The influence of light, dark, and ambient CO2 on nitrate assimilation in 8-to 9-day-old barley seedlngs was studied. To develop the photosynthetic apparatus fully, the seedings were grown in nitrogen-free Hoagland solution for 5 days in darkness followed by 3 days in continuous ight.The It is well established that light energy is intimately involved in the assimilation of nitrate (1,(4)(5)(6)24). The influence of light is not well understood, however, and reports are contradictory. An obligatory requirement of light for nitrate assimilation has been reported for several species (6,18,19), whereas others report nitrate reduction in darkness in tissue slices or detached leaves of many species under both anaerobic (3,13,(17)(18)(19) and aerobic (12, 13) conditions. Sawhney et aL (18) recently proposed that reduction of nitrate in light only avoids the accumulation of toxic levels of nitrite in the dark. In contrast, Jones and Sheard (12) and placed in the dark at room temperature. On the 6th day, the seedlings were transferred to aerated one-fourth-strength Hoagland solution (9) lacking nitrogen and placed in continous light of 500 ,uE m-2 s-1 for 3 more days at 25 C and 70 to 75% RH to develop the photosynthetic apparatus (10). When carbohydratedeficient seedlings were used, they were given 3 days of light treatment and then placed in darkness at 25 C and 70 to 75% RH for the desired period (24-48 h).Nitrate and Nitrite Uptake. Uptake of nitrate and nitrite was measured as the amounts disappearing from the substrate solution with time. Ten seedlings per treatment (each treatment replicated twice and each experiment repeated two times) were placed in 140 ml of one-fourth-strength Hoagland solution containing 1.0 mm KNO3 or 1.0 mm NaNO2 and 5.0 mm CaSO4. The initial pH of the solutions was 5.8. The solutions were renewed once after a 12-h absorption period. By this time, nitrate concentration from the uptake medium was reduced to about 0.5 mm. Previous studies in this laboratory have shown that uptake rates between 0.5 and 1.0 mm nitrate were constant (7). When excised roots were used, they were excised at the scutellar node and submerged in the uptake medium for the desired periods. Two g of excised roots per treatment were used. All solutions were aerated during uptake.The addition of 50 jig/ml chloramphenicol to uptake solutions had no effect on the results showing that bacterial contamination was not a problem.To study the effect of CO2 on nitrate uptake, the seedlings were placed in a 15-liter Plexiglas chamber. The light intensity inside the chamber at the top of the seedling canopy was 450 ,uE m-2 s-'.Normal or CO2-free air was passed through the chamber at 2 liters/min, giving about eight exchanges/h, with a positive pressure inside the chamber. The treatment solutions were aerated with the same gas. The air from the Plexiglas chamber was passed through a cold bath and recirculated back into the chamber at 6 liters/min. This maintained temperature and RH at about the same level as in the growth cham...

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