Aim
This study aimed to investigate the effects of different copper sulfate concentrations and irrigation levels on bell pepper growth parameters in a subsurface irrigation system using copper-coated porous clay emitters. The research examined how four copper sulfate concentrations (0, 1000, 5000, and 10000 ppm) and three irrigation volumes (low: 200 mL, moderate: 300 mL, and high: 500 mL) influence root characteristics including volume, length, and dry weight, as well as shoot and leaf parameters including fresh and dry weights of leaves and stems, and root-to-shoot dry weight ratios. The study addresses the critical problem of root intrusion into porous clay emitters causing clogging and reduced irrigation efficiency over time. Given increasing global population, declining groundwater reserves, increasing droughts due to climate change, and excessive water resource consumption, water scarcity has become a critical crisis in many countries worldwide. In low-rainfall countries like Iran, high evaporation rates cause severe water losses in irrigated agricultural lands. Subsurface irrigation using porous clay capsules offers an efficient solution by gradually releasing water near plant roots and minimizing evaporation and deep percolation losses. However, root intrusion into clay emitters and pore clogging due to algae, fungi, and bacterial growth have limited the widespread adoption of this otherwise promising technique. Copper has been shown to inhibit root growth by reducing cell division and longitudinal root elongation, making it a potential solution for preventing root intrusion into emitters. This research optimizes copper concentrations for bell pepper cultivation in subsurface irrigation systems while minimizing phytotoxic effects and maintaining emitter functionality.
Materials and Methods
The experiment was conducted as a completely randomized design with 4 copper sulfate treatments (0 ppm as control, 1000 ppm, 5000 ppm, and 10000 ppm CuSO₄·5H₂O) and 3 irrigation levels (low: 200 mL, moderate: 300 mL, and high: 500 mL) with 3 replications, totaling 36 experimental units. The experiment was carried out in the research greenhouse of Tarbiat Modares University on bell pepper (Capsicum annuum L. cv. 302). Porous clay emitters measuring 20 cm in length and 3.5 cm in diameter were manufactured using clay material from Nasrabad village in Gorgan and rice bran filler using an extrusion machine. The emitters were immersed in the respective copper sulfate solutions for 24 hours and subsequently fired in a gas furnace at 600°C to fix the copper onto the ceramic surfaces. Soil samples were collected from an almond orchard in Tehran province at 0-30 cm depth and analyzed for physicochemical properties. Soil texture was loam with pH 8.14, electrical conductivity 2.36 dS/m, organic matter 0.97%, cation exchange capacity 11.50 cmol/kg, and nutrient contents including nitrogen (0.3 mg/kg), phosphorus (17.02 mg/kg), potassium (303.9 mg/kg), iron (10.46 mg/kg), zinc (2.37 mg/kg), copper (1.23 mg/kg), and manganese (11.91 mg/kg). Bell pepper seedlings were transplanted into 16 kg soil-filled pots on December 6, 2022, with one seedling per pot at 10 cm depth. The porous emitters were installed 12 cm from the plant root using plastic rods, with the emitter cap remaining above the soil surface. Plastic bottles with cut bottoms were attached to the emitters for water delivery. Irrigation was applied weekly at the specified volumes for approximately 70 days until before fruit set. Plants were then harvested, and roots were separated from shoots at the crown. Root fresh and dry weights were measured after drying at 105°C for 48 hours. Root volume was determined using a 1-liter graduated cylinder by water displacement, and root length was measured using a ruler from the cut point to the longest root. Statistical analysis was performed using SAS software.
Results
Analysis of variance revealed that copper sulfate levels and the interaction between copper sulfate and irrigation had no significant effect on root volume and length. However, irrigation volume significantly affected root volume at P<0.01 and root length at P<0.05. Copper sulfate significantly influenced root dry weight (P<0.05), leaf fresh weight (P<0.05), stem dry weight (P<0.05), and leaf-to-root dry weight ratio (P<0.01), but had no significant effect on total dry weight and stem fresh weight. Irrigation significantly affected all measured parameters at P<0.01, including leaf and stem dry weights, leaf and stem fresh weights, and total dry weight. The interaction between copper sulfate and irrigation was significant for leaf dry weight (P<0.05), stem dry weight (P<0.01), total dry weight (P<0.01), leaf-to-root dry weight ratio (P<0.01), and stem fresh weight (P<0.01). Regarding root characteristics, the highest root dry weight (3.27 g) was observed at the highest copper concentration (10000 ppm), representing approximately a 33% increase compared to other concentrations and the control treatment. For leaf fresh weight, the highest value (29.61 g) and lowest value (22.73 g) were recorded at the highest and lowest copper concentrations, respectively, with the 10000 ppm treatment reducing leaf fresh weight by approximately 23% compared to the 1000 ppm treatment. Stem dry weight at 10000 ppm copper showed a 22% reduction compared to the 1000 ppm treatment. The highest copper concentration significantly reduced the leaf-to-root dry weight ratio by approximately 26% compared to the control. For irrigation effects, the low irrigation level (200 mL) resulted in significantly higher root volume, root dry weight, leaf fresh and dry weights, stem fresh and dry weights, and total dry weight compared to the high irrigation level (500 mL). The high irrigation level reduced root volume by 23% and 43% compared to moderate and low irrigation levels, respectively, and reduced root length by approximately 13% compared to the low irrigation level. Low irrigation developed larger and deeper root systems through increased secondary root formation due to wetting-drying cycles. Interaction effects showed that at low irrigation, 5000 ppm copper significantly increased leaf dry weight compared to the control, while at moderate irrigation, all copper concentrations reduced leaf dry weight. For stem dry weight, maximum value was obtained at moderate irrigation with the control treatment. At low irrigation, increasing copper concentration led to increased stem dry weight compared to the control. For total dry weight at low irrigation, 1000 and 10000 ppm copper significantly increased this parameter compared to 5000 ppm treatment, while at high irrigation, 5000 and 10000 ppm copper reduced total dry weight. Leaf-to-root dry weight ratio was significantly reduced at moderate irrigation with 5000 and 10000 ppm copper, while at high irrigation, all copper treatments reduced this ratio compared to the control.
Conclusion
This research successfully demonstrated that copper sulfate application in porous clay emitters and irrigation management significantly influence bell pepper growth parameters in subsurface irrigation systems. Copper treatments did not significantly affect root volume and length, but the highest copper concentration significantly increased root dry weight, likely due to secondary root proliferation rather than primary root elongation. This suggests copper-induced inhibition of primary root growth is compensated by enhanced lateral root development. Irrigation volume proved to be a more dominant factor than copper concentration for most growth parameters, with low irrigation producing superior root and shoot characteristics due to wetting-drying cycles stimulating secondary root formation. Interaction effects indicated copper responses depended on irrigation levels, suggesting optimal concentrations must be determined based on specific regimes. These findings suggest copper-coated emitters at low to moderate concentrations combined with low irrigation volumes can maintain emitter functionality while promoting beneficial root proliferation. This approach offers sustainable solutions to improve water use efficiency, reduce water losses, and address emitter clogging in arid regions. Future research should evaluate long-term copper accumulation in soils, environmental impacts, and economic feasibility of large-scale implementation in commercial production.