The Effect of Reduction in the Number of Nodules on Nodule Activity of Faba Bean (Vicia faba cv. Fiord)

Faba bean (Vicia faba L. cv. Fiord) plants were raised in agrowth room for 5 weeks and then transplanted to a hydroponicsystem. After 48 h for acclimation, nine plants were removed(day 0) for the measurement of nitrogenase activity by acetylenereduction (AR), for determination of nodule number and noduleweight, volume of the active N₂ fixing region (VAR), and volumeof the senescent N₂, fixing region (VSR). Half the nodule populationon a further 18 plants was excised, and nine of these plantswere assayed for AR. The nine plants from which the noduleshad been removed (treated plants) and nine control plants witha full complement of nodules, were left to grow for 5 d, afterwhich they were all harvested and assayed. The average weight of nodules and VAR remained constant in thecontrol plants between day 0 and day 5, whereas the nodulesleft on the treated plants increased in weight by 1.2 timesand VAR by 2.2 times. By day 5, VAR per plant was the same inthe control plants as in the treated, whereas VSR of controland treated plants increased by 4.6 and 2.2 times, respectively.Removal of half of the nodules at day 0 halved the AR activityper plant, but specific activity remained the same. After 5d, however, the nodules of the treated plants showed the sametotal activity as those of the control. Thus the specific activityof the nodules left on the treated plants doubled after 5 din response to excision. The indeterminate nodule of faba bean appears to be able toincrease its specific activity substantially in response toincrease in the demand for fixed N. N₂ fixation per nodule wasