of N per person per day is excreted in urine by humans who eat a typical Western diet [182]. Along with
this, however, nearly 180 mmol of Na is excreted in urine [182]. Thus, Na inputs from human liquid
wastes (which include urine, gray water, etc.) could amount to nearly 200 mmol per person per day. A nu-
trient solution volume of 17 L m^2 for a hydroponic crop production system and 40 m^2 growing are per
person are assumed (A. Drysdale, personal communication), Na inputs from waste recycling could lead
to 0.28 mM increase per day [10,168]. If plants fail to remove this Na and recycle it to the crews through
the food chain, Na concentrations could reach 100 mM in the nutrient solutions within 1 year from the
urine input alone; this would kill most crops, resulting in a collapse of the food production system
[10,168]. Thus, appropriate nutrient management approaches, coupled with crop selection strategies, are
required to manage the cycling of Na between humans and plants in tightly closed systems. One solution
to this would involve getting Na into the edible portions of the crops and returning Na to the humans
through the food loop so that there would be no reason for an external supply of Na (Figure 6).
IX. CONCLUDING REMARKS
In spite of many decades of research, the full role of Na in plant metabolism remains unresolved. The de-
termination that Na is an essential nutrient under Arnon and Stout’s [1] definition has been convincingly
established only for certain C 4 plants but not for all higher plants. Numerous reports indicate a significant
stimulation of the growth of selected crops in response to Na even under conditions of apparent K suffi-
ciency. There is convincing evidence that Na can substitute for K in nonspecific functions such as an os-
moticum during cell enlargement and a counterion in long-distance transport. The functional role of Na
in plant nutrition is large, multifaceted, and sufficiently important in many biosystems and plant produc-
tion that there needs to be a category of essentiality to designate such a role. The term functional nutrient
seems to be appropriate, when defined as a nutrient that is essential for maximal yield and functional in
metabolic functions without being unique for any one function.
ACKNOWLEDGMENTS
We would like to acknowledge the extensive support and discussions we had with Dr. Mike Shannon
(USDA Salinity Laboratory, Riverside, CA) during the course of preparing this chapter, which has helped
in our interpretation of various issues related to sodium nutrition. Also, we would like to thank Dr.
378 SUBBARAO ET AL.Figure 6 Some considerations for Na flows in closed systems in space where crops might be used for life
support.