Journal of Experimental Botany, Vol. 52, No. 354, pp. 113-121,
January 2001
© 2001 Oxford University Press
Original Papers |
Rapid disruption of nitrogen metabolism and nitrate transport in spinach plants deprived of sulphate
IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK
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 
and the uptake of 
. The former declined by approximately 50%, the effect being most significant at higher []ext. while the latter increased 6-fold over a 4 d period. Growth in the absence of external resulted in exhaustion of internal 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 [] 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 from outside the plant.
Key words: Spinach, nitrate, nitrate reductase, sulphate.
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