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Article

Combined Effects of Drought and Soil Fertility on the Synthesis of Vitamins in Green Leafy Vegetables

by
Taewan Park
1,
Sahrah Fischer
1,
Christine Lambert
2,
Thomas Hilger
1,*,
Irmgard Jordan
3 and
Georg Cadisch
1
1
Institute of Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute), University of Hohenheim, 70593 Stuttgart, Germany
2
Department of Nutritional Science, University of Hohenheim, 70593 Stuttgart, Germany
3
Alliance Bioversity International and CIAT, Food Environment and Consumer Behaviour Lever, 00621 Nairobi, Kenya
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(5), 984; https://doi.org/10.3390/agriculture13050984
Submission received: 14 March 2023 / Revised: 22 April 2023 / Accepted: 27 April 2023 / Published: 29 April 2023
(This article belongs to the Special Issue Crop Responses and Tolerance to Abiotic Stress)
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Versions Notes

Abstract

:
Green leafy vegetables, such as Vigna unguiculata, Brassica oleraceae, and Solanum scabrum, are important sources of vitamins A, B1, and C. Although vitamin deficiencies considerably affect human health, not much is known about the effects of changing soil and climate conditions on vegetable vitamin concentrations. The effects of high or low soil fertility and three drought intensities (75%, 50%, and 25% pot capacity) on three plant species were analysed (n = 48 pots) in a greenhouse trial. The fresh yield was reduced in all the vegetables as a result of lower soil fertility during a severe drought. The vitamin concentrations increased with increasing drought stress in some species. Regardless, the total vitamin yields showed a net decrease due to the significant biomass loss. Changes in vitamin concentrations as a result of a degrading environment and increasing climate change events are an important factor to be considered for food composition calculations and nutrient balances, particularly due to the consequences on human health, and should therefore be considered in agricultural trials.

Graphical Abstract

1. Introduction

Changes in crop growth conditions do not only affect yields, they also influence the nutrient content of the crop. In areas with a high prevalence of nutritional deficiencies in the human population, even small differences in the amounts of vitamins due to less favorable growing conditions may influence the deficiency rate. Low (pro-)vitamin A (including pro-vitamin A carotenoids, such as β-carotene) intake is one of the most significant threats to health in Sub-Saharan Africa (SSA). In this region, up to 20% of children below five years of age suffered from a vitamin A deficiency in 2016 [1]. Deficient intake of vitamin A impairs visual functions and growth, and increases the risk for respiratory infections, which increases the risk of morbidity and mortality [2]. Vitamin B1 (thiamine) is a common deficiency in staple food-dominated diets, exacerbated by practices such as milling grains [3]. A vitamin B1 deficiency can lead to severe neurological and cardiovascular effects and even death [3]. Vitamin C (ascorbic acid) deficiency is relatively common in middle- to low-income countries. In Kampala, Uganda, for example, 70% of pregnant women were deficient in vitamin C [4]. Vitamin C deficiencies in the diet, in addition to the negative effects of an actual vitamin C deficiency, may contribute to a lower bioavailability of essential minerals from diets, such as Fe and Zn [5].
Green leafy vegetables are rich in micronutrients and other phytochemicals, such as vitamins, which are vital for human health. Many studies show the benefits of increasing green leafy vegetable consumption on human health. An increased and regular intake of green leafy vegetables is, for example, associated with a lower risk for cardiovascular diseases (CVDs) [6]. The authors of a nutrition study in Tanzania confirmed that an increased consumption of green leafy vegetables can contribute to a lower prevalence of anemia and micronutrient deficiencies, particularly in resource-poor communities [7].
Plants contain a wide range of metabolites that have antioxidant potential [8,9] and are pre-cursors of vitamins, essential to human health. Plants produce co-enzymes and antioxidants, such as ascorbic acid, thiamine, and β-carotene, for a specific purpose [10], e.g., to protect themselves from environmental stressors, such as drought or poor soil fertility. Drought causes the production of reactive oxygen species (ROS), which, in turn, can lead to oxidative damage to the photosynthesis apparatus, and, hence, to reduced primary production [11]. Different carotenes and ascorbic acid are the main ROS detoxifying compounds [10,11]. Thiamine is also an important component of plant stress response, in addition to its other functions [12]. For lack of a unifying term in plant nutrition, the vitamin pre-cursors will be referred to as vitamins in this study.
Soil fertility provides the basis of plant production by providing plants with sufficient nutrients and water. Varying levels of soil fertility affect food yields, as well as food quality in terms of mineral nutrient concentrations [13]. Furthermore, vitamin concentrations were found to be inversely affected by nitrogen (N) fertilizer application [14,15], thereby suggesting the possibility that soil fertility could also affect vitamin concentrations.
Climate change has led to an increase in extreme weather events, such as prolonged and more frequently occurring droughts [16]. While the effects of different climate change variables (increasing temperatures, drought, increasing CO2 and O3) on mineral and macronutrient concentrations in different plant parts have been measured [17,18,19], the effects on vitamin concentrations have not been covered thus far.
Both climate change and soil degradation represent two of the biggest challenges for the sustained production of high quantity and quality foods [20], while concurrently being partially responsible for both occurrences. This paper will focus on the following research question: (i) Does soil fertility, (ii) water stress (drought), and the combination of drought stress and varying soil fertility significantly affect the contents of β-carotene, ascorbic acid, and thiamine in the leaves of three green leafy vegetables (Brassica oleraceae L., Solanum scabrum L., Vigna unguiculata L.)?
As the production of vitamins is a stress reaction, especially to stressors such as drought, an increase in vitamin concentrations in all green leafy vegetables with increasing drought is expected. Soil fertility is key for plant nutrient intake, and largely provides the nutrients needed to produce vitamins. Therefore, it is expected that in high soil fertility, the vitamin concentrations will be higher in all the drought severities than in low soil fertility of the same drought severity. The fresh weight is expected to decrease with increasing drought and decreasing soil fertility.

