CONSTITUTIVE AND INDUCIBLE NET NH4-VULGARE L) SEEDLINGS( UPTAKE OF BARLEY (HORDEUM)

High initial NH4+ depletion rates were measured immediately after expo sure of N-free grown and NH4+-induced barley to NH4+. We provide evide nce that this rapid initial NH4+ depletion was due primarily to the fi lling of the Apparent Free Space of the root cell wall. The high initi al depletion rates depended on the NH4+ concentration of the uptake so lution and on the degree of prior NH4+ saturation of the Apparent Free Space. N-free grown barley seedlings were able to take up NH4+ consti tutively with a V-max of 3.7 mu mol.(g root FW)(-1) h(-1) and a K-s of 153 mu M. However, an induction of NH4+ uptake by the NH4+ reserves o f the grain (105 nmole per dry grain; 74% of which was stored in the d ormant seed) during early germination cannot be excluded. Exposure of N-free grown seedlings to external NH4+ (< 1 mM) resulted in an induct ion of NH4+ uptake after a 80 min lag; the rates were higher (V-max = 6-8.3 mu mol.(g root FW)(-1).h(-1)) than the constitutive rates, but K , remained unchanged (154 to 166 mu M after induction and 153 mu M pri or to induction). This indicates that the same NH4+ uptake system (low -capacity uptake system) was operating in N-free and NH4+-induced barl ey and that during induction the amount of transport proteins was incr eased. A linear uptake component (high-capacity system) was measured a t high external (> 1 mM) NH4+ concentrations with NH4+-induced and N-f ree grown plants. The steady-state rates were about 10-fold higher tha n that of the low-capacity system although no additional protein synth esis was required.