Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4  + Toxicity in Plant Roots    

Densely rooted rhizosphere hotspots induced around subsurface NH4 +-fertilizer depots: a home for soil PGPMs?

“…In contrast to plant and animal cells, bacteria preferentially use NH 4 + as a source of N and can be tolerant even to molar concentrations of NH 4 -N, at which point growth limiting effects may be related more to osmotic or ionic imbalances than to direct toxic effects of NH 4 + ions per se [38]. The threshold concentration range of 50-250 mM NH 4 -N at which cultured PGPMs showed that inhibited growth is above the range for NH 4 + toxicity in sensitive crop species like barley (Hordeum vulgare L.) and tolerant ones like rice (Oryza sativa L.) grown hydroponically [39][40][41]. The NH 4 + tolerance threshold for our tested PGPMs (50-250 mM NH 4 -N) is also above the range of 2-20 mM that can be common in soil solution of most agricultural soils [42].…”

Measuring Fluxes of Mineral Nutrients and Toxicants in Plants with Radioactive Tracers

“…Figure 1 shows that NH 3 influx can reach in excess of 225 µmol g -1 hr -1 , which is perhaps the highest bona fide transmembrane flux ever reported in a plant system 13 , but the magnitude of this flux would not be visible if only net fluxes were measured. This is because a large efflux of NH 3 occurs at the same time as influx, in a futile cycling scenario that can result in a pronounced underestimate of influx that increases with labeling time 13 .…”

“…Figure 1 shows that NH 3 influx can reach in excess of 225 µmol g -1 hr -1 , which is perhaps the highest bona fide transmembrane flux ever reported in a plant system 13 , but the magnitude of this flux would not be visible if only net fluxes were measured. This is because a large efflux of NH 3 occurs at the same time as influx, in a futile cycling scenario that can result in a pronounced underestimate of influx that increases with labeling time 13 . By supplementing the tracer technique with electrophysiological analysis, we were able to demonstrate that under the conditions of Figure 1, both influx and efflux of 13 N is primarily of the neutral gas NH 3 , and not of its conjugate acid NH 4 + (for details, see 13 ).…”

“…This is because a large efflux of NH 3 occurs at the same time as influx, in a futile cycling scenario that can result in a pronounced underestimate of influx that increases with labeling time 13 . By supplementing the tracer technique with electrophysiological analysis, we were able to demonstrate that under the conditions of Figure 1, both influx and efflux of 13 N is primarily of the neutral gas NH 3 , and not of its conjugate acid NH 4 + (for details, see 13 ). This is the first in planta demonstration of rapid NH 3 gas fluxes in roots, and as such, provides important preliminary evidence towards unraveling the transport mechanism that lies at the heart of NH 3 /NH 4 + toxicity in higher plants 2,13 .…”

“…Immerse roots in pre-labeling (non-radioactive) solution for 5 min, to pre-equilibrate plants under test conditions (see e.g., 10,13,14 ), Q* is the quantity of tracer accumulated in tissue (cpm, usually in root, shoot, and basal shoot, combined), S o is the specific activity of the labeling solution (cpm µmol -1 ), w is the root fresh weight (g), and t L is the labeling time (hr). NOTE: More sophisticated calculation can be made to account for simultaneous tracer efflux from roots during labeling and desorption, based on parameters obtained from CATE (see below; for details, see 4 ).…”