2. Materials and Methods

This study was part of the project “Education and Training for Sustainable Agriculture and Nutrition in East Africa (EaTSANE)”. Since the main research area for the EaTSANE project was East Africa, namely the areas of Teso South, Kenya (a region with low soil fertility) and Kapchorwa, Uganda (a region with high soil fertility), the trial was set up to mimic the local conditions. Three local green leafy vegetables were analysed: sukuma wiki (Brassica oleraceae L.), black nightshade (Solanum scabrum L.), and cowpea (Vigna unguiculata L.), using soils with properties similar to the soils found in the East African research areas.
The temperature in a greenhouse of the University of Hohenheim, Germany was adjusted to 22 °C, which is the mean of the average temperature during the growing season in Kapchorwa and Teso South (Table S1). Daylight was allowed from 6 am to 6 pm, as in the target areas. The greenhouse temperature and humidity were monitored during the entire trial using a TGP-4500 Tiny Tag Plus 2 (Gemini Data Loggers, Ltd., Chichester, UK). Two soils were selected to represent the soils of Kapchorwa, Uganda and Teso South, Kenya [21]. The first soil collected showed similarities to the soils with lower fertility ferralsols in Teso South and was classified as an endostagnic alisol collected in Tauchenweiler, Germany (48°47′13.3″ N and 10°02′18.9″ E). In this paper, the soil with lower fertility will be referred to as “infertile”. The second soil, comparable to a higher fertility nitisol, such as from Kapchorwa, Uganda, was an endoleptic cambisol, and was collected in Höwenegg, Germany (47°54′55.7″ N and 8°44′25.7″ E) (Table S3). In this paper, the soil with higher fertility will be referred to as “fertile”.
The soils gathered in Germany show similar properties to the soils of Teso South, Kenya and Kapchorwa, Uganda. The low fertility soils both feature sandy soils, whereas the high fertility soils feature loamy clays. While the pH of both of the low fertility soils is acidic (German alisol: 4.0; Kenyan ferralsol: 4.94), the pH of the high fertility soil was 5.6 in both cases [13]. Both of the low fertility soils showed a very low amount of soil organic matter, whereas the soils of higher fertility showed a higher amount.
The three green leafy vegetables were planted into pots on 17 September 2019. The plastic pots (37.5 cm height and 16 cm diameter) were filled with 1.5 kg gravel as drainage, with a depth of 5 cm. The two soil types were dried and sieved (0.9 cm2 sieve type), and 5.5 kg of each soil was placed into the different pots. After sowing, the soil was covered by a thin layer of sand (~10 mm, 80 g) to prevent excessive evaporation and soil cracking. Each pot was sown with four seeds of each green leafy vegetable, and then later thinned out, leaving one seedling per pot.
The pot water capacity (PC) was analysed using the gravimetric methods provided by [22]. Three treatments in the watering regime were used: 75% PC as control, 50% PC as mild stress, and 25% as severe drought stress. A total of 144 pots were used, 48 per plant species, with two soil fertilities (fertile and low fertility soil), and three drought intensities (control 1, control 2, mild, and severe) (Table S2). The double control group was used to improve the statistical power [23]. Each treatment had six replicates organised into a randomized complete block design (Figure S1). After plant germination, all the pots were weighed and watered every two days to maintain the assigned drought conditions.
Fifty-one days after sowing, flower buds of the control plants became visible. The entire aboveground part of all the pots was harvested and frozen after recording their fresh weight. The belowground biomass (roots) were washed, then dried with paper towels, and the fresh weight recorded. The fresh leaves were stored at −80 °C and then lyophilized using a freeze dryer (LyoQuest laboratory freeze dryer, Telstar, Spain) for 24 h, and the dry weight was recorded. The samples were then ground, homogenized, and stored in the dark at −20 °C.
The plant water use efficiency (WUE) was calculated as the ratio between the aboveground biomass (yield) and water use (evapotranspiration). WUE is generally high when the plants are exposed to drought conditions or are drought tolerant [24], and is, therefore, used to evaluate drought resistance [25]. The WUE of the yield was determined by the division of the fresh yield by water consumption [24].
The concentrations of thiamine, β-carotene, and ascorbic acid were measured in the leaf samples using high performance liquid chromatography (HPLC) at the Institute of Nutritional Sciences, University of Hohenheim. The HPLC data were recorded and analysed using the Shimadzu LabSolutions Software (Version 5.54, Shimadzu Deutschland GmbH, Duisburg, Germany). The chromatographic analysis of the vitamins was conducted using the Shimadzu HPLC system. The Shimadzu HPLC system consisted of a DGU 20A3 Degassing Unit, an LC-20AT Pump, a SIL-20AC HT AutoSampler, and a CBM-20-A Communication Module. The method for thiamine analysis was based on the European Standard (DIN EN 14122:2014) of vitamin B1 determination [26]; however, minor adaptations were made. β-carotene was measured using the method of [27]. The retinol equivalents (REs) of β-carotene were calculated using a factor 1 μg RE = 6 μg β-carotene [28]. Ascorbic acid was measured using the method of [29].
Since C3 plants close their stomata to reduce water loss during water-limiting conditions, 13CO2 fixation is decreased and δ13C is discriminated due to reduced CO2 diffusion in and out of the leaves [30]. Therefore, δ13C isotope discrimination has been used to measure water stress in C3 plants [31]. The δ13C contents of green leafy vegetables were analysed by comparing the control (75% PC) to the drought treatments (50% and 25% PC) using randomly selected subsamples (n = 4 per PC of each species) of the green leafy vegetable samples. The selected subsamples were measured with a Euro EA Elemental Analyser (Euro Vector, Pavia, Italy) coupled to a Finnigan Delta IRMS (Thermo Fischer Scientific, Waltham, MA, USA) at the core facility of the University of Hohenheim.
The fulfilment of the recommended nutrient intake (RNI) of vitamin A, ascorbic acid, and thiamine by consuming 150 g of green leafy vegetables (fresh weight) as an average serving size was calculated for female adults (19–50 years) [28].
SAS (SAS® University Edition, SAS Institute Inc., Cary, NC, USA) was used for the statistical analysis. The fresh yield, belowground biomass, number of nodules, irrigation water added, water use efficiency (WUE), pot capacity (PC), thiamine, β-carotene, and ascorbic acid were subjected to analysis of variance (ANOVA) for each treatment. ANOVA using PROC GLIMMIX was used to compare the treatments between the plant species and the significance of factors by an F-test at α = 0.05. A two-factorial model was fitted as an equation (SUPPL MAT). The interaction of the treatment and species was significant; therefore, the cell means were compared using a SLICE statement with the SLICEBY options, i.e., SLICEBY = treatments and SLICEBY = species, in the GLIMMIX procedure. The means of yield and WUE differences between the different soil fertility and drought conditions were compared. The vitamin concentrations were expressed on a fresh weight basis. The absolute vitamin amounts (mg per pot) were calculated for thiamine, β-carotene, and ascorbic acid by the multiplication of the fresh leaf yield (g/pot) by the vitamin content (mg/100 g FW) and divided by 100 [32]. The vitamin data were used to analyse whether the changed vitamin contents compensate for the treatments’ changed yield. An analysis of covariance (ANCOVA) using PROC GLM was used to compare the treatment means adjusted for a covariate soil fertility within the plant species.

