Diurnal Variation in the Functioning of Cowpea Nodules

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Abstract
Nitrogenase (EC 1.7.99.2) activity of nodules of cowpea (V. unguiculata [L.] Walp), maintained under conditions of a 12-h day at 30.degree. C and 800 1000 microeinsteins/s m2 s-1 (photosynthetically active radiation) and a 12-h night at 20.degree. C, showed a marked diurnal variation with the total electron flux through the enzyme at night being 60% of that in the photoperiod. This diurnal pattern was, however, due to changes in hydrogen evolution. The rate of N fixation, measured by short-term 15N2 assimilation or estimated from the difference in hydrogen evolution in air or Ar:O2 (80:20; vol/vol), showed no diurnal variation. CO2 released from nodules showed a diurnal variation synchronized with that of nitrogenase functioning and, as a consequence, the apparent respiratory cost of N fixation in the photoperiod was almost double that at night (9.74 .+-. 0.38 vs. 5.70 .+-. 0.90 mol CO2 evolved per mole N2 fixed). Separate C and N balances constructed for nodules during the photoperiod and dark period showed that, at night, nodule functioning required up to 40% less carbohydrate to achieve the same level of N fixation as during the photoperiod (2.4 vs. 1.4 mol hexose/mol N2 fixed). Stored reserves of nonstructural carbohydrate of the nodule only partly satisfied the requirement for C at night, and fixation was dependent on continued import of translocated assimilates at all times. Measurements of the soluble N pools of the nodule together with 15N studies indicated that during the day and night nitrogenous products of fixation were effectively translocated to all organs of the host plant despite low rates of transpiration at night. Reduced fluxes of water through the plant at night were apparently counteracted by increased concentration of N, especially as ureides, in the xylem stream.