 June 24 June
59 48
4 April DOT30
4 April24 April 22 June
14 May 53 13 July 4 April 60 3024April
April 15 21
MayJune 30 June
52 47
14 April 7 May DOT = date of55
1 July transplanting; DTH = days to harvest.
14 April 7 May 24 June 48
24 April 14 May 13 July 60 24 April 15 May 30 June 47
Soil temperature at the time of transplanting is presented in Table 5. In 2021, the soil
DOT = date of transplanting; DTH = days to harvest.
temperature at DOT 4 was 1.1 ◦ C warmer than DOT 2 and DOT 3, and 2.8 ◦ C warmer than
the first date of transplanting.
Table 5. Regional soil temperatures for Deerfield, MA.
Table 5. Regional soil
Soiltemperatures for Deerfield,
Temperature °C MA.
DOT 2021 5-Year AverageSoil Temperature10-Year
◦C Average
23/4 12.2 12.0 11.0
DOT 2021 5-Year Average 10-Year Average
30/4 13.3 11.3 11.8
23/4
07/5 13.3 14.0 12.2 12.0
14.5 11.0
30/4 13.3 11.3 11.8
14/5 15.0 12.11 13.9
07/5 13.3 14.0 14.5
DOT = date of transplanting. 14/5 15.0 12.11 13.9
DOT = date of transplanting.
3.3. Bulb and Leaf Yield
3.3. Bulb and Leaf
The date of transplanting hadYield
significant effects on both the total yield and bulb yield
of kohlrabi. However, Theleafdate
yield
of was not statistically
transplanting significantly
had significant effects ondifferent
both theamong DOT.
total yield and bulb yield
of kohlrabi. However,
Bulb yield, and consequently leaf yield
total yield, was not
increased statistically
with significantly different
later transplanting. Kohlrabiamong DOT.
transplanted on 14 May produced the highest bulb yields, equal to 20,297 kg ha (Table 4 Kohlrabi
Bulb yield, and consequently total yield, increased with later transplanting.
−1
and Figure 2). transplanted on 14 May produced the highest bulb yields, equal to 20,297 kg ha−1 (Table 4
and Figure 2).
30,000.00
y = 2597.4x + 13885
25,000.00
R² = 0.8683
Predicted yield kg ha-1
20,000.00 y = 1091.2x2 - 3051.8x + 15425
R² = 0.9213
15,000.00
10,000.00
Total Yield
5,000.00
Bulb Yield
0.00
23/4 30/4 07/5 14/5
DOT
Figure 2. The influence of date of transplanting on kohlrabi bulb and total (bulb and leaf) yield;
DOT = date of transplanting.
Figure 2. The influence of date of transplanting on kohlrabi bulb and total (bulb and leaf) yield; DOT
= date of transplanting.
3.4. Leaf Number and Total Leaf Area
The date of transplanting had a significant effect on the average number of leaves perAgronomy 2022, 12, 770 6 of 11
3.4. Leaf Number and Total Leaf Area
The date of transplanting had a significant effect on the average number of leaves per
plant, as well as the total leaf area and the cumulative area of all leaves per plant (Figure 3).
, x FOR PEER REVIEW While the number of leaves increased as DOT was delayed, the total leaf area demonstrated
6 of 11
a parabolic response and reached its highest value in DOT 3 (7 May). Correlations between
bulb yield, leaf yield, leaf area, and leaf number are presented in Table 6.
16
14
(a)
12
Number of leaves plant-1
10 y = 0.2143x - 9563
R² = 0.8824
8
6
4
2
0
23/4 30/4 07/5 14/5
DOT
1200
(b)
1000
Total leaf area plant-1 (cm2)
800
y = -69.23x2 + 412.24x + 237.81
600 R² = 0.9631
400
200
0
23/4 30/4 07/5 14/5
DOT
Figure 3. (a) Number of leaves and (b) total leaf area of kohlrabi influenced by date of transplanting;
Figure 3. (a) Number of
DOTleaves and
= date of (b) total leaf area of kohlrabi influenced by date of transplanting;
transplanting.
DOT = date of transplanting.
Table 6. Pearson correlation matrix of bulb yield, leaf yield, leaf area and leaf number. Results rep-
resent the averages of two growing seasons.
Leaf Yield Leaf Area Leaf Number
Bulb Yield −0.016 ns 0.047 ns 0.470 **
Leaf Yield 0.560 *** 0.310 nsAgronomy 2022, 12, 770 7 of 11
Table 6. Pearson correlation matrix of bulb yield, leaf yield, leaf area and leaf number. Results
represent the averages of two growing seasons.
Leaf Yield Leaf Area Leaf Number
Bulb Yield −0.016 ns 0.047 ns 0.470 **
Leaf Yield 0.560 *** 0.310 ns
Leaf Area 0.653 ***
**, *** Significant at the 0.01 and 0.001 probability levels, respectively; ns = non-significant.
3.5. Nutrient Concentration
The date of transplanting significantly impacted kohlrabi nutrient concentrations,
including the Mn, Fe, Cu, and Ca in bulbs, as well as Ca and Mn concentrations in leaves.