3. Results

3.1. Plant Yield Analysis

All the fresh leaf yields of the three green leafy vegetables were significantly higher (B. oleraceae: p < 0.01, V. unguiculata: p < 0.001, S. scabrum: p < 0.0001) in fertile soil than in low fertility soil with the same watering regime, with the exception of B. oleraceae with a 75% pot capacity (PC), where the difference was not significant (Table 1). Regardless of the soil fertility, the fresh leaf yields of the green leafy vegetables decreased with an increasing level of drought (Table 1 and Figure S1). However, the yield losses caused by drought did not differ significantly between the fertile and low fertility soils (Figure 1).
B. oleraceae showed the highest yield loss with increasing drought regardless of the soil fertility, as evidenced in the steeper slope increase in the linear regression (Figure 1). Nevertheless, B. oleraceae obtained a higher total fresh yield under no drought (control, 75% PC) (p < 0.0001) and mild drought (50% PC) (p < 0.0001) conditions than the other two green leafy vegetables (Table 2). In contrast, S. scabrum had the highest yield loss with decreasing soil fertility under all watering regimes, shown by the highest interceptor differences between the regression lines of fertile and low fertility soils, while B. oleraceae showed the lowest yield loss by different soil fertility (Figure 1). The belowground biomass was significantly lower in the low fertility soil than in the fertile soil for all the green leafy vegetables (all p < 0.0001) (Table 1). Only severe drought (25% PC) significantly (p < 0.05) decreased the belowground fresh weight of all the species in the fertile soil (Table 1).
Within all the vegetables, the WUE changed more drastically in the drought treatments in fertile soil than in the drought treatments on infertile soil with increasing drought intensity. The exception to this is S. scabrum, where the changes in fertile and infertile soil with increasing drought intensity were similar.
The WUE of B. oleraceae was higher than V. unguiculata and S. scabrum under severe drought (25% PC) in the fertile soil, which was the highest WUE at 43.2 g/L (F-test, p < 0.0001) among the three green leafy vegetables under all the treatments, while the yield was not significantly different (p < 0.05) (Table 1). In the low fertility soil, B. oleraceae had the highest water demand under control (75%), and the yield was higher than the other green leafy vegetables (Table 1).

3.2. Δ13C Measurement as an Indicator for Water Stress

All three green leafy vegetables showed an increase in their δ13C signature, i.e., less negative δ13C values, with the decreasing irrigated water amount, i.e., increasing drought intensities (Figure S2). Under drought conditions, V. unguiculata had the highest changes of δ13C values, ranging between −31.76‰ and −26.67‰ (R2 = 0.611, p = 0.0027) for leaves under both soil fertilities, followed by B. oleraceae with ranges between −34.48‰ and −32.62‰ (R2 = 0.619, p = 0.0024). S. scabrum also showed an increase of δ13C values, but the change was not significant (Figure S2).

3.3. Effects of Drought and Soil Fertility on Vitamin Concentrations

3.3.1. Thiamine

The thiamine concentrations of B. oleraceae (R2 = 0.47; p < 0.05) and V. unguiculata (R2 = 0.54; p < 0.01) nearly doubled in response to increasing drought intensities in the low fertility soil (Figure 2). In V. unguiculata, the thiamine concentrations were two times higher than B. oleraceae with the combination of low fertility soil and drought, as the regression line’s slope was double (Figure 2). However, in the fertile soil, the thiamine concentrations did not respond to drought in B. oleraceae or only slightly increased in V. unguiculata (R2 = 0.46; p < 0.05) (Figure 2). In S. scabrum, the thiamine concentration was not affected by drought, but was significantly higher (p = 0.005) in the low fertility soil compared to the fertile soil (Figure 2). Despite the increase in the thiamine concentration, the total thiamine yields (fresh leaf yield (g/pot) x thiamine concentration as mg/100 g fresh weight (FW)) of V. unguiculata (R2 = 0.60; p < 0.001) and S. scabrum (R2 = 0.64; p < 0.001) were significantly lower in the low fertility soil than in the fertile soil and fell with increasing drought (Figure 3).
There was no interaction between drought and soil fertility in the thiamine yield (Figure 3). In B. oleraceae, an increased thiamine concentration through the drought and low soil fertility compensated for the decline in fresh yield in the lower soil fertility, as the thiamine yield of B. oleraceae showed no differences between soil fertilities (Figure 1, Figure 2 and Figure 3).

