Drought leads to several critical dysfunctions in the photosynthesis process32, including damage to plant pigments, disruption of stomatal performance and reduced CO2 flow to the photosynthetic machinery32. Additionally, reducing the crop’s water requirement impairs nutrient uptake and utilisation, while disturbing osmotic potential33. The generation of reactive oxygen species (ROS) under deficit irrigation further exacerbates plant damage, injuring pigments, photosynthetic systems, and overall metabolism34. Consequently, deficit irrigation ultimately results in reduced crop growth and yield35,36.
On the other hand, the soil water content showed significant variation among the three water regimes: well-watered, moderate water stress, and severe water stress. As anticipated, the well-watered treatment displayed the highest soil water content, followed by moderate water stress, with severe water stress showing the lowest soil water content. These results align with previous findings14,37, confirming the expected relationship between water regime and soil water content.
The reduction in stomatal conductance of selected fodder genotypes under drought stress has been extensively documented in the literature38,39. However, contrary to this prevailing observation, certain studies have reported an improvement in stomatal conductance under moderate and severe water stress conditions40,41, which aligns with the findings of the current study where drought stress led to higher stomatal conductance. This discrepancy may be attributed to the response of crops to stress, wherein their stomata, small pores on the surface of leaves, tend to close to prevent excessive water loss through transpiration42.
The importance of water use efficiency (WUE) in plant production cannot be overstated, as it plays a critical role in ensuring better productivity, especially in challenging environmental conditions43. High WUE allows plants to maintain essential metabolic processes crucial for their growth and development44. Interestingly, Mandal45 observed lower WUE in well-watered plots of certain forage brassicas; this finding was partially supported by our study, where plots under moderate stress exhibited higher WUE compared to both well-watered and severely stressed plots. Similarly, Akram38 documented higher WUE in water-stressed radish plots. However, Henschel39 reported no significant difference in WUE between well-watered and drought-stressed radish plots. Despite limited studies focusing specifically on WUE in fodder radish or forage brassicas, research across various crops indicates an increase in WUE under water-stressed conditions46,23,47.
The reduction in WUE observed under well-watered conditions could be attributed to amplified soil water evaporation rather than transpiration48. Additionally, the higher values of WUE under moderate water stress may be due to more efficient stomatal regulation in water-stressed plots, as the closure of stomata reduces water loss through transpiration. Conversely, well-watered plants might keep their stomata open longer, leading to higher transpiration rates and lower water use efficiency. Water use efficiency measures the biomass or yield produced per unit of water the crop uses. Our expectations were that WUE would be higher under stressed conditions, as the crop is expected to produce more with less water. This higher WUE under stress implies that the crop can make more efficient use of available water resources, potentially leading to increased productivity per unit of water.
When crops experience stress, their stomata, which are small pores on the surface of leaves, tend to close to prevent excessive water loss through transpiration. While this mechanism helps conserve water, it also affects crop productivity. When stomata close, photosynthesis is inhibited, reducing carbon dioxide uptake and decreasing yield potential. The extent of yield reduction under stressed conditions varies depending on factors such as the severity and duration of stress, crop species, and growth stage. In general, prolonged or severe stress can lead to significant yield losses. Our study findings concurred with severe water stress having low biomass and higher stomatal conductance values. However, moderate water stress in both seasons for ENDURANCE resulted in higher biomass. In contrast, for the LINE 2 genotype in both seasons, well-watered and moderate stress conditions resulted in similar biomass. Nyathi14 observed a reduction in both total aboveground and edible aboveground biomass of leafy vegetables under severe water-stressed conditions.
Unsurprisingly, well-watered treatments resulted improved biomass because the leaf area index was improved by well-watered conditions, given that the higher the LAI, the higher the biomass. The higher light use efficiency (LI) might have facilitated increased CO2 assimilation and subsequently improved biomass; this might explain the favourable outcomes in moderate stress and well-watered treatments due to their higher LI, Fractional interception (FI), and superior Radiation Use Efficiency (RUE) in mean values49,60 (Table 4). Throughout both seasons, the average daily maximum temperature remained around 25 °C (Table 2), which remained optimal for radish growth. Stagnari40 reported similar findings in radish genotypes, where well-watered plots exhibited significantly higher mean values for aboveground biomass. The high total biomass under moderate stress in both seasons reveals that these genotypes ENDURANCE and LINE 2 conserve water because for 2021/2022 season W1 = 305 mm, W2 = 221 mm and 2022/2023 season W1 = 366 mm, W2 = 245 mm. Therefore, using these genotypes, farmers can save between 84 and 121 mm to achieve maximum yield. This implies that the goal of WUE in crops is to maximise the yield or productivity of the crop while minimising the amount of water used. In the South African context, which is deemed as a water-stressed country ranked 30th in the world, WUE is a crucial factor for ensuring sustainable water management practices and improving overall crop performance.