The mean concentrations of nutrients in kohlrabi bulbs and leaves for each DOT are shown
in Table 7. Among the measured nutrients, Mg and K concentrations in both bulbs and
leaves were not affected by DOT. For all other nutrients in which DOT had a significant
effect, higher concentrations were detected in the later DOTs. Correlations among the
nutrients in bulbs and leaves are presented in Table 8.
Table 7. The influence of DOT on nutrient concentrations of kohlrabi bulbs and leaves.
Bulb Nutrient Concentration mg g−1 Leaf Nutrient Concentration mg g−1
DOT Ca K Mg Cu Fe Mn Ca K Mg Cu Fe Mn
23/4 7.136 55.098 2.450 0.004 0.033 0.011 32.216 32.682 3.284 0.005 0.068 0.033
30/4 7.277 59.311 2.580 0.005 0.039 0.013 30.862 31.803 3.587 0.005 0.068 0.039
07/5 7.233 56.025 2.568 0.005 0.043 0.013 33.115 32.836 3.720 0.005 0.061 0.043
14/5 7.993 60.367 2.827 0.006 0.057 0.016 35.595 32.729 3.670 0.005 0.074 0.057
Trend L ** NS NS L ** L ** C ** L* NS NS NS NS L **
DOT = date of transplanting; L = linear, C = cubic; *, **, significant at the 0.05, and 0.01 probability level,
respectively; ns = non-significant.
Table 8. Pearson correlation matrix of nutrients in bulbs and leaves of kohlrabi. Results represent the
average of two growing seasons.
Bulb K Bulb Mg Bulb Cu Bulb Fe Bulb Mn Leaf Ca Leaf K Leaf Mg Leaf Cu Leaf Fe Leaf Mn
ns ns ns
Bulb Ca 0.725 *** 0.786 *** 0.636 *** 0.562 *** 0.403 * 0.600 *** 0.149 −0.007 0.565 *** 0.040 −0.002 ns
Bulb K 0.733 *** 0.381 * 0.508 ** 0.196 ns 0.416 * 0.443 * −0.127 ns 0.283 ns −0.121 ns −0.199 ns
Bulb Mg 0.640 *** 0.405 * 0.477 ** 0.550 ** 0.319 ns 0.373 * 0.457 ** 0.144 ns 0.054 ns
Bulb Cu 0.620 *** 0.664 *** 0.488 ** 0.112 ns 0.168 ns 0.495 ** 0.041 ns 0.041 ns
Bulb Fe 0.467 ** 0.388 * 0.029 ns −0.217 ns 0.237 ns 0.202 ns 0.192 ns
Bulb Mn 0.379 * −0.042 ns 0.308 ns 0.449 ** 0.297 ns 0.605 ***
Leaf Ca 0.224 ns 0.273 ns 0.257 ns 0.073 ns 0.243 ns
Leaf K −0.082 ns 0.221 ns −0.420 * −0.290 ns
Leaf Mg 0.263 ns 0.236 ns 0.276 ns
Leaf Cu 0.091 ns 0.104 ns
Leaf Fe 0.485 **
*, **, *** Significant at the 0.05, 0.01, and 0.001 probability level, respectively; ns = non-significant.
4. Discussion
The growing season in the Northeastern United States is relatively short, resulting in
limited opportunities for double cropping. As a cool-season vegetable crop, kohlrabi can
potentially be planted as early as the beginning of April, making it a promising candidate
for double cropping. This study evaluated the impacts of spring transplanting dates on
the yield and nutrient concentration of kohlrabi bulbs and leaves. Higher yields and
nutrient concentrations were observed in kohlrabi transplanted in May rather than inAgronomy 2022, 12, 770 8 of 11
April. Earlier spring transplanting allowed for an earlier kohlrabi harvest but resulted in a
considerable yield penalty. Averaging two growing seasons, we found bulb yield increased
by 307 kg ha−1 for each day that transplanting was delayed, beginning 23 April. This delay
in transplanting also resulted in a greater accumulation of nutrients in both the bulbs and
leaves of kohlrabi. Among the measured nutrients, the accumulation of Ca, Cu, Fe, and Mn
in bulbs, as well as Ca and Mn in leaves, increased with the delay in transplanting time.
Although the number of leaves per plant increased with the time of transplanting,
the total leaf area was highest in the two intermediate transplanting dates: 30 April and
7 May. Leaf yield was not significantly different among transplanting dates and showed
no significant correlation with leaf number. This suggests that, although there are more
kohlrabi leaves per plant at later dates of transplanting, they are generally smaller and
lighter than the leaves of those transplanted earlier in the spring. The number of leaves
showed a moderate correlation with bulb yield, suggesting that leaf position may play a
greater role in photosynthetic efficiency than the total leaf area.
Lower yields and nutrient concentrations of kohlrabi transplanted earlier in the spring
can likely be explained, at least in part, by cooler soil temperatures in earlier DOTs (Table 3).
