Université de Caen, UMR 950, Ecophysiologie Végétale Agronomie et Nutritions NCS, INRA/Université de Caen, Esplanade de la Paix, F-14032 Caen Cedex, France.
J Exp Bot. 2010;61(1):225-34. doi: 10.1093/jxb/erp303.
The role of S in legume growth, N uptake, and N2 fixation was investigated using white clover (Trifolium repens L.) as a model species. We examined whether the effect of sulphate addition on N fixation resulted from a stimulation of host plant growth, a specific effect of S on nodulation, or a specific effect of S on nodule metabolism. Clones of white clover, inoculated with Rhizobium leguminosarum, were grown for 140 d in a hydroponic system with three levels of sulphate concentration (0 mM, 0.095 mM, and 0.380 mM). Nodule morphological and biochemical traits, such as root length, nodule biomass and volume, nodule protein contents (nitrogenase and leghaemoglobin obtained by an immunological approach), and root amino acid concentrations, were used to analyse the effect of sulphate availability on N2 fixation. The application of sulphate increased whole plant dry mass, root length, and nodule biomass, expressed on a root-length basis. N uptake proved less sensitive than N2 fixation to the effects of S-deficiency, and decreased as a consequence of the lower root length observed in S-deficient plants. N2 fixation was drastically reduced in S-deficient plants as a consequence of a low nodule development, but also due to low nitrogenase and leghaemoglobin production. This effect is likely to be due to down-regulation by a N-feedback mechanism, as, under severe S-deficiency, the high concentration of whole plant N and the accumulation of N-rich amino acids (such as asparagine) indicated that the assimilation of N exceeded the amount required for plant growth.
我们使用白三叶草（Trifolium repens L.）作为模式物种，研究了 S 元素在豆科植物生长、N 吸收和 N2 固定中的作用。我们考察了添加硫酸盐对 N2 固定的影响是源于对宿主植物生长的刺激、S 元素对结瘤的特殊作用，还是 S 元素对根瘤代谢的特殊作用。接种根瘤菌（Rhizobium leguminosarum）的白三叶草克隆体在含有三种硫酸盐浓度（0 mM、0.095 mM 和 0.380 mM）的水培系统中生长了 140 天。我们利用根长、根瘤生物量和体积、根瘤蛋白含量（通过免疫方法获得的固氮酶和豆血红蛋白）以及根氨基酸浓度等根瘤形态和生化特性来分析硫酸盐供应对 N2 固定的影响。硫酸盐的添加增加了整株植物干重、根长和根瘤生物量，而根长是基础。与 S 缺乏相关的 N 吸收比 N2 固定对 S 缺乏的敏感性更低，而且由于在 S 缺乏植物中观察到的根长较短，N 吸收也随之减少。由于根瘤发育不良以及固氮酶和豆血红蛋白产量较低，S 缺乏植物中的 N2 固定严重减少。这种效应可能是由于 N 反馈机制的下调所致，因为在严重的 S 缺乏条件下，整株植物 N 浓度的升高和富含 N 的氨基酸（如天冬酰胺）的积累表明，N 的同化量超过了植物生长所需的量。
