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 timecourse of events following this treatment was monitored, The first responses were found in the uptake and translocation of NO3- and the uptake of SO42-. The former declined by approximately 50%, the effect being most significant at higher [NO3-]ext. while the latter increased 6-fold over a 4 d period. Growth in the absence of external SO42- resulted in exhaustion of internal SO42- 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 [NO3-] 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 SO42- from outside the plant.