Editorial: Nutrients Recycling in Hydroponics: Opportunities and Challenges toward Sustainable Crop Production under Controlled Environment Agriculture, Volume II

Md Asaduzzaman^1*Toshiki ASAO²

• ¹Kyoto University of Advanced Science (KUAS), Kyoto, Japan
• ²Setsunan University, Osaka, Japan

Hammock et al. investigated the effects of LED and high-pressure sodium (HPS) lighting at different daily light integrals (DLIs) to enhance winter greenhouse crop production. They assessed edible biomass, nutrient bioaccumulation, and light use efficiency (LUE). The study included nine lighting treatments: one non-supplemented natural light (NL) control; two end-ofday (EOD) HPS treatments applied for 6 and 12 hours; five EOD LED treatments with a 20% blue and 80% red spectrum applied for 3, 6, 9, 12, and 18 hours; and one continuous LED treatment lasting 24 hours. Each supplemental lighting (SL) treatment delivered a consistent intensity of 100 μmol•m⁻²•s⁻¹. The DLI of the NL control averaged 9.9 mol•m⁻²•d⁻¹ throughout the growth period, with a range from 4 to 20 mol•m⁻²•d⁻¹. Both supplemental lighting and growing season had significant impacts on biomass production and nutrient accumulation. Interestingly, some SL treatments actually resulted in lower yields than the non-supplemented NL control. Among the different seasons, January produced the lowest fresh and dry mass, while November yielded the highest. Mineral analysis showed that both lighting type and season influenced macro-and micronutrient uptake. Additionally, light use efficiency varied widely among treatments. This study did not evaluate plant quality or secondary metabolites, which are critical for consumer appeal. Future research should address this gap by using metabolomic and transcriptomic tools to explore how different lighting strategies impact plant physiology and biochemical pathways. Zinc biofortification of hydroponically grown basil Herbs, medicinal and aromatic plants are rich in bioactive metabolites that benefit human health. Agronomic biofortification of hydroponically grown crops like basil offers a practical and sustainable approach to combat zinc deficiency in humans. Ciriello et al. reported that biofortification through nutrient solutions of different Zn concentration (12.5, 25.0, 37.5, and 50 μM) on the yield, physiological indices, quality, and Zn accumulation in two basil cultivars 'Aroma 2' and 'Eleonora' grown in a floating raft hydroponics. Although increasing the concentration of Zn in the nutrient solution significantly reduced the yield, this reduction was less evident in 'Aroma 2'. However, the highest Zn dose (50 μM) led to increases in carotenoids, polyphenols, and antioxidant activity by 19.76%, 14.57%, and 33.72%, respectively, compared to the control. The significant positive correlation between Zn levels in the nutrient solution and Zn content in plant tissues confirmed basil's potential for effective Zn biofortification in soilless systems. In this study, strong cultivar-dependent response highlights the importance of variety selection in biofortifying non-hyperaccumulating crops to boost micronutrient content without reducing yield. Biofortified Genovese basil can enhance Zn intake while also delivering beneficial phytochemicals like carotenoids and polyphenols. Nitrogen and light quality modulate plant growth and resource allocation. CEA enables year-round and high-yield production is the key for high value horticultural crops grown hydroponically. Nitrogen supply and light intensity and spectral composition are important in these intensive production systems. The individual effects of nitrogen and light quality on plant growth, nitrogen uptake, and allocation of both dry matter and reduced nitrogen attract significant research efforts. Liang et al. analyzed the growth and allocation of dry matter and nitrogen using lettuce as a plant model in combining three light regimes (100% red light, R; 50% red light + 50% blue light, RB; 100% blue light, B) and two nitrogen rates (low, 0.1 mM N; high, 10 mM N). Red light increased shoot dry weight in relation to both B and RB irrespective of nitrogen supply. Blue light favored root growth under low nitrogen. Allometric analysis showed lower allocation to leaf in response to blue light under low nitrogen and similar leaf allocation under high nitrogen. A difference in allometric slopes between low nitrogen and high nitrogen in treatments with blue light reflected a strong interaction effect on root-to-shoot biomass allocation. Shoot nitrate concentration increased