Measurement of Net Photosynthetic and Transpiration Rates of Spinach and Maize Plants under Hypobaric Condition.

Abstract

An environmental control system for measuring net photosynthetic and transpiration rates was constructed to study the feasibility of growing plants under hypobaric condition in order to achieve crop production for a controlled ecological life support system (CELSS) in space. The total pressure of the system's assimilation chamber was reduced to 10kPa, with the rates for spinach and maize plants being measured under total air pressures of 10, 40, 70, and 100kPa at the same CO2 partial pressure condition. Photosynthetic rates of spinach and maize increased until a total pressure of 10kPa. The rate of increase in the photosynthetic rate under low total pressures was distinctive in maize. In addition, the rate of increase was found to be affected by the CO2 gas diffusion rate. Photosynthesis is considered to be enhanced by a decrease in boundary layer resistance and stomatal resistance to CO2 transfer due to an increase in the CO2 diffusion coefficient. The transpiration rate of spinach slightly increased under low total pressures, whereas that of maize also increased, though much less; both plants had lower rates than those simulated using water vapor diffusion theory.