Mechanisms for N-uptake and their running costs; Is there scope for more efficiency?

The current state of knowledge about transport of NO3- and NH4+ across the plasma membranes is reviewed. Thermodynamic considerations show that NO3- influxes by both high-affinity (HATS) and low-affinity (LATS) transport systems need to be coupled to the diffusion of protons down their electrochemical potential gradient. The cost of such transport is largely derived from the expulsion of H+ from the cytosol by the plasma-membrane H+-ATPase. At concentrations higher than about 50 mu M, NH4+ uptake may be diffusive and occur via channel proteins. This movement is also dependent on the membrane potential (E-m) created by the H+-ATPase. Evidence is presented that there can often be substantial effluxes of NO3- and 'ammonia' from root cells. The former may occur via outwardly rectifying channels while ammonia efflux may represent diffusion of the uncharged species through the lipid bilayer. The experimental difficulties in measuring efflux in plants receiving dilute or growth-limiting supplies of NO3- are discussed. Methods for estimating the costs of transport are outlined. It is concluded that costs based on thermodynamic considerations do not estimate reliably the actual costs. 'Biochemical' costs, based simply on the coupling of H+ to the observed NO3- uptake rates, underestimate ATP consumption if no allowance is made for the leakage of NO3- by efflux. Respiratory budget estimates of the cost of NO3- transport look the most reasonable since they take into account automatically the 'inefficiencies' caused by NO3- efflux. If efflux decreases proportionally more than influx as [NO3-](ext) decreases, it follows that net NO3- transport will become more efficient in its use of ATP. The above ideas are discussed in relation to the transport costs of slow- and fast-growing species growing with abundant or growth-limiting N supply.