Feasibility of substituting sodium for potassium in crop plants for advanced life support systems

Recycling of nutrients, air, and water is an integral feature of life support systems designed for long-term space missions. Plants can play a major role in supplying the basic life support requirements, which include providing the crew's food, clean water, and air, and recycling their wastes. The nutrient flux through the plant and human systems needs to be matched in order for nutrients to recycle between humans and plants without an excessive buildup in any one section of the system. Sodium, which is essential at the macronutrient level for human metabolism, has only been shown to be a micronutrient for some plants, with only very limited uptake in most plants. Thus, when Na is added from the outside to meet the human demand in these closed life support systems it will accumulate someplace in the overall system. In simple systems such as these, without a complete biogeological cycle, the buildup of Na could occur in the nutrient solution of the plant system. Various concepts related to the substitution of sodium for potassium in crop plants are currently being investigated by NASA. Results to date suggest that Na concentrations up to 100 g kg-1 dry weight may be achievable in the edible portions of Na-tolerant crops (e.g., red beet and chard). A flow path for nutrient solution high in Na wastes has been suggested for optimizing Na and nitrogen incorporation and utilization from such solutions. Options for further improvements include selecting plant genotypes tolerant to high salinity, which are efficient in Na uptake. This should also be combined with environmental manipulations to maximize Na uptake by crop plants.