3.3.2. Beta-Carotene (Pro-Vitamin A)

Interactions between soil fertility and drought were found in the β-carotene concentrations of V. unguiculata (p = 0.0038) and S. scabrum (p < 0.0001) (Figure 2). The β-carotene concentrations in V. unguiculata (R2 = 0.24; p < 0.05) and S. scabrum (R2 = 0.40; p < 0.01) fell significantly due to the combination of low soil fertility and increasing drought, while the concentrations rose in the fertile soil under drought conditions, but only significantly in S. scabrum (R2 = 0.40; p < 0.001) (Figure 2). When comparing the β-carotene concentration changes in the low fertility soil, the β-carotene concentrations of V. unguiculata and S. scabrum decreased significantly from the control (75% PC) to severe drought (25% PC) (Table 2). In the fertile soil (S), the β-carotene concentration of S. scabrum increased during the drought treatment (Table 2). B. oleraceae β-carotene concentrations were not significantly affected by soil fertility and drought treatments (Figure 2 and Table 2). Despite the inverse effects of the soil fertility by drought on the β-carotene concentrations of V. unguiculata and S. scabrum (Figure 2), there was no interaction between the soil fertility and drought on the β-carotene yields (Figure 3). Furthermore, the β-carotene yields of all three green leafy vegetables were higher in the fertile soil than in the low fertility soil (S) (Figure 3).

3.3.3. Ascorbic Acid

In B. oleraceae, the ascorbic acid content significantly decreased with increasing drought (R2 = 0.39; p < 0.01), and the concentration was consistently higher in the fertile soil regardless of the drought intensity (Figure 2). In contrast, in V. unguiculata, the ascorbic acid concentration increased with severe drought (25% PC) in the low fertility soil (R2 = 0.45; p < 0.01), while the concentration was not significantly affected by drought in the fertile soil (Figure 2). The ascorbic acid concentration of S. scabrum rose with drought (R2 = 0.13; p < 0.05) but was not affected by soil fertility (Figure 2). The ascorbic acid yield was higher in the fertile soil than in the low fertility soil and decreased with drought in each green leafy vegetable (Figure 3). The three green leafy vegetables showed no interaction between the soil and drought on the ascorbic acid yields (Figure 3).

3.4. Fulfilment of Recommended Nutrient Intake (RNI) by Green Leafy Vegetables under Different Growth Conditions

For β-carotene intake, 150 g of fresh leaves of V. unguiculata and S. scabrum reached more than 200% of vitamin A’s RNI, regardless of the treatments, while B. oleraceae reached more than 100% of the RNI for vitamin A in 150 g of fresh leaves (Table 3). The thiamine concentrations of B. oleraceae and V. unguiculata significantly increased with drought in the low fertility soil (Figure 2). However, S. scabrum could only contribute less than 6% of the RNI for vitamin B1 by 150 g fresh leaves (Table 3). For ascorbic acid intake, B. oleraceae reached more than 200% of RNI in 150 g of the fresh leaves under a well-watered condition in the fertile soil. Additionally, B. oleraceae could provide more than 100% of vitamin C’s RNI, regardless of the soil fertility, even when the ascorbic acid concentration was decreased by drought (Table 3). In V. unguiculata, the ascorbic acid content in 150 g of fresh leaves reached 170% of RNI in the low fertility soil under severe drought (25% PC) (Table 3). In addition, S. scabrum could provide over 160% of RNI for ascorbic acid concentration in 150 g of fresh leaves under severe drought conditions (25% PC), regardless of the soil fertility (Table 3).

3.5. Effects of Drought under Two Soil Fertilities on Fresh Yield and Vitamin Contents

Table 4 shows a summary of the drought’s effect on the fresh yield and vitamin contents of three green leafy vegetables under two different soil fertility conditions. For the fresh yield, the three green leafy vegetables were significantly decreased by drought (Figure 1 and Table 4). The β-carotene content was significantly increased in S. scabrum in fertile soil, while the content was significantly decreased by drought in V. unguiculata and S. scabrum in infertile soil (Figure 2 and Table 4). The ascorbic acid content was significantly increased in S. scabrum during drought, regardless of the soil condition, and in V. unguiculata in interfile soil. However, the ascorbic acid content was significantly decreased in B. oleraceae by drought (Figure 2 and Table 4). For thiamine, the content was increased by drought in V. unguiculata in both fertile and infertile soils and in B. oleraceae in infertile soil (Figure 2 and Table 4).

