Effects of oxygenated brackish water on light response characteristics and yield of pakchoi (Brassica chinensis L.)

Abstract: In arid and semiarid areas, brackish water irrigation can be a great potential approach to alleviate the shortage of fresh water resources in agriculture. However, the irrigation of agricultural land with brackish waters can also reduce crops production due to salt stress, while lead to soil degradation, such as secondary salinization. To accurately evaluate soil salinity, various methods have been developed to safely utilize brackish water for farmland irrigation system. The oxygenated treatment technology can be expected to improve the adverse effects of salt content on soil structure and crop growth, due to easy operation, low energy consumption, cost-saving, friendly environment and high efficiency. In this study, the pakchoi was taken as the research object under the combination of the brackish water irrigation and oxygenation technology. A mathematical model was proposed to investigate the impact of dissolved oxygen in brackish water on light-response characteristics and pakchoi yield under the condition of hydroponics. Four light-response models, including rectangular hyperbolic, non-rectangular hyperbolic, modified rectangular hyperbolic and exponential model, were used to analyze the photosynthetic characteristics of pakchoi leaves. Five levels of dissolved oxygen concentrations were measured, 9.5 (CK), 12.5, 15.5, 18.5, 22.5mg/L. In the light-response model, the optimal parameters of the photosynthetic characteristics were obtained under the different dissolved oxygen concentration of brackish waters. The results showed that there was significant difference between the fitting light-response curves in four light-response models at the five levels of dissolved oxygen concentration in brackish water during the error tests. The optimal accuracy can be obtained in the non-rectangular hyperbolic model for all treatments. Specifically, the photosynthetic parameters indicated that the maximum net photosynthetic rate (Pnmax), apparent quantum efficiency (α), and dark respiration rate (Rd) under 18.5 mg/L treatment were significantly higher than those under other treatments. In 18.5 mg/L dissolved oxygen concentration, the net photosynthetic rate and fresh quality for the aboveground part of pakchoi were also significantly higher than those in other levels. Therefore, the 18.5 mg/L dissolved oxygen concentration can be optimal level for pakchoi cultivating with brackish water. The reason is that the dissolved oxygen in brackish water can enhance the shade tolerance and the ability to endure high light intensity of pakchoi, as well the available range of light intensity. In addition, the oxygenated treatment can also improve the physiological activity and efficient photosynthesis of pakchoi leaves, and thereby to achieve the high yield of pakchoi. The findings can provide a theoretical basis for the safe and efficient utilization of brackish water, while the alleviation of oxygen stress in crop roots for brackish water irrigation in arid areas.