Enhancing Bioactive Compound Levels in Habanero Peppers through Foliar Application of Zinc Oxide Nanoparticles and Zinc Sulfate -

Enhancing Bioactive Compound Levels in Habanero Peppers through Foliar Application of Zinc Oxide Nanoparticles and Zinc Sulfate

The incorporation of fertilizers is crucial for modern crop production, ensuring optimal soil fertility. Zinc (Zn) is a vital micronutrient for plants, influencing various physiological processes. However, traditional Zn fertilization methods, such as Zn sulfate application, often exhibit low efficiency. In regions like Mexico, where habanero peppers are in high demand, the current production faces limitations due to technology constraints and inadequate fertilizer supply. To address this, optimized fertilization methods, particularly through Zn-based foliar fertilizers, are explored for increased crop yield and reduced environmental impact. Nanotechnology, specifically zinc oxide nanoparticles (ZnO NPs), emerges as a promising solution, showing positive effects on plant growth and bioactive compound content. Despite previous studies on ZnO NPs in various crops, their interaction with habanero peppers, especially in comparison with Zn sulfate, remains unexplored. This research aims to fill this gap by evaluating the growth responses and impact on bioactive compounds in habanero peppers treated with ZnSO4 and ZnO NPs through foliar applications, shedding light on their potential effects on plant physiology.

Materials and Methods:

Plant Material: The Chichen Itza variety of Capsicum chinense (Seminis, St. Louis, MO, USA) was chosen for this study due to its early maturity, vigor, and distinctive orange, three-lobed fruits, known for their spiciness (10,500 SHU fresh, up to 200,000 SHU dry).

Characteristics of the ZnO NPs Used in This Experiment:  The ZnO NPs utilized in this study were characterized by Garcia-Lopez et al., revealing particles with diameters ranging from 12 to 24 nm (75%) and 30% with sizes between 12 nm and 20 nm.

Preparation of Suspensions: ZnSO4 and ZnO NPs suspensions were freshly prepared in deionized water, homogenized using a probe sonicator, and adjusted to pH 6.5–6.7 before plant exposure. Concentrations used were 1000 mg L−1 and 2000 mg L−1 for both ZnSO4 and ZnO NPs, with electrical conductivities (EC) specified for each.

Foliar Exposure to Zn: Foliar applications were conducted at different stages of crop development (vegetative growth, flowering, fruit development, and maturity), with approximately 0.125 L of Zn suspensions sprayed twice per stage using a Hudson RL Flomaster sprayer.

Growth of Habanero Pepper Plants and Greenhouse Conditions: Habanero pepper plants were cultivated in an Israeli-type greenhouse under controlled conditions for 208 days. Pumicite served as the growth medium, and an automated drip-irrigation system provided nutrient solutions based on Steiner.

 Performance Variables and Relative Chlorophyll Content: Various parameters, including plant height, stem diameter, and chlorophyll content, were measured at specific intervals throughout the plant’s life cycle. Performance metrics, such as fruit yield, weight, and biomass, were recorded at 208 days.

Preparation for the Analysis of Habanero Pepper Fruits: Fruits were harvested at full maturity, washed, and stored for subsequent physical, chemical, and functional analyses.

 Extraction of Soluble and Bound Phenolic Compounds: Soluble phenolics were extracted in 80% methanol, while bound phenolics were extracted using NaOH, followed by ethyl acetate. Extracts were stored for subsequent analyses of total phenols, flavonoids, and antioxidant capacity.

Phenolic Compound and Antioxidant Capacity Analysis: Total phenols, flavonoids, and antioxidant capacity were determined using spectrophotometric methods. Chromatic evaluations, firmness, and cutting force were measured for physical evaluations. Capsaicinoids were extracted, quantified by HPLC, and  transformed into Scoville Heat Units.

Results and Summary:

In this greenhouse study, the impact of foliar applications of zinc sulfate (ZnSO4) and zinc oxide nanoparticles (ZnO NPs) on habanero pepper plants (Capsicum chinense Jacq.) was meticulously examined, with a particular focus on fruit quality. The experimental setup involved treating habanero pepper plants with Zn concentrations of 1000 and 2000 mg L−1, administered as ZnSO4 and ZnO NPs, throughout various phenological stages.

Effect on Plant Growth and Biomass Accumulation: ZnO NPs at 1000 mg L−1 exhibited positive effects on plant height, stem diameter, and chlorophyll content, leading to increased fruit yield and biomass. However, at a concentration of 2000 mg L−1, ZnO NPs had contrasting effects, showing a negative impact on plant growth but significantly enhancing fruit quality parameters.

Impact on Fruit Quality: At 2000 mg L−1, ZnO NPs significantly increased capsaicin content by 19.3%, dihydrocapsaicin by 10.9%, and Scoville Heat Units by 16.4%. Moreover, this concentration positively influenced the accumulation of total phenols and total flavonoids (soluble + bound) in fruits by 14.50% and 26.9%, respectively. This resulted in elevated antioxidant capacity measured through ABTS, DPPH, and FRAP assays by 15.4%, 31.8%, and 20.5%, respectively.

Concentration-Dependent Effects: The study underscored the concentration-dependent nature of ZnO NPs’ impact on habanero pepper plants. At 1000 mg L−1, ZnO NPs promoted positive effects on plant growth and fruit characteristics. In contrast, at 2000 mg L−1, while growth-related effects were adverse, there was a notable enhancement in fruit quality and nutraceutical properties.

Comparison with ZnSO4: Compared to ZnSO4 treatments at both concentrations (1000 and 2000 mg L−1), ZnO NPs exhibited superior effects on crop yield and fruit quality. ZnSO4 had a comparatively lower impact on crop yield and showed minor effects on plant development.

Improvements in Fruit Quality: Foliar application of ZnO NPs at 2000 mg L−1 resulted in significant improvements in various fruit quality parameters, including titratable acidity, soluble solids, and pH. The concentration of ZnO NPs also led to substantial increases in total capsaicinoids, resulting in higher Scoville Heat Units.

Conclusion: In conclusion, this study highlights the significant influence of ZnO NPs on the development, yield, and quality of habanero pepper plants. The concentration-dependent effects underscore the importance of carefully selecting application concentrations. The findings suggest that ZnO NPs, especially at 2000 mg L−1, could be a valuable tool in habanero pepper production, contributing to improved yield, enhanced fruit quality, and elevated nutraceutical attributes.

Reference: GARCÍA-LÓPEZ, J.I., NIÑO-MEDINA, G., OLIVARES-SÁENZ, E., LIRA-SALDIVAR, R.H., BARRIGA-CASTRO, E.D., VÁZQUEZ-ALVARADO, R., RODRÍGUEZ-SALINAS, P.A. AND ZAVALA-GARCÍA, F., 2019. Foliar application of zinc oxide nanoparticles and zinc sulfate boosts the content of bioactive compounds in habanero peppers. Plants, 8(8), p.254.