Elevated CO2 Decreases the Photorespiratory NH3 Production but Does not Decrease the NH3 Compensation Point in Rice Leaves

摘要

The exchange of gaseous NH3 between the atmosphere and plants plays a pivotal role in controlling the global NH3 cycle. Photorespiration generates NH3 through oxygenation instead of carboxylation by the CO2-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The future increase in the atmospheric CO2 concentration, [CO2], is expected to reduce plant NH3 production by suppressing RuBisCO oxygenation (V-o). We measured the net leaf NH3 uptake rate (F-NH3) across NH3 concentrations in the air (n(a)) ranging from 0.2 to 1.6 nmol mol(-1) at three [CO2] values (190, 360 and 750 mu mol mol(-1)) using rice plants. We analyzed leaf NH3 gas exchange using a custom-made whole-leaf chamber system, and determined the NH3 compensation point (gamma), a measure of potential NH3 emission, as the x-intercept of the linear relationship of FNH3 as a function of n(a). Our gamma values were lower than those reported for other plant species. gamma did not decrease under elevated [CO2], although leaf NH4+ content decreased with decreasing V-o at higher [CO2]. This was also the case for gamma estimated from the pH and NH4+ concentration of the leaf apoplast solution (gamma'). gamma' of rice plants, grown at elevated [CO2] for months in a free-air CO2 enrichment facility, was also not decreased by elevated [CO2]. These results suggest that suppression of RuBisCO oxygenation by elevated [CO2] does not decrease potential leaf NH3 emission in rice plants.