Document: ARN-3-33-24

Effect of CO₂ on nitrate reduction and nitrite transport into the chloroplast.

NGUYEN, D.T.*, A.J.BLOOM and D.R.SMART

University of California, Davis, CA 95616 ¹

Abstract:
In long term growth experiments and short term studies of NO₃^- reduction using intact wheat plants, we found that elevated CO₂ diminishes NO₃^- assimilation. We proposed that in elevated CO₂ conditions, 1) HCO₃^- may inhibit NO₂^- transport into the chloroplast or 2) that CO₂ directly competes with NO₃^- and NO₂^- for reductant. To examine these hypotheses, chloroplasts were isolated from mature wheat leaf tissue and mature pea leaf tissue. The intact chloroplasts were then incubated with ¹⁵N potassium nitrite only (control experiment) and ¹⁵N potassium nitrite plus potassium bicarbonate (competition experiment). We found that nitrite was actually transported into the chloroplast faster in the competition experiment than in the control experiment. Thus, bicarbonate stimulated rather than inhibited nitrite transport into the chloroplast. Chloroplasts were washed with magnesium chloride and potassium chloride before the incubation to remove nitrate reductase activity. The data showed that removing the enzyme activity disrupted NO₂^- transport into the chloroplast. Our observations do not support competition of NO₃^- and NO₂^- with CO₂ for reductant at the intact chloroplast level. However, combining chloroplast data with plant results suggest that nitrate reduction and NO₂- transport into the chloroplast are closely linked.

Keywords: Chloroplast, reductant, competition, nitrite, bicarbonate.

This abstract is being presented at: 10:30 AM in session:
Poster Session #18: Elevated CO₂.