1. Introduction
Kohlrabi (
Brassica oleracea var. gongylodes) is a cool-season, biennial vegetable revered for its mildly sweet flavor and many health benefits. Kohlrabi is known by many names around the world and is cultivated as an annual crop in many parts of Europe, Asia, and North America. Like other cruciferous vegetables, kohlrabi is a potent source of essential nutrients and bioactive compounds produced by plant secondary metabolites [
1,
2]. The regular consumption of kohlrabi can aid in reducing the risk of chronic disease, depressive disorders, and many types of cancer [
2,
3,
4,
5].
Kohlrabi is primarily cultivated for its round, thickened stem, often referred to as the ‘bulb’, which resembles a turnip at maturity. Cultivars can be white, pale green, or purple and differ significantly in size and storage ability. Fresh market spring varieties 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 sweeter in taste, can be cooked, pickled, or consumed raw in salads [
1]. The flavorful leaves of kohlrabi, comparable to kale in appearance and texture, are also highly nutritious and can provide an additional commodity to growers and consumers.
Secondary metabolites found in kohlrabi, including anthocyanins, phenolic compounds, and glucosinolates contribute to its antioxidant and anti-inflammatory properties [
4,
6]. Unlike pale green varieties, purple kohlrabi cultivars contain anthocyanins, bioactive flavonoids important for human health, as well as higher concentrations of phenolic compounds resulting in greater anti-inflammatory and antidiabetic effects [
7,
8,
9]. While the majority of research has focused on the health benefits of kohlrabi bulbs, the antibacterial 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,
15,
16,
17,
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,
20,
21,
22,
23,
24]. The growing season in the Northeastern United States is relatively short; therefore, identifying 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 productivity. 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 Massachusetts 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 between 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.
2.3. Data Collection
Kohlrabi plants were harvested when 50% of the bulbs in each DOT treatment reached 6.4 cm in diameter. Yield was determined by measuring the fresh leaf and bulb weight (kg) of ten randomly chosen plants per plot immediately after harvest. Two plants from each plot were randomly selected for further analysis. Leaves were removed from bulbs, and fresh weight of bulbs and leaves were separately determined. The leaf area (cm2) of each plant was measured using a LI-3100C Area Meter (LI-COR, Lincoln, NE, USA). Total leaf area was calculated by adding the areas of all leaves per plant. Leaf and bulb samples were dried in a forced air oven at 109 °C until they maintained a constant weight. Moisture content of leaves and bulbs was calculated by subtracting the dry weight value from the fresh weight.
2.4. Nutrient Analysis
The nutrient content of kohlrabi bulbs was determined using a dry ashing procedure. Dried bulb and leaf tissue samples were ground in a stainless steel container using a Vitamix 5200 high-power blender to pass through a 20-mesh sieve (Dual Manufacturing Company, Inc., Franklin Park, IL, USA) and homogenized. An amount of 0.2 g of the powdered samples were weighed into porcelain crucibles and placed into a combustion oven at a temperature of 500 °C for 6 hours. Afterwards, the crucibles were allowed to cool to room temperature and 15 ml of 10% HCl was added to each sample. The resulting mixture was filtered through Whatman #2 filter paper. Finally, the Cu, Mn, Fe, Ca, K, and Mg concentrations of each sample were quantified using microwave plasma-atomic emission spectroscopy (4210 MP-AES, Agilent Technologies, Santa Clara, CA, USA).
2.5. Statistical Analysis
Statistical analyses were performed using the GLM and CORR procedures in SAS, 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.
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 in 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,
26,
27,
28,
29,
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,
32,
33,
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,
37,
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.
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.