4. Discussion

Our research demonstrates that green leafy vegetables respond differently to soil fertility, drought stress, or its combination in regard to the yield and vitamin concentrations. The observed increase in the δ13C values, i.e., less negative δ13C values, in the three green leafy vegetables with a decreasing irrigated water amount typically indicate the experienced water stress for C3 plants [17,30,31].
The yields of the three green leafy vegetables most strongly decreased in the low fertility soil (endostagnic alisol) compared to the fertile soil (endoleptic cambisol) under all watering regimes due to the lower total soil fertility in the low fertility soil. B. oleraceae had the highest fresh biomass yield among all the three green leafy vegetables in both soil treatments under control and mild drought, therefore indicating its lower sensitivity to soil fertility compared to the other two vegetables. However, the decreasing yield rate by drought was higher in B. oleraceae than V. unguiculata and S. scabrum, confirming that B. oleraceae had the highest susceptibility to water stress among the three vegetables measured. Likewise, Brassica spp., such as kale (B. oleracea) [33] and Chinese cabbage (B. rapa) [34], are sensitive to drought stress.
Among the three green leafy vegetables, S. scabrum was the most vulnerable to low soil fertility regarding the fresh biomass yield. However, in the fertile soil, the S. scabrum yield was higher than V. unguiculata under mild drought and in the control. Although the fresh yield of V. unguiculata was lower than that of B. oleraceae and S. scabrum under the control (75% PC), the decreasing yield rate by drought was lower than the others. Therefore, V. unguiculata was the most drought-tolerant vegetable among the three green leafy vegetables. High drought tolerance under the severe drought of V. unguiculata was also identified in a comparison with two other green leafy vegetables: Amaranthus spp. and Corchorus olitorius [35].
The thiamine concentration was significantly increased by the combination of severe drought and low soil fertility in the leaves of B. oleraceae and V. unguiculata, whereas in S. scabrum, the thiamine concentration was only increased in low soil fertility, but not by drought. The increased thiamine concentrations were possibly due to the effect of induced oxidative stress through severe drought stress [36] and/or proton (H+) rhizotoxicity (lower pH in the low fertility soil) [37]. Oxidative stress induces the precursors of thiamine; hence, thiamine is upregulated [12,36,38].
The authors of [14] observed that β-carotene concentrations of kale responded positively to increasing soil N content, which was also observed in this trial (33% higher mineral N level in the fertile soil than in the low fertility soil). However, there were no significant β-carotene concentration changes in the leaves of the investigated green leafy vegetables by soil fertility under well-watered conditions in our trial. The different results compared to the literature might be explained by the N-fertilizer dose in the experiment by [14] being 217% higher between treatments, therefore much larger than the 33% difference in the present trial. In addition, as the soil materials of this study were under acidic conditions, the availability of soil nitrogen (N) might also be restricted by soil acidity, the form of nitrogen (NH4 vs. NO3) thereby not allowing for the full uptake of N present in the soil [39].
In contrast to thiamine, β-carotene showed a different reaction to the drought treatments between the fertile and interfile soils. The β-carotene concentrations were significantly decreased under the combination of drought and low soil fertility treatments in the leaves of V. unguiculata and S. scabrum. In the fertile soil, the β-carotene concentration was increased significantly in S. scabrum under severe drought. The latter result is consistent with the effect of severe drought (30% field capacity) on β-carotene concentrations in the leaves of amaranth, where the total antioxidant capacity was increased with induced drought stress [40]. The opposite effect that was observed in S. scabrum in low soil fertility can be attributed to the fact that S. scabrum is most affected by lower soil fertility, implying that it may be less susceptible to drought than to nutrient stress.
The ascorbic acid concentration increased with more intensive drought conditions in S. scabrum, regardless of the soil fertility, and in V. unguiculata in low fertility soil. A higher concentration of ascorbic acid during drought was also found in amaranth leaves, with an elevation of 163% during a severe drought treatment [40].
The ascorbic acid concentration was significantly reduced in the leaves of B. oleraceae by increasing drought intensity in both soil types. Similar results of lower ascorbic acid concentrations by drought were identified in rosemary (Rosmarinus officinalis L.), sage (Salvia officinalis L.), lemon balm (Melissa officinalis L.) [41], and soybean (Glycine max L. Merr) [42]. Our observed highly variable species response of ascorbic acid content to drought stress confirms results by [42]; however, this does not allow us to make general recommendations.
Regardless of the soil fertility, the absolute amounts of vitamins decreased with lower yields, although the concentrations of the analysed vitamins were mostly higher. The reason for this was the loss of total plant biomass as a result of drought severity. A similar drought reaction was observed regarding the mineral nutrient concentrations in maize (Zea mays L.) grains and cassava (Manihot esculenta Crantz) tubers during a mild drought in Kenya [17], i.e., while the nutrient concentrations of minerals increased as a result of mild drought, only the total mineral amount (yield x concentration) of calcium was significantly higher compared to a normal season. Therefore, higher vitamin concentrations under drought stress, observed particularly for the thiamine and ascorbic acid levels of V. unguiculata and S. scabrum, could not compensate for the loss of the total biomass.
Droughts are known to affect human health, particularly through the lower production of foods [17,43]. Droughts, often caused by El Niño–southern oscillation (ENSO) events, occur with increasing frequency in the southern hemisphere [43], which unfortunately also coincides with areas with high nutrient deficiencies [44]. The results of this study show that drought would also affect the total production of the vitamins A, B1, and C in both areas of high and low soil fertility, thereby reducing vitamin levels in many already nutritional deficient geographic areas. In areas with lower soil fertility, the negative impact on diets worsens as they show both lower mineral nutrient concentrations [13] and predominantly (vegetable-dependent) significantly lower vitamin concentrations and yields.
Increasing agrobiodiversity by including different species, even of the same food group (in this case, green leafy vegetables), can, however, increase food and nutrition security. This study shows that green leafy vegetables are differently adapted to stress, while producing one or two vitamins in a higher concentration than the others. B. oleraceae, for example, is less susceptible to low fertility soils; however, it is more susceptible to drought. Therefore, to optimally cover all nutrient needs, the focus must be placed on rehabilitation of soil fertility linked with intelligent agrobiodiversity and dietary diversity to enhance the probability of adequate nutrient uptake and to minimize the risk for malnutrition.