The study findings supported our first hypothesis that leaf harvesting significantly reduces fodder radish storage root/tuber yield in the 2021/22 and 2022/23 seasons. This reduction persisted across different irrigation water regimes, highlighting the consistent negative effect of leaf harvesting on root/tuber yield. The observed phenomenon aligns with global trends, as similar root/tuber yield reductions due to leaf harvesting have been documented in different regions. Studies from India50, and Congo51 revealed decreased yields with multiple leaf harvests, while research in South Africa, Nyathi15 reported losses in root or tuber yield with repeated harvesting. However, there were contrasting findings by52, indicating that leaf harvest did not influence root/tuber yield. Hauser53 even noted positive effects on root or tuber yield in specific conditions, claiming that shorter dry seasons could be attributed to their results.
The second hypothesis was that no leaf harvesting paired with a well-watered irrigation water regime would improve fodder radish storage root yield, and the study results did not consistently support it. The well-watered and moderate stress treatments were not significantly different. Both well-watered and moderate stress with no leaf harvest improved root/tuber yield during the 2021/22 and 2022/23 seasons. The unexpected result might be attributed to that radish crops store water in their tubers, allowing them to withstand periods of water stress, so moderate stress conditions positively affected the tuber yield, implying that water applied in the moderate stress plots combined with the water reservoir in the tubers was enough to produced yield same as in the well-watered conditions. Although not all were conducted on fodder radish, some studies align with these findings15,54. Stagnari40 noted the reduction in radish’s root storage/tuber yield under water stress treatments. Access soil resources for regrowth, possibly explaining the lack of significant differences among water regimes, particularly in the fodder radish from the severely water-stressed treatment paired with leaf harvest. This indicates that the ENDURANCE and LINE 2 fodder radish genotypes can achieve higher tuber yield without excessive water requirements. The total water supplied for well-watered conditions was 305 mm and 366 mm for the 2021/22 and 2022/23 seasons, respectively, while moderate stress received 221 mm and 245 mm across both seasons.
Both well-watered and moderate stress, coupled with no leaf harvest, yielded larger tuber diameters, as expected, given the strong correlation between stem diameter and plant biomass7. The well-watered regime improves radish tuber diameter55. Cunha56 found no genotype-specific effects on radish tuber diameter, indicating that the total water applied may have a more pronounced impact, which aligns with our study’s results. In our study results, the lack of genotype impact might be due to limited genomic diversity between ENDURANCE and LINE 2. Tuber length remained unaffected by leaf harvest, water regime, and genotype. The root/tuber, essential for nutrient and water uptake from the soil, may prioritise growth by enlarging to access soil resources for regrowth, possibly explaining the lack of significant differences among water regimes, particularly in the fodder radish from the severely water-stressed treatment paired with leaf harvest. Similar findings of no water regime effects on radish tuber length were reported by57,58, consistent with the results of this study. This indicates that the ENDURANCE and LINE 2 fodder radish genotypes can achieve high tuber yield without excessive water requirements. The total water supplied for well-watered conditions was 305 mm and 366 mm for the 2021/22 and 2022/23 seasons, respectively, while moderate stress received 221 mm and 245 mm across both seasons.
Crop growth indicators such as total aboveground biomass, leaf area index (LAI), harvest index (HI), relative growth rate (RGR), relative height rate (RHR), plant height, and the number of leaves all showcased positive responses to well-watered conditions as hypothesised. Stagnari40 reported similar findings in radish genotypes, where well-watered plots exhibited significantly higher mean values for aboveground biomass, RGR, and LAI. As noted by Silva42, water stress treatments impacted the growth indicators of Salvia hispanica L., including RGR, suggesting a reduction due to water stress. Galmes59 also noted reduced RGR in different genotypes under water-stressed conditions, potentially linked to leaf senescence and decreased leaf area index60.
