Raven John A, Franco Avilio Antonio, de Jesus Eli Lino, Jacob-Neto Jorge
Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, UK.
EMBRAPA, CNPBS, Seropedica, 23851 -Rio de Janeiro, Brazil.
New Phytol. 1990 Mar;114(3):369-389. doi: 10.1111/j.1469-8137.1990.tb00405.x.
An analysis of published data suggests that the N -fixing symbiotic vascular plants extrude more H per unit N fixed than would be expected from data on the same genotypes growing on NH if the plants had the same chemical composition when grown on the two N sources. The H /N ratio with urea as the N source is similar to that with N . The higher H /N ratio and higher organic acid/N ratio with N or urea as N source implies higher whole-plant energy and water costs per unit of biomass and, ultimately, inclusive fitness, produced. The rhizosphere acidification resulting from H extrusion may serve to change rhizosphere pH to some 'optimal' value, and to increase the availability of such limiting resources as P, Mo and Fe which are especially needed in diazotrophy. Data in the literature are consistent with these possibilities in the few cases examined. Within the plant, data on xylem and phloem sap composition in conjunction with shoot composition, of diazotrophically-growing legumes suggest that shoot acid-base homoiostasis can be maintained via the import of appropriate solutes in the xylem and the export of appropriate solutes in the phloem. Acid-base regulation of the nodules in the absence of any H exchange with their environment can also probably be explained in terms of the solutes supplied in the phloem and exported in the xylem. This conclusion is based on data in the literature on the composition of stem phloem sap and of xylem sap exuding from detached nodules of diazotrophic vascular plants. These considerations do not exclude the possibility of net H efflux from nodules fixing N in contact with an aqueous medium. The limited data available are consistent with extrusion of some of the H generated in nodules as an alternative to their neutralization by metabolism of organic anions entering in the phloem. Such H extrusion by nodules could aid in their acquisition of Fe from the medium, albeit not always at a phase in the life or the nodule when there is a net requirement for Fe.
对已发表数据的分析表明，与以铵态氮（NH）为氮源生长的相同基因型植物相比，如果这些植物在两种氮源上生长时具有相同的化学成分，那么固氮共生维管植物每固定单位氮所排出的质子（H）更多。以尿素为氮源时的H/N比与以铵态氮为氮源时相似。以铵态氮或尿素为氮源时较高的H/N比和较高的有机酸/N比意味着每单位生物量的全株能量和水分成本更高，最终意味着所产生的综合适合度更高。质子排出导致的根际酸化可能有助于将根际pH值改变到某个“最佳”值，并增加诸如磷、钼和铁等在固氮过程中特别需要的限制资源的有效性。文献中的数据在少数已研究的案例中与这些可能性相符。在植物体内，关于固氮生长的豆科植物木质部和韧皮部汁液成分以及地上部分成分的数据表明，地上部分的酸碱稳态可以通过木质部中适当溶质的输入和韧皮部中适当溶质的输出得以维持。在不与环境进行任何质子交换的情况下，根瘤的酸碱调节也可能可以用韧皮部供应并在木质部输出的溶质来解释。这一结论是基于文献中关于固氮维管植物茎韧皮部汁液成分以及从离体根瘤中渗出的木质部汁液成分的数据得出的。这些考虑并不排除与水介质接触的固氮根瘤有净质子外流的可能性。现有的有限数据与根瘤排出部分产生的质子以替代通过韧皮部进入的有机阴离子代谢来中和质子的情况相符。根瘤的这种质子排出可能有助于其从介质中获取铁，尽管并非总是在根瘤生命中的某个对铁有净需求的阶段。