5. Conclusions

The research conducted in this study covers a new area of research, in that it not only studies the effects of the combination of drought and soil fertility on the quantity of plant production, but also observes the resulting vitamin production.
The results of the trial show that both drought and varying soil fertility can strongly affect vitamin concentrations and final amounts in plants. The varying results observed during the trials show the importance of consuming a variety of foods, particularly when consuming foods from regions with environmental stressors, to fulfill nutritional needs.
The co-existence of increasing drought events with increased soil degradation in areas with high levels of malnutrition makes it vital to include research on food quality into agricultural trials. The results of such an inclusion could be used to form recommendations for the plants that are best to cultivate in different geographic regions, and could allow particularly rural areas to fulfill their nutritional needs.
The results of this study can be used to derive recommendations as to which green leafy vegetables can be used under which conditions to maximize vitamin production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture13050984/s1, Table S1: Mean temperature and humidity of the greenhouse and the first growing season (the data recorded from 21 April to 2 August in 2019) of the project areas in Uganda and Kenya.; Table S2: The gravimetric amount of 100% pot capacity (PC), 75% PC, 50% PC, 25% PC, and a single pot with two soil types (fertile soil; cambisol and infertile soil; alisol).; Table S3: Summary of chemical properties of collected soil samples in Tauchenweiler and Höwenegg, Germany (Stahr & Böcker, 2014).; Figure S1: A sample layout of a 4 * 6 factorial experiment involving three species (SW: B. oleracea, CP: V. unguiculata, and BN: S. scabrum), two soil fertility (L: fertile soil and S: unfertile soil), and two drought treatments with a double number of the control (DC: drought control, DM: drought mild, and DS: drought severe) in a randomized complete block design with six replications. The layout was designed using the SAS program.; Figure S2: δ13C measurements of samples of B. oleracea (SW, ●), V. unguiculata (CP, +), and S. scabrum (BN, △) according to the mean of irrigated water amount for each treatment in the greenhouse trial (n = 4 per pot capacity of species). Regression respective formula and R2 are given. The triple asterisk indicates significance at p < 0.001.

Author Contributions

Conceptualization, S.F.; Data curation, T.P.; Formal analysis, T.P., S.F., C.L., T.H. and G.C.; Funding acquisition, S.F., T.H., I.J. and G.C.; Investigation, S.F., C.L., I.J. and G.C.; Methodology, T.P., S.F. and C.L.; Project administration, S.F., T.H., I.J. and G.C.; Resources, S.F.; Supervision, S.F., C.L., T.H., I.J. and G.C.; Validation, T.P., C.L. and T.H.; Visualization, T.P.; Writing—original draft, T.P. and S.F.; Writing—review & editing, S.F., C.L., T.H., I.J. and G.C. All authors have read and agreed to the published version of the manuscript.

Funding

The study was conducted in the project “Education and Training for Sustainable Agriculture and Nutrition in East Africa (EaTSANE)”. The project is part of the LEAP Agri program, a joint Europe Africa Research and Innovation (R&I) initiative related to Food and Nutrition Security and Sustainable Agriculture (FNSSA). This study was financially supported by the German Federal Ministry of Food and Agriculture (BMEL) based on the decision of the Parliament of the Federal Republic of Germany through the Federal Office of Agriculture and Food (BLE) (2817LEAP03).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

On request, data can be made available.

Acknowledgments

The authors gratefully acknowledge the assistance of Alexander Koza for facilitating the work with the HPLC in the midst of the pandemic. We would also like to acknowledge and thank Sang-eun Bae for her help in the greenhouse.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study, the collection, analysis or interpretation of the data; in the writing of the manuscript or in the decision to publish the results.