Low root zone temperatures have been shown to negatively impact yield, growth rate, and
nutrient accumulation in vegetable crops, including cucumber, red leaf lettuce, tomato, and
several brassica species [25–30]. Chinese broccoli (Brassica oleracea var. alboglabra) grown at
10 ◦ C root zone temperature, as compared to 20 ◦ C, resulted in a 26% reduction in yield
and accumulated less K, Ca, and Mg in the leaves [30].
It has been well-documented that low soil temperatures reduce water absorption by
crop roots and hinder plant growth by limiting respiration, and thus, the metabolic activity
of root cells [31–34]. However, the specific mechanisms by which low soil temperature
impacts plant physiology vary by crop species. In red leaf lettuce, decreased root oxygen
consumption caused by low root zone temperatures (10 ◦ C) led to oxidative stress in the
leaves, resulting in reduced final yield [20]. Low root zone temperatures have been shown
to cause damage to photosystem II in African snake tomato, resulting in photochemical
inhibition and a decreasing net photosynthetic efficiency [35]. By contrast, root zone
temperature has been shown to have no direct effect on the photosynthesis of Brassica
rapa, despite negatively impacting crop growth rate and biomass accumulation [36–38].
The negative impacts of low root zone temperatures on Brassica species can likely be
explained by a decreased nitrate uptake efficiency, and water and solute flow rates through
the roots [31,39]. In the present study, neither kohlrabi leaf yield nor leaf area were
significantly correlated with bulb yield, indicating that photosynthetic efficiency was
sufficient across all transplanting dates. Additionally, there were no statistically significant
differences in specific leaf area among transplanting dates, further suggesting that the
reductions in yield and nutrient concentrations in earlier transplanting dates were unrelated
to photosynthetic efficiency.
Although kohlrabi is primarily grown for its enlarged, rounded stems, the leaves
of kohlrabi can provide an additional commodity to growers, reduce processing labor,
and increase the variety of nutrient-dense leafy greens available to consumers. Table 9
demonstrates a comparison between the nutrient content of fresh kohlrabi leaves and those
of kale, which are comparable in taste and texture.Agronomy 2022, 12, 770 9 of 11
Table 9. Comparison of selected nutrient content of leaves of kohlrabi (Brassica oleracea var. sabellica)
and kale (Brassica oleracea var. sabellica).
Leaf Nutrient Content (mg 100 g−1 Fresh Weight)
Kohlrabi Kale *
Ca 427.00 254.00
K 393.00 348.00
Mg 44.00 29.00
Cu 0.06 0.05
Fe 0.88 1.60
Mn 0.47 0.92
* Kale nutrient content was obtained from the FoodData Central USDA database (USDA, 2019). Kohlrabi nutrient
content was obtained from oven-dried leaves and then adjusted to 88% moisture content. The presented nutrient
contents of the kohlrabi leaves are based on the values measured in the latest date of transplanting (DOT).
As shown in Table 9, the concentrations of macro minerals (Ca, Mg, K) are higher
in kohlrabi leaves, with the concentrations of Ca and Mg over 1.5 times higher than the
concentrations found in those of kale. By contrast, kale leaves are generally richer in
micronutrients than kohlrabi leaves. These findings suggest that kohlrabi leaves can be
consumed as a rich edible source of nutrients and provide an additional commodity to
growers who may wish to market both the bulbs and fresh leaves of kohlrabi. Except for Fe,
a highly immobile nutrient, concentrations of nutrients in bulbs and leaves were positively
correlated, suggesting that nutrients acquired by the plant are consistently distributed
throughout the plant and that higher kohlrabi bulb nutrient concentrations correspond to
higher nutrient concentrations in the leaves. Therefore, the optimum time of harvest is the
same for leaves and bulbs, allowing them to be simultaneously harvested and marketed.
5. Conclusions
The results of the current study revealed that although the early transplanting of
kohlrabi provides an opportunity for earlier harvest, and thus double cropping, this
advantage comes with significant reductions in the yield and nutritional value of both
bulbs and leaves. Therefore, in northern climates, transplanting kohlrabi in May rather than
April optimizes crop yield and quality. Further research is needed to better understand
the exact mechanisms by which kohlrabi yield responds to a low soil temperature in early
DOTs. Additionally, this study confirmed kohlrabi leaves to be a promising fresh market
commodity by comparing their nutrient content with related brassica leaves such as kale, a
popular leafy vegetable known for its health benefits.
Author Contributions: Conceptualization, A.S. and M.H.; methodology, A.S. and M.H.; validation,
A.S.; formal analysis, A.S.; investigation, A.S.; data curation, A.S.; writing—original draft preparation,
A.S.; writing—review and editing, M.H.; visualization, A.S.; supervision, M.H. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: The authors thank the Center for Agriculture, Food, and the Environment for
all their support, including graduate and undergraduate summer research funding. The authors
would also like to acknowledge Olivia Larrivee for technical assistance. The success of the project
was dependent on the field support of the Crop and Animal Research Center crew, University of
Massachusetts Amherst.
Conflicts of Interest: The authors declare no conflict of interest.Agronomy 2022, 12, 770 10 of 11
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