The reduction in dry weight growth in ENDURANCE and LINE 2 could be largely attributed to diminished light interception, radiation use efficiency, and fractional interception under severe stress treatments. In a radish study, Marcelis and Van Hooijdonk60 observed decreased light interception under water stress treatments, attributing it to low growth rates in the leaf area. Understanding light interception, radiation use efficiency, and fractional interception is pivotal in comprehending the photochemical efficiency of plant photosynthesis. While our findings on light interception align with Nyathi14, differences were noted in radiation use efficiency, where they found a reduction with increased water stress levels, contrary to our study where only season influenced RUE, although on Amaranthus.
The decline in chlorophyll content in Raphanus sativus L. due to drought stress is a well-documented38,39. However, contrary to this, certain studies have reported improved chlorophyll content under moderate and severe water stress conditions40,41. This opposes our study result, which found no effect on water regimes. This discrepancy might be attributed to the inherent genetic traits that ENDURANCE and LINE 2 possess to maintain consistent chlorophyll content levels in different water availability conditions. Similarly, some studies, consistent with our findings, observed no significant difference in chlorophyll content under various water regimes.
Brassica microgreens are a rich source of microelements Fe and Zn61. The lack of micronutrients and vitamin A in human diets leads to “hidden hunger”, a condition whose effects may not manifest immediately. Still, it can result in severe consequences such as stunted growth, delayed cognitive development, and reduced immunity62. Inadequacy of essential nutrients can directly influence the body’s immune response. These nutrients act as antioxidants, safeguarding cells, fortifying immune cells’ growth and function, and initiating antibodies’ production. Our study strongly supports the hypothesis that well-watered conditions lead to elevated zinc concentrations in both genotypes. Our findings align with those of Maseko63, who observed increased zinc concentration under well-watered conditions. However, our results contradict those of Schlering64, who reported a rise in zinc levels under water-stressed treatments in radish leaves. Furthermore, a similar pattern was noted by Nyathi14, where severe water stress resulted in higher concentrations of iron and zinc in Amaranth and Spider flower.
Although the study hypothesis was partially supported for the iron, well-watered and moderately water-stressed treatments exhibited comparable values in LINE 2. Conversely, ENDURANCE under severe water-stressed treatment demonstrated higher values than well-watered and moderately water-stressed treatments. This contradicts the findings of Nyathi14, who reported an approximately 46% decrease in iron concentration for Swiss chard under severe water stress. The disparity between these genotypes could be attributed to the resilience of ENDURANCE to water scarcity, as it maintained or even increased iron concentration as a stress response. At the same time, LINE 2 experienced a decline in iron levels.
Our study results supported our hypothesis that well-watered will improve the β-carotene and vitamin C concentration. The water-stressed treatments reduced the concentration of β-carotene in both genotypes and seasons. Maluleke65 assessed cucumber under different water regimes and found 1.6 to 1.5 mg 100 g− 1 DW β-carotene under moderate and severe water stress treatments; this aligns with our findings. While Park66 found no effect of different water regimes β-carotene of green leafy vegetables. In an extensive review conducted by Gamba67, radish leaves’ mineral and vitamin composition was compared to various vegetables such as cabbage, cauliflower, broccoli, arugula, and turnip. The β-carotene concentration in radish leaves was particularly remarkable, which reached the highest level at 3.96, surpassing other vegetables by up to threefold. This high β-carotene content establishes radish leaves as a significant source of vitamin A within the Brassicaceae family, highlighting their nutritional importance compared to the studied vegetables.
For vitamin C, the current study results showed a reduction in the concentration of vitamin C under water-stress treatments, aligning with the findings of Ahmed68, who observed a decrease in vitamin C in fruits under water-stress treatments. Conversely, Park65 have demonstrated increased vitamin C under water stress conditions. Stagnari69 have acknowledged the limited data reporting on vitamin C under varying water regimes.
Regarding vitamin E, our hypothesis was not supported because moderate water stress treatment had higher values of vitamin E than well-watered treatment. This might be attributed to that moderate water stress conditions impacted photosynthetic processes in plants. Vitamin E protects the photosynthetic apparatus from oxidative damage, and an increase in its levels may be a response to maintaining or enhancing photosynthetic efficiency under stress conditions. An increase in vitamin E in soybean under water stress treatments was found70. This finding aligns with our study results, indicating a higher concentration of vitamin E under moderate stress. However, Oh71, in their study on lettuce, found no effect of water regime on Vitamin C and E. Our findings suggest that under well-watered conditions, both ENDURANCE and LINE 2 varieties can exceed the daily recommended nutrient intake (DRNI) for vitamin A across all age groups. Meanwhile, under moderate stress conditions, they meet the DRNI for the most vulnerable groups, including women and infants aged 1 to 3 years. Under severe water stress conditions, both ENDURANCE and LINE 2 varieties surpass the daily recommended nutrient intake (DRNI) for vitamins E and iron and zinc across all age groups and genders. Vitamin C only meet the daily recommended nutrient intake (DRNI) for 1–3 years old.
Nutritional yield is the amount of essential nutrients a crop provides per area, considering both how much it produces and how concentrated the nutrients are within it14. The study on different types of fodder radish explored how water levels affect the nutritional yield of these crops. Under moderate water stress conditions (W2), the levels of essential nutrients, including vitamin E, CP yield and other important micronutrients, were typically found to be ideal. In contrast, severe water stress (W3) often resulted in lower nutrient levels per unit, but still met the necessary vitamin requirements for extreme dietary needs. These findings align with the observations of Nyathi14, who reported a decrease in zinc and β-carotene nutritional yield under conditions of severe water stress. The results highlight how effectively managing water is crucial for maximising the amount of essential nutrients provided by crops in regions with limited water resources72. Nutritional yield is essential for ensuring food and fodder security, particularly in regions such as South Africa, where dealing with water scarcity necessitates growing crops like fodder radish, valued for efficient water use and nutritional advantages for both humans and livestock. Therefore, nutritional yield is a crucial measure for evaluating how strategic water management and selecting plant varieties contribute to improving the sustainability of diets and agriculture.
Conclusion and future research
This study examined how efficiently water was used, along with the growth of biomass and nutrient levels, in two types of fodder radish, “LINE 2” and “ENDURANCE,” under different water conditions and leaf harvesting practices. Conducted over two growing seasons (2021/22 and 2022/23) in South Africa, the research assessed the impact of well-watered, moderate water stress, and severe water stress environments. The results showed that when experiencing moderate water stress, the efficiency of water use and biomass yield were around 90% compared to full irrigation, while also increasing vitamin E concentrations and CP yield. Under severe stress, total biomass decreased by 30%, tuber yield by 25%, and nutrient content by 15%. LINE 2 showed 5% better resilience than ENDURANCE in this scenario. Additionally, leaf harvesting substantially lowered tuber yield. Our findings proposes that using moderate water stress as an irrigation method can optimise resource use efficiency in environments with limited water supply. For instance, moderate water stress encourages the growth of deeper roots, which enhances water absorption and utilization, ultimately boosting plant health and productivity.
The study concludes that moderate water stress is a sustainable alternative to full irrigation for cultivating fodder radish. It optimises both water use efficiency and nutrient concentration, leading to improved plant health and nutritional value, all while maintaining a competitive yield. Severe water stress had negative effects, leading to a significant decrease in both biomass and nutritional value. Both genotypes showed resilience under moderate stress, but LINE 2 outperformed ENDURANCE in specific aspects. This research contributes to developing climate-smart agricultural practices tailored for arid and semi-arid regions. For instance, implementing precision irrigation techniques can optimise water use in arid environments, ensuring sustainable crop production and food security while preserving soil quality.
Future research should broaden its evaluation to include additional fodder radish genotypes and evaluate their performance over the long term in various climatic conditions. Investigating the effects of different cooking methods on nutrient bioavailability and exploring soil amendments to mitigate nutrient loss under stress conditions are crucial for promoting human health and sustainable agricultural practices. Additionally, integrating multi-season trials across various agro ecological zones will enhance the generalisability of findings. Studying scalable water management technologies and their economic viability for farmers with limited resources will strengthen the position of fodder radish in sustainable agriculture.