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Agriculture 13 00984 g001 550
Figure 1. Fresh yield (g/pot) of the green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, along with the pot capacity (%) for the fertile (L) and infertile (S) soil. Analysis of covariate (ANCOVA) was performed. The regression lines in each plant species are parallel. Regression formulas and R² are given. The triple asterisk indicates significance at p < 0.001.
Figure 1. Fresh yield (g/pot) of the green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, along with the pot capacity (%) for the fertile (L) and infertile (S) soil. Analysis of covariate (ANCOVA) was performed. The regression lines in each plant species are parallel. Regression formulas and R² are given. The triple asterisk indicates significance at p < 0.001.
Agriculture 13 00984 g001
Agriculture 13 00984 g002 550
Figure 2. (A–I) Relationship between vitamin content (mg/100 g FW) of green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, and pot capacity (%) in two soils, fertile and infertile soils. Analysis of covariate (ANCOVA) was performed. Boxplots (A,I) showing the comparison of vitamin content by soil fertility. A fit plot (F) was adapted to S. scabrum ascorbic acid contents due to no significant effect by the soil fertility. Regression respective formula and R2 are given. Triple, double, and single asterisks indicate significance at p < 0.001, <0.01, and <0.05, respectively. L refers to fertile soil, whereas S refers to infertile soil.
Figure 2. (A–I) Relationship between vitamin content (mg/100 g FW) of green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, and pot capacity (%) in two soils, fertile and infertile soils. Analysis of covariate (ANCOVA) was performed. Boxplots (A,I) showing the comparison of vitamin content by soil fertility. A fit plot (F) was adapted to S. scabrum ascorbic acid contents due to no significant effect by the soil fertility. Regression respective formula and R2 are given. Triple, double, and single asterisks indicate significance at p < 0.001, <0.01, and <0.05, respectively. L refers to fertile soil, whereas S refers to infertile soil.
Agriculture 13 00984 g002
Agriculture 13 00984 g003 550
Figure 3. Relationship between vitamin yields (mg/pot) of green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, and pot capacity (%) in two soils, fertile and infertile soils. Analysis of covariate (ANCOVA) was performed. The regression lines in each plant species are parallel. A fit plot (bottom-left) was adapted to the thiamine yield of B. oleraceae due to no significant effect of soil fertility. Regression respective formula and R2 are given. The triple asterisk indicates significance at p < 0.001.
Figure 3. Relationship between vitamin yields (mg/pot) of green leafy vegetables B. oleraceae, V. unguiculata, and S. scabrum, and pot capacity (%) in two soils, fertile and infertile soils. Analysis of covariate (ANCOVA) was performed. The regression lines in each plant species are parallel. A fit plot (bottom-left) was adapted to the thiamine yield of B. oleraceae due to no significant effect of soil fertility. Regression respective formula and R2 are given. The triple asterisk indicates significance at p < 0.001.
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Table
Table 1. Fresh leaf yield (g/pot), belowground biomass (g/pot FW), nodule number per pot, total irrigation water added (ml/pot), and water use efficiency (WUE: fresh yield/water use, g/L) in Vigna unguiculata, Brassica oleraceae, and Solanum scabrum under two soil fertilities with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
Table 1. Fresh leaf yield (g/pot), belowground biomass (g/pot FW), nodule number per pot, total irrigation water added (ml/pot), and water use efficiency (WUE: fresh yield/water use, g/L) in Vigna unguiculata, Brassica oleraceae, and Solanum scabrum under two soil fertilities with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
Fresh Yield
(g/pot)		Belowground
(g/pot FW)		No. of NodulesIrrigation Water Added (mL/pot)WUE
(g/L)
Vigna unguiculata
Fertile soil
75% PC25.2 ± 1.3 c12.0 ± 1.1 bc11.2 ± 3.4 a2882 ± 115 a8.8 ± 2.0 def
50% PC18.6 ± 1.9 d9.0 ± 1.5 cd6.3 ± 2.1 b1871 ± 162 bc10.1 ± 2.9 cdef
25% PC12.2 ± 1.9 ef5.2 ± 1.5 de1.2 ± 0.5 bc832 ± 162 de15.3 ± 2.9 bcd
Infertile soil
75% PC13.8 ± 1.3 e2.0 ± 1.1 ef0.0 c2192 ± 165 b6.3 ± 2.0 ef
50% PC7.9 ± 1.9 fg1.0 ± 1.5 ef0.0 c1239 ± 163 d6.2 ± 2.9 ef
25% PC2.5 ± 1.9 h0.9 ± 1.5 f0.0 c405 ± 162 ef8.8 ± 2.9 cdef
Brassica oleraceae
Fertile soil
75% PC41.5 ± 1.7 a9.4 ± 1.3 c-2842 ± 142 a14.9 ± 2.5 bcd
50% PC32.8 ± 2.1 b10.2 ± 1.6 c-2080 ± 179 bc16.4 ± 3.2 bc
25% PC12.6 ± 2.1 ef1.4 ± 1.6 ef-398 ± 179 ef43.2 ± 3.2 a
Infertile soil
75% PC40.3 ± 1.4 a3.9 ± 1.1 ef-2928 ± 120 a13.9 ± 2.1 bcde
50% PC15.0 ± 1.9 de1.0 ± 1.5 ef-1204 ± 162 d11.8 ± 2.9 cdef
25% PC4.7 ± 1.9 gh0.6 ± 1.5 f-342 ± 162 f14.1 ± 2.9 bcde
Solanum scabrum
Fertile soil
75% PC35.0 ± 1.3 b21.5 ± 1.1 a-2844 ± 115 a12.4 ± 2.0 cde
50% PC25.9 ± 1.9 c15.4 ± 1.5 b-1783 ± 162 c14.8 ± 2.9 bcd
25% PC17.4 ± 1.9 de10.4 ± 1.5 c-917 ± 162 d20.6 ± 2.9 b
Infertile soil
75% PC16.5 ± 1.3 de3.0 ± 1.1 ef-1804 ± 115 c9.0 ± 2.0 cdef
50% PC4.8 ± 1.9 gh0.9 ± 1.5 f-1013 ± 162 d4.8 ± 2.9 f
25% PC1.1 ± 1.9 h0.7 ± 1.5 f-209 ± 162 f14.4 ± 2.9 bcd
Analysis of variance (ANOVA) using PROC GLIMMIX was used to compare the treatments between plant species and the significance of factors by an F-test at α = 0.05. LS-means ± SE within the column with the same letter are not significantly different (p < 0.05).
Table
Table 2. Levels of β-carotene contents and retinol activity equivalents (RAE) of V. unguiculata, B. oleraceae, and S. scabrum under two soil fertilities with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
