SUPRAOPTIMAL CARBON-DIOXIDE EFFECTS ON GROWTH OF SOYBEAN [GLYCINE-MAX(L) MERR]

In tightly closed environments used for human life support in space, c arbon dioxide (CO2) partial pressures can reach 500 to 1000 Pa, which may be supraoptimal or toxic to plants used for life support. To study this, soybeans [Glycine max (L.) Merr. cvs. McCall and Pixie] were gr own for 90 days at 50, 100, 200, and 500 Pa partial pressure CO2 (500, 1000, 2000, and 5000 ppm). Plants were grown using recirculating nutr ient film technique with a 12-h photoperiod, a 26-degrees-C/20-degrees -C thermoperiod, and approximately 300 mumol m-2 s-1 photosynthetic ph oton flux (PPF). Seed yield and total biomass were greatest at 100 Pa for cv. McCall, suggesting that higher CO2 levels were supraoptimal. S eed yield and total biomass for cv. Pixie showed little difference bet ween CO2 treatments. Average stomatal conductance of upper canopy leav es at 50 Pa CO2 almost-equal-to 500 Pa > 200 Pa > 100 Pa. Total water use over 90 d for both cultivars (combined on one recirculating system ) equalled 822 kg water for 100 Pa CO2, 845 kg for 50 Pa, 879 kg for 2 00 Pa, and 1194kg for 500 Pa. Water use efficiences for both cultivars combined equaled 3.03 (g biomass kg-1 water) for 100 Pa CO2, 2.54 g k g-1 for 200 Pa, 2.42 g kg-1 for 50 Pa, and 1.91 g kg-1 for 500 Pa. The increased stomatal conductance and stand water use at the highest CO2 level (500 Pa) were unexpected and pose interesting considerations,fo r managing plants in a tightly closed system where CO2 concentrations may reach high levels.