Dakora F D, Atkins C A
Botany Department, University of Western Australia, Nedlands WA 6009, Australia.
Plant Physiol. 1991 Jul;96(3):728-36. doi: 10.1104/pp.96.3.728.
Nodulated soybean (Glycine max L. Merr. cv White Eye inoculated with Bradyrhizobium japonicum strain CB 1809) plants were cultured in the absence of combined N from 8 to 28 days with their root systems maintained continuously in 1, 2.5, 5, 10, 20, 40, 60, or 80% O(2) (volume/volume) in N(2). Plant dry matter yield was unaffected by partial pressure of oxygen (pO(2)) and N(2) fixation showed a broad plateau of maximum activity from 2.5 to 40 or 60% O(2). Slight inhibition of nitrogenase activity occurred at 1% O(2) and as much as 50% inhibition occurred at 80% O(2). Low pO(2) (less than 10%) decreased nodule mass on plants, but this was compensated for by those nodules having higher specific nitrogenase activities. Synthesis and export of ureides in xylem was maintained at a high level (70-95% of total soluble N in exudate) over the range of pO(2) used. Measurements of nitrogenase (EC 1.7.99.2) activity by acetylene reduction indicated that adaptation of nodules to low pO(2) was largely due to changes in ventilation characteristics and involved increased permeability to gases in those grown in subambient pO(2) and decreased permeability in those from plants cultured with their roots in pO(2) greater than ambient. A range of structural alterations in nodules resulting from low pO(2) were identified. These included increased frequency of lenticels, decreased nodule size, increased volume of cortex relative to the infected central tissue of the nodule, as well as changes in the size and frequency of extracellular voids in all tissues. In nodules grown in air, the inner cortex differentiated a layer of four or five cells which formed a band, 40 to 50 micrometers thick, lacking extracellular voids. This was reduced in nodules grown in low pO(2) comprising one or two cell layers and being 10 to 20 micrometers thick in those from 1% O(2). Long-term adaptation to different external pO(2) involved changes which modify diffusive resistance and are additional to adjustments in the variable diffusion barrier.
将接种了日本慢生根瘤菌CB 1809菌株的根瘤大豆（大豆品种白眼睛，Glycine max L. Merr. cv White Eye）植株在无化合态氮的条件下培养8至28天，其根系持续保持在氮气中1、2.5、5、10、20、40、60或80%的氧气（体积/体积）环境中。植株干物质产量不受氧分压（pO₂）影响，固氮作用在2.5%至40%或60%的氧气环境中呈现出广泛的最大活性平稳期。在1%的氧气环境中，固氮酶活性略有抑制，而在80%的氧气环境中，抑制率高达50%。低pO₂（低于10%）会使植株上的根瘤质量下降，但根瘤较高的比固氮酶活性可对此进行补偿。在所使用的pO₂范围内，木质部中脲类的合成与输出维持在较高水平（占渗出液中总可溶性氮的70 - 95%）。通过乙炔还原法测定固氮酶（EC 1.7.99.2）活性表明，根瘤对低pO₂的适应主要归因于通气特性的变化，包括在低于环境pO₂中生长的根瘤气体渗透性增加，以及在根系培养于高于环境pO₂中的植株根瘤气体渗透性降低。已确定低pO₂导致根瘤出现一系列结构改变。这些改变包括皮孔频率增加、根瘤大小减小、皮层相对于根瘤受感染中央组织的体积增加，以及所有组织中细胞外空隙大小和频率的变化。在空气中生长的根瘤，内皮层分化出一层由四到五个细胞组成的带，厚度为40至50微米，没有细胞外空隙。在低pO₂中生长的根瘤中，这一层减少为一到两个细胞层，在1%氧气环境中的根瘤中厚度为10至20微米。对不同外部pO₂的长期适应涉及改变扩散阻力的变化，这是对可变扩散屏障调整之外的额外变化。
