, Volume 185, Issue 2, pp 269-278

Sulphate deprivation depresses the transport of nitrogen to the xylem and the hydraulic conductivity of barley (Hordeum vulgare L.) roots

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During the first 4 d after the removal of SO 4 2- from cultures of young barley plants, the net uptake of 15N-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 [3H]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 SO 4 2- 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 NO 3 - uptake and N translocation which occurred during continued SO 4 2- deprivation. This treatment had relatively little effect in lowering amide levels in roots. Experiments with excised root systems indicated that SO 4 2- deprivation progressively lowered the hydraulic conductivity, Lp, of roots; after 4 d the Lp of SO 4 2- -deprived excised roots was only 20% of that of +S controls. In the expanding leaves of intact plants, SO 4 2- 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 NO 3 - and 13NO 3 - 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.

J.L.K. is extremely grateful to the British Council for supporting his working visit to Long Ashton. We thank John Radin for helpful discussion and encouragement.