Proportional control of CO₂ by higher plants in a life support system

Abstract

Controlled Ecological Life Support System (CELSS) is a life support system for space application which depends upon biological systems, either wholly or in part, for regeneration of air, water and food and for the recycling of wastes. Control can be maintained using exogenous control, which depends upon human manipulation of the system, or endogenous control, which uses biological control inherent in natural ecosystems. Higher plants have physiological responses to the environment which can be utilized in CELSS for endogeous control. The response of carbon uptake to CO₂ concentration can be used as a proportional controller for atmospheric CO₂ in the CELSS. A CELSS simulation model was developed which incorporated random crew CO₂ output based upon metabolic activity levels and the flux of carbon to and from the plants, as well as its flow through sinks internal to the plants. It showed that endogenous control was possible. A theoretical model of a plant growth chamber with a proportional control system for CO₂ incorporated into the chamber response was developed. An analogy was drawn between proportional control and the response of the rate of net CO₂ uptake of plants to changes in CO₂ concentration. Experimental methods were suggested from this theoretical model that could be used to test for the existence of proportional control capability of the plants. Experiments were conducted in a plant growth chamber using wheat at three temperatures, 15, 20 and 25 °C, and at various CO₂ concentrations. The experimental evidence confirmed that endogenous CO₂ control capability exists in higher plants. The main evidence lies with the theoretical predictions that a signal of CO₂ concentration passing through the chamber will be more reduced if endogenous control is present compared to when it is not. The theoretical model also suggested two practical methods for monitoring the endogenous control capability in a CELSS: (1) proportional control of CO₂ will cause a reduction in correlation of succesive CO₂ concentration measurements, and (2) analysis of the low frequency components of the signal passing through the system. The experiments failed to show that these methods would give consistent or useable results.