Effects of Superabsorbent Polymers, Fish Waste Hydrolysates, and Amino Acids on the Quality and Yield of Bell Pepper in a Calcareous Soil Under Greenhouse Conditions

Background and Objectives
Bell pepper (Capsicum annuum L.) is a globally important vegetable crop valued for its high nutritional content and antioxidant compounds. However, its production faces significant challenges in arid and semi-arid regions where soils typically have high pH (alkaline), low organic matter, poor structure, and low water retention capacity. These characteristics reduce the availability of essential nutrients, impairing physiological processes and ultimately diminishing yield. In recent years, biostimulants such as fish hydrolysate (rich in amino acids and bioactive peptides) and amino acid supplements have emerged as sustainable approaches to improve plant performance under stress conditions. Additionally, superabsorbent polymers can absorb water several times their weight and gradually release it in the root zone, improving soil water status and nutrient uptake. The main objective of this research was to investigate the simultaneous effect of superabsorbent polymer application with fish hydrolysate and amino acids on improving physiological, morphological, and yield characteristics of bell pepper plants under greenhouse conditions.
Materials and Methods
This research was conducted in the research greenhouse of the University of Zanjan in a completely randomized design with three replications. The treatments included: T₁ (control), T₂ (fish hydrolysates), T₃ (amino acids), T₄ (superabsorbent polymer + fish hydrolysates), and T₅ (amino acids + superabsorbent polymer at 1 mg L⁻¹ concentration). The soil used had an electrical conductivity of 0.97 dS/m, pH of 7.79, sandy loam texture, organic matter content of 0.7%, total nitrogen of 0.04%, available phosphorus of 11 mg/kg, and available potassium of 151 mg/kg. The superabsorbent polymer (Aquasorb) with 0.5-1 mm particle size was used. Harvesting and trait evaluation were performed 92 days after planting. Parameters measured included chlorophyll a, b, total chlorophyll, carotenoids, photosynthesis rate, relative water content, stem and root dry weight, plant height, leaf number and area, and fruit number and weight. Data were analyzed using SPSS software and means comparisons were done by Duncan’s multiple range test at the 1% probability level.
Results
All treatments showed significant effects on measured traits compared to the control. Combined treatments (T4 and T5) demonstrated significant superiority over individual applications. The highest total chlorophyll content (2.15 mg/g fresh weight) and carotenoid content (0.36 mg/g fresh weight) were observed in T4 treatment (superabsorbent + fish hydrolysate), showing increases of 21.5% and 24%, respectively, compared to the control (1.77 mg/g for total chlorophyll). T4 treatment also exhibited the highest plant height (73.6 cm) compared to control (63.24 cm), and the highest leaf number (57.5 leaves per plant) and leaf area (713.5 cm²), representing increases of 20% and 22%, respectively, over control.
Total plant dry weight in T4 and T5 treatments were 25.93 g and 24.78 g, respectively, showing significant increases compared to control (20.9 g), with T5 treatment showing a 24% increase. The highest photosynthesis rate (22.74 μmol CO₂ m⁻² s⁻¹) and relative water content (90.66%) were measured in T4 treatment, representing increases of 16.6% and 10.2%, respectively, compared to control. The increased relative water content in treatments containing superabsorbent polymers demonstrates the ability of these polymers to maintain soil moisture and provide stable water supply to plants, maintaining stomatal opening and enhancing gas exchange.
Fruit yield was highest in T4 treatment with average fresh fruit weight of 104.75 g, showing a 17.7% increase compared to control. The number of fruits, fresh weight, and dry weight in T4 treatment were 16.57, 104.75 g, and 10.51 g, respectively, representing increases of 33%, 17.7%, and 18% compared to control. T5 treatment ranked second with fruit number of 15.55, fresh weight of 100.56 g, and dry weight of 10.8 g. The increase in fruit number indicates improved flowering and fruit set processes, while increased fruit weight reflects enhanced translocation of assimilates to reproductive organs. This results from combining the positive effects of biostimulants on photosynthesis and superabsorbent polymers on plant water status.
Conclusions
Based on the results, the application of amino acids and fish hydrolysate significantly improved the quantitative and qualitative characteristics of bell pepper plants and their yield. The combined application of fish hydrolysate with superabsorbent polymer proved particularly effective due to improved soil water status and plant nutrition. The synergistic effects observed between biostimulants and superabsorbent polymers led to coordinated improvements in vegetative growth, physiological performance, and fruit production. Heat map and correlation matrix analyses confirmed that the positive effects of treatments were not limited to specific traits but harmoniously improved all measured parameters. Therefore, the integrated application of fish hydrolysate and superabsorbent polymer can be recommended as an effective strategy for enhancing both the quantity and quality of bell pepper production, especially in calcareous soils with limited water availability.