| Successful crop production is vital to manned missions to Mars. Plants play integral roles in conceptual life-support systems as sources of food, oxygen, and waste treatment. Constraints of building a structure on the Martian surface to withstand Earth-similar interior air pressures make it necessary to develop plant growth systems capable of operating in air pressures as low as 0.1 to 0.3 atm (10--30 kPa). Research has shown that plants are capable of surviving in such environments, but have increased rates of water loss. The enormous costs associated with launching a manned mission to Mars make it crucial that plants be not only capable of survival, but also of producing fruit and seed. Plant growth and development, and thus, performance of a biological life-support system are highly dependent on plant environmental responses. Therefore, it is important that the interactions between plants and the environment of a Mars greenhouse are well understood.; A model was used to predict the rate of evapotranspiration in response to changes in pressure, CO2, and light. The model was compared to empirical data obtained in experiments performed in a system of three small-scale low pressure controlled environment chambers built for this research. The system provided control of pressure, CO2 concentration, air temperature, and relative humidity and measured plant weight and leaf temperature.; The rate of evapotranspiration changed little when pressure was 33 kPa and greater, but increased significantly at 12 kPa. Plants quickly wilted when pressure was 12 kPa and CO2 was 40 Pa. Reduced pressure increased the rate of evapotranspiration by decreasing resistances to sensible and latent heat loss as well as reducing the effectiveness of convection. However, when CO2 concentration was increased from 40 to 150 Pa, stomates closed and evapotranspiration decreased even at the lowest pressure. Thus, plants are capable of growing at extreme low pressures, but are more sensitive to changes in other environmental parameters. In a low-pressure Mars greenhouse, failure of the control system will likely result in crop failure. |