Table 2. Levels of β-carotene contents and retinol activity equivalents (RAE) of V. unguiculata, B. oleraceae, and S. scabrum under two soil fertilities with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
β-Carotene
(mg/100 g FW)		RAE
(μg/100 g FW)		β-Carotene
(mg/Fresh Yield)		RAE
(μg/Fresh Yield)
Vigna unguiculata
Fertile soil
75% PC7.37 ± 0.26 bc614 ± 21 bc1.86 ± 0.09 bc155 ± 7 bc
50% PC7.64 ± 0.36 bc636 ± 30 bc1.42 ± 0.13 de118 ± 10 de
25% PC8.04 ± 0.36 b670 ± 30 b0.97 ± 0.13 f80 ± 10 f
Infertile soil
75% PC6.89 ± 0.26 cd574 ± 21 cd0.97 ± 0.09 f81 ± 7 f
50% PC6.41 ± 0.36 de534 ± 30 de0.57 ± 0.13 g47 ± 10 g
25% PC4.92 ± 0.36 gh410 ± 30 gh0.12 ± 0.13 h10 ± 10 h
Brassica oleraceae
Fertile soil
75% PC4.02 ± 0.32 hi335 ± 26 hi1.66 ± 0.11 cd138 ± 9 cd
50% PC4.11 ± 0.40 ghi343 ± 33 ghi1.36 ± 0.14 de114 ± 12 de
25% PC3.66 ± 0.40 i305 ± 33 i0.46 ± 0.14 gh38 ± 12 gh
Infertile soil
75% PC4.03 ± 0.27 hi336 ± 22 hi1.62 ± 0.10 cd135 ± 8 cd
50% PC3.48 ± 0.36 i290 ± 30 i0.55 ± 0.13 g46 ± 10 g
25% PC3.61 ± 0.36 i301 ± 30 i0.17 ± 0.13 h14 ± 10 h
Solanum scabrum
Fertile soil
75% PC7.59 ± 0.26 bc632 ± 21 bc2.66 ± 0.09 a221 ± 7 a
50% PC8.09 ± 0.36 b674 ± 30 b2.11 ± 0.13 b176 ± 10 b
25% PC9.48 ± 0.36 a790 ± 30 a1.62 ± 0.13 cd135 ± 10 cd
Infertile soil
75% PC7.08 ± 0.26 cd590 ± 21 cd1.17 ± 0.09 ef97 ± 7 ef
50% PC5.99 ± 0.36 ef499 ± 30 ef0.31 ± 0.13 gh26 ± 10 gh
25% PC5.13 ± 0.45 fg428 ± 37 fg0.06 ± 0.16 h5 ± 13 h
Analysis of variance (ANOVA) using PROC GLIMMIX was used to compare the treatments between plant species and the significance of factors by an F-test at α = 0.05. LS-means ± SE within the column with the same letter are not significantly different (p < 0.05).
Table
Table 3. Percentage of recommended nutrient intakes (RNI) for vitamin content (mg/150 g FW for thiamine and ascorbic acid and μg/150 g FW for RAE) and vitamin yield (mg/plant for thiamine and ascorbic acid and μg/plant for RAE) of V. unguiculata, B. oleracea, and S. scabrum under two soil fertilities (fertile; L and infertile soil; S) with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
Table 3. Percentage of recommended nutrient intakes (RNI) for vitamin content (mg/150 g FW for thiamine and ascorbic acid and μg/150 g FW for RAE) and vitamin yield (mg/plant for thiamine and ascorbic acid and μg/plant for RAE) of V. unguiculata, B. oleracea, and S. scabrum under two soil fertilities (fertile; L and infertile soil; S) with three watering regimes (75% pot capacity (PC), 50% PC, and 25% PC).
% of RNI 1
for Vitamin Content
(mg or μg/150 g FW)		% of RNI 1
for Vitamin Yield
(mg or μg/Plant)
ThiamineAscorbic AcidRAE 2ThiamineAscorbic AcidRAE 2
Vigna unguiculata
Fertile soil (L)
75% PC13.01181842.42131
50% PC14.71011911.81124
25% PC20.21052011.7916
Infertile soil (S)
75% PC15.5751721.6816
50% PC19.5921601.169
25% PC33.01701230.522
Brassica oleraceae
Fertile soil (L)
75% PC9.82381012.76828
50% PC9.72371032.35323
25% PC10.2132921.2518
Infertile soil (S)
75% PC11.31891013.01527
50% PC12.0117871.5149
25% PC20.6135900.643
Solanum scabrum
Fertile soil (L)
75% PC2.91521900.73744
50% PC3.41272020.62235
25% PC4.31802370.41727
Infertile soil (S)
75% PC4.91221770.51319
50% PC5.31251500.255
25% PC5.51661280.111
1 RNI, recommended nutrient intake; values for female adults (19–50 years) (WHO & FAO, 2004); 2 RAE, retinol activity equivalent, bioconversion of ingested β-carotene to retinol based on equivalency factors, for which 1 µg RAE = 12 µg β-carotene (WHO & FAO, 2004).
Table
Table 4. Comparison of effects of increasing drought in two soil fertility conditions (fertile, L and infertile soil, S) on fresh yield and vitamin content (mg/100 g FW) of V. unguiculata, B. oleraceae, and S. scabrum.
Table 4. Comparison of effects of increasing drought in two soil fertility conditions (fertile, L and infertile soil, S) on fresh yield and vitamin content (mg/100 g FW) of V. unguiculata, B. oleraceae, and S. scabrum.
Brassica oleraceaeVigna unguiculataSolanum scabrum
Fresh yieldFertile soil (L)Agriculture 13 00984 i001Agriculture 13 00984 i002Agriculture 13 00984 i003
Infertile soil (S)Agriculture 13 00984 i004Agriculture 13 00984 i005Agriculture 13 00984 i006
β-caroteneFertile soil (L)--Agriculture 13 00984 i007
Infertile soil (S)-Agriculture 13 00984 i008Agriculture 13 00984 i009
Ascorbic acidFertile soil (L)Agriculture 13 00984 i010-Agriculture 13 00984 i011
Infertile soil (S)Agriculture 13 00984 i012Agriculture 13 00984 i013Agriculture 13 00984 i014
ThiamineFertile soil (L)-Agriculture 13 00984 i015-
Infertile soil (S)Agriculture 13 00984 i016Agriculture 13 00984 i017-
The arrows show trends of interaction (upwards blue arrow: positive, downwards red arrow: negative). Hyphen sign indicates no interaction.
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Park, T.; Fischer, S.; Lambert, C.; Hilger, T.; Jordan, I.; Cadisch, G. Combined Effects of Drought and Soil Fertility on the Synthesis of Vitamins in Green Leafy Vegetables. Agriculture 2023, 13, 984. https://doi.org/10.3390/agriculture13050984

AMA Style

Park T, Fischer S, Lambert C, Hilger T, Jordan I, Cadisch G. Combined Effects of Drought and Soil Fertility on the Synthesis of Vitamins in Green Leafy Vegetables. Agriculture. 2023; 13(5):984. https://doi.org/10.3390/agriculture13050984

Chicago/Turabian Style

Park, Taewan, Sahrah Fischer, Christine Lambert, Thomas Hilger, Irmgard Jordan, and Georg Cadisch. 2023. "Combined Effects of Drought and Soil Fertility on the Synthesis of Vitamins in Green Leafy Vegetables" Agriculture 13, no. 5: 984. https://doi.org/10.3390/agriculture13050984

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