SULFATE DEPRIVATION DEPRESSES THE TRANSPORT OF NITROGEN TO THE XYLEM AND THE HYDRAULIC CONDUCTIVITY OF BARLEY (HORDEUM-VULGARE L) ROOTS

During the first 4 d after the removal of SO42- from cultures of young barley plants, the net uptake of N-15-nitrate and the transport of labelled N to the shoot both decline. This occurred during a period in which there was no measurable change in plant growth rate and where the incorporation of [H-3]leucine into membrane and soluble proteins was unaffected. Reduced N translocation was associated with six- to eightfold increases in the level of asparagine and two- to fourfold increases in glutamine in root tissue; during the first 4 d of SO42- deprivation there were no corresponding increases in amides in leaf tissue. The provision of 1 mol . m-3 methionine halted, and to some extent reversed the decline in NO3- uptake and N translocation which occurred during continued SO42- deprivation. This treatment had relatively little effect in lowering amide levels in roots. Experiments with excised root systems indicated that SO42- deprivation progressively lowered the hydraulic conductivity, Lp, of roots; after 4 d the Lp of SO42--deprived excised roots was only 20% of that of + S controls. In the expanding leaves of intact plants, SO42- deprivation for 5 d was found to lower stomatal conductance, transpiration and photosynthesis, in the order given, to 33%, 37% and 18% of control values. The accumulation of amides in roots is probably explained by a failure to export either the products of root nitrate assimilation or phloem-delivered amino-N. This may be correlated with the lowered hydraulic conductivity. Enhanced glutamine and-or asparagine levels probably repressed net uptake of NO3- and (NO3-)-N-13 influx reported earlier (Clarkson et al. 1989, J. Exp. Bot. 40, 953-963). Attention is drawn to the similar hydraulic signals occurring in the early stages of several different types of mineral-nutrient stresses.