Rapid disruption of nitrogen metabolism and nitrate transport in spinach plants deprived of sulphate.

Hydroponically grown spinach plants were deprived of an external source of sulphate after an initial period when the S-supply was sufficient. The time-course of events following this treatment was monitored. The first responses were found in the uptake and translocation of NO(3)(-) and the uptake of SO(4)(2-). The former declined by approximately 50%, the effect being most significant at higher [NO(3)(-)](ext.) while the latter increased 6-fold over a 4 d period. Growth in the absence of external SO(4)(2-) resulted in exhaustion of internal SO(4)(2-) pools, the effect being seen first in roots, then in young leaves and, after a marked delay, in mature leaves. In young leaves, there were dramatic increases in the [NO(3)(-)] and the content of arginine in the first 2 d of S-deprivation. The concentration of glutamine, the most abundant amino acid in S-sufficient conditions, also more than doubled in S-deficient young leaves. The changes in arginine levels were also found in older leaves, but the change in glutamine level was not seen. Assays of nitrate reductase activity (NRA) and nitrate reductase (NR) mRNA from young leaves of S-replete and S-deprived plants revealed a divergence in activity and content only late in the experiments (between days 4 and 8) when results were expressed on a unit leaf basis. However, there were also time-dependent changes in the protein content that kept the specific activities (NRA:protein and RNA:protein) more or less unchanged. The results imply that the impact of S-deficiency on N-utilization are more sensitively monitored by simple measurements of the chemical composition of young leaves than by measurements of NRA or NR transcript abundance. They also suggest that protein synthesis in young leaves is strongly dependent on a continuous supply of SO(4)(2-) from outside the plant.

Sulphate 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 SO 4 (2-) from cultures of young barley plants, the net uptake of (15)N-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 [(3)H]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 (13)NO 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.