## 1 Introduction  
The study of the effect of the atmospheric composition of the atmosphere on the atmosphere of the atmosphere  
## 1 Introduction  
Ionic silver (Ag\({}^{+}\)) is being investigated as a residual biocide for use in NASA spacecraft potable water systems on future crewed missions and has been used for many years on the Russian Mir and Russian modules of the International Space Station (ISS) [1; 2]. NASA currently uses iodine (I\({}_{2}\)) as a biocide in the potable water system on the ISS. Iodine is removed before use because excessive amounts can lead to thyroid dysfunction [3]. The advantage of ionic silver is that it does not need to be removed from potable water before consumption. The design specifications for a silver dosing device in a spacecraft potable water system include the ability to add between 200 to 500 ppb (0.2-0.5 mg/L) of silver [2]. The final amount of silver that will be dispensed from the potable water dispenser for end use is currently unknown and may vary due to the propensity for silver to plate out of solutions onto system materials [3]. In addition to providing clean water to the crew and other life-support system functions, potable water will be used to irrigate future spaceflight crop production systems. Substrate-based watering systems are currently used on the ISS (e.g., Veggie and Advanced Plant Habitat (APH)) and use arcillite--a calculated clay with high cation ion exchange capacity [4; 5]. Surface-tension effects exhibited in reduced gravity can lead to thick boundary layers of water around plant roots and, in turn, lead to root hypoxia [6]. Substrate-free nutrient delivery systems such as hydroponics can reduce waste and up-mass in future missions [7] and can provide efficient absorption of nutrients as roots are directly in contact with the nutrient solution [8]. Recent work to evaluate this efficiency and overcome the challenges of watering plants in the microgravity environment has included spaceflight testing of hydroponics systems such as Plant Water Management (PWM) [9] and the eXposed Root On-Orbit Test System (XROOTS) [10]. In this study, the Ag\({}^{+}\) in our irrigation water is leached from a foam containing silver chloride currently being tested for use in spacecraft water processor assemblies [2]. However, research concerning the effect of silver on plants has focused on silver nanoparticles (AgNPs) due to their ubiquitous use in nanomaterials. In agriculture, AgNPs have been used to stimulate plant growth, inhibit fungal growth, and modify ripening in fruits [11; 12]. Conversely, AgNP exposure is known to affect plant morphology by inhibiting seed germination and root growth, while reducing biomass and leaf area and increasing the production of reactive oxygen species (ROS) in plant tissue [11]. As an example, 5-10 nm particles tested at a concentration of 100-500 ppb caused a reduction in the elongation and fresh weights of shoots in roots, total chlorophyll, protein, and sugar content of _Lupinus termis_[13]. Additionally, the uptake of AgNP in radish was associated with a decrease in the water content of the plant tissue as well as nutrients in the form of Ca, Mg, B, Cu, Mn, and Zn [14]. AgNPs also affect plant hormones and are known to interfere with root gravitropism in _Arabidopsis_ by reducing the auxin receptor-related genes for the signaling hormone, auxin [15]. Conversely, cytokinin [16] and ethylene [17] levels are known to increase in response to AgNP exposure, although Ag\({}^{+}\) has also long been known to block the action of ethylene in plants [18]. The mechanism of toxicity for AgNPs is attributed to the induction of oxidative stress [19; 20] in plant cells which can damage cell membranes, structure, protein, and DNA through the excessive production of reactive oxygen species (ROS) from the chloroplasts, mitochondria, and peroxisomes. Water from the environment oxidizes AgNPs and Ag\({}^{+}\) is released, resulting in more ROS.