Photosynthesis and respiration of a wheat stand at reduced atmospheric pressure and reduced oxygen

A 34-day functional test was conducted in Johnson Space Center's Variable Pressure Growth Chamber (VPGC) to determine responses of a wheat stand to reduced pressure (70 kPa) and modified partial pressures of carbon dioxide and oxygen. Reduced pressure episodes were generally six to seven hours in duration, were conducted at reduced ppO(2) (14.7 kPa), and were interrupted with longer durations of ambient pressure (101 kPa). Daily measurements of stand net photosynthesis (Pn) and dark respiration (DR) were made at both pressures using a ppCO(2) of 121 Pa. Corrections derived from leakage tests were applied to reduced pressure measurements. Rates of Pn at reduced pressure averaged over the complete test were 14.6 % higher than at ambient pressure, but rates of DR were unaffected. Further reductions in ppO(2) were achieved with a molecular sieve and were used to determine if Pn was enhanced by lowered O-2 or by lowered pressure. Decreased ppO(2) resulted in enhanced rates of Pn, regardless of pressure, but the actual response was dependent on the ratio of ppO(2)/ppCO(2). Over the range of ppO(2)/ppCO(2) of 80 to 200, the rate of Pn declined linearly. Rate of DR was unaffected over the same range and by dissolved O-2 levels down to 3.1 ppm, suggesting that normal rhizosphere and canopy respiration occur at atmospheric ppO(2) levels as low as 11 kPa. Partial separation of effects attributable to oxygen and those related to reduced pressure (e.g. enhanced diffusion of CO2) was achieved from analysis of a CO2 drawdown experiment. Results will be used for design and implementation of studies involving complete crop growth tests at reduced pressure. (C) 1997 COSPAR. Published by Elsevier Science Ltd.