Prosser I M, Purves J V, Saker L R, Clarkson D T
IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK.
J Exp Bot. 2001 Jan;52(354):113-21.
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)(-) 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.
在最初一段时间提供充足硫供应后，对水培菠菜植株停止供应外源硫酸盐。监测了此处理后一系列事件的时间进程。首先观察到的反应是硝酸根（NO₃⁻）的吸收和转运以及硫酸根（SO₄²⁻）的吸收发生了变化。前者下降了约50%，在较高的外界硝酸根浓度下这种影响最为显著，而后者在4天内增加了6倍。在没有外源硫酸根的情况下生长导致内部硫酸根库耗尽，这种影响首先在根部出现，然后在幼叶中出现，经过明显延迟后在成熟叶中出现。在幼叶中，缺硫的头两天硝酸根浓度和精氨酸含量急剧增加。在硫充足条件下最丰富的氨基酸谷氨酰胺的浓度，在缺硫的幼叶中也增加了一倍多。在老叶中也发现了精氨酸水平的变化，但未观察到谷氨酰胺水平的变化。对硫充足和缺硫植株幼叶的硝酸还原酶活性（NRA）和硝酸还原酶（NR）mRNA进行测定，结果表明只有在实验后期（第4天至第8天），以单位叶面积计算时，活性和含量才出现差异。然而，蛋白质含量也有随时间的变化，使比活性（NRA：蛋白质和RNA：蛋白质）或多或少保持不变。结果表明，通过简单测量幼叶的化学成分比测量NRA或NR转录本丰度，能更灵敏地监测缺硫对氮利用的影响。它们还表明，幼叶中的蛋白质合成强烈依赖于植物外部持续供应的硫酸根。
