Elevated atmospheric CO(2) decreases the ammonia compensation point of barley plants

The ammonia compensation point ([Image: see text]) controls the direction and magnitude of NH(3) exchange between plant leaves and the atmosphere. Very limited information is currently available on how [Image: see text] responds to anticipated climate changes. Young barley plants were grown for 2 weeks at ambient (400 μmol mol(–1)) or elevated (800 μmol mol(–1)) CO(2) concentration with [Image: see text] or NH(4)NO(3) as the nitrogen source. The concentrations of [Image: see text] and H(+) in the leaf apoplastic solution were measured along with different foliar N pools and enzymes involved in N metabolism. Elevated CO(2) caused a threefold decrease in the [Image: see text] concentration in the apoplastic solution and slightly acidified it. This resulted in a decline of the [Image: see text] from 2.25 and 2.95 nmol mol(–1) under ambient CO(2) to 0.37 and 0.89 nmol mol(–1) at elevated CO(2) in the [Image: see text] and NH(4)NO(3) treatments, respectively. The decrease in [Image: see text] at elevated CO(2) reflected a lower N concentration (–25%) in the shoot dry matter. The activity of nitrate reductase also declined (–45 to –60%), while that of glutamine synthetase was unaffected by elevated CO(2). It is concluded that elevated CO(2) increases the likelihood of plants being a sink for atmospheric NH(3) and reduces episodes of NH(3) emission from plants.