Mitigating Salinity Stress in Red Kale (Brassica napus L. var. Pabularia ‘KX-1’) Through Split-root: Implications for Brackish Water Aquaponics

Abstract

Brackish water aquaponics is a promising approach to expand the scope of aquaculture-hydroponics integration. However, progress on brackish water aquaponics is limited due to low salinity tolerance of most high-value vegetable crops. Salinity can significantly affect plant growth by reducing plant height, decreasing leaf number and leaf size, leading to early leaf senescence, and decreasing biomass accumulation in salt-sensitive plant species. Additionally, salinity can negatively impact plant physiological performance, resulting in reduced stomatal conductance, lower CO2 assimilation, reduced photosynthetic activity, and ultimately lower plant productivity. To address this limitation in brackish water aquaponics systems, two studies were conducted to investigate the effectiveness of split-root system in deep-water culture (DWC) hydroponics system to mitigate salinity effects on red kale (Brassica napus L. var. pabularia). The focus of the first study was to evaluate the effect of increasing salinity levels (0, 3, 6, and 9 g L-1 or ppt) on red kale growth and physiology in whole-root and split-root systems. Results indicated that red kale plants grown in split-root system had a higher growth index than whole-root system when compared to the control treatment (0 ppt) in each condition. Stomatal conductance was similar in split-root system with increasing salinity level but was greatly decreased in the whole-root system as salinity level increased. The second study evaluated the effect of brackish water aquaculture effluents on the growth and physiology of red kale in two split-root systems: homogeneous and heterogeneous. In homogeneous split-root system, both parts of the plant root system received the same treatment, while in the heterogeneous system, one part was treated and the other exposed to clear water. Treatments included hydroponic solution containing 0 ppt salinity, saltwater-based hydroponics at 11 ppt, and shrimp effluents at 14 ppt. Significant interactions were found between split-root systems (conditions) and treatments in kale growth traits including height, leaf number, size index, and shoot fresh and dry weights. The heterogeneous split-root system showed positive effects on mitigating salinity stress on red kale growth, compared to the homogeneous split-root system. Plants treated with shrimp effluents showed lower stress levels, evidenced by improved leaf stomatal conductance compared to saltwater-based hydroponic in split-root system. The split-root system demonstrated in this thesis under the DWC system offers a practical solution to mitigate salinity effects on high-value vegetable crops cultivated with brackish water aquaculture effluents. Further research can explore and refine split-root systems for commercial application in brackish water aquaponics systems.