with light exposure up to 14 h in both nitrogen treatments, was higher under blue light with high nitrogen, and varied little with light quality under low nitrogen. Shoot nitrogen concentration, nitrogen nutrition index, and shoot NR activity increased in response to blue light. Thus, the interaction between blue light and nitrogen supply modulates dry mass and nitrogen allocation between the shoot and root. This research highlights the effects of the interactions between nitrogen supply and light quality on dry matter and nitrogen accumulation and allocation between shoot and root. Resource allocation is central to plant adaptation and crop yield. These findings have potential applications in the design of light/nitrogen regimes for CEA. Physiological and biochemical responses of halophyte to NaCl concentrations Limonium (L.) tetragonum (Thunb.) A. A. Bullock, a halophyte that grows all over the southwest coast of Korea, is a medicinal plant with various pharmacological effects. The salt defense mechanism stimulates the biosynthesis of various secondary metabolites and improves functional substances. Jang S-N et al. investigated the optimal NaCl concentration for the growth and enhancement of secondary metabolites in hydroponically grown L. tetragonum. The seedlings grown for 3 weeks in a hydroponic cultivation system were treated with 0-, 25-, 50-, 75-, and 100-mM NaCl in Hoagland's nutrient solution for 8 weeks. No significant effect on the growth and chlorophyll fluorescence was observed for the NaCl concentrations below 100-mM. The increase in the NaCl concentration resulted in the decrease in the water potential of the leaves. The Na + content accumulated in the aerial part increased rapidly and the content of K + , which acts as an antagonist, decreased with the increase in NaCl concentrations in hydroponics. The total amino acid content decreased compared to the 0-mM NaCl, and most of the amino acid content decreased as the NaCl concentration increased. In contrast, the content of urea, proline, b-alanine, ornithine, and arginine was increased with an increase in NaCl concentration. The proline content at 100-mM NaCl accounted for 60% of the total amino acids and was found to be a major osmoregulator as an important component of the salt defense mechanisms. The top five compounds identified in this halophyte were classified as flavonoids while the flavanone compound was detected only in the NaCl treatments. A total of four myricetin glycosides were increased in comparison to the 0-mM NaCl. This study revealed that NaCl treatment was necessary to improve the flavonoid myricetin glycoside. The optimal NaCl concentration for the enhancement of the secondary metabolites of L. tetragonum in the present study is 75-mM NaCl. Reuse of plant based soilless substrate for strawberry production Production of strawberries in greenhouses and polytunnels is gaining popularity worldwide. Woznicki et al. investigated the effect of reuse of coir and peat, two substrates commonly adapted to soilless strawberry production, as well as stand-alone wood fiber from Norway spruce, a promising substrate candidate. They evaluated the feasibility of substrate reuse for soilless culture system (SCS) of strawberry production. Two commonly used substrates, peat and coir, as well as a novel alternative, wood fiber from Norway spruce, revealed potential for successful implementation even after two cycles of production. While plants grown in peat and wood fiber had highest yield in the first year of production, the strawberry yield was slightly reduced when these substrates were utilized for the second and third time. However, yield was comparable to the yield level attained in new and reused coir. Interestingly, strawberries grown in wood fiber tended to a higher sugar accumulation which also produced the highest plants. Stand-alone wood fiber was the substrate with the highest accumulation of nitrogen during the three consecutive production cycles. All three investigated materials revealed a trend for decreased potassium accumulation. Wood fiber is characterized by the highest percentage of cellulose, however after three years of production the cellulose content was reduced to the same levels as for coir and peat. Implementation of wood fiber as a growing medium, as well as general practice of substrate reuse can be therefore an achievable strategy for more sustainable strawberry production. Author contributions MA: Writingoriginal draft, Writingreview & editing, Conceptualization, TA: Writingreview & editing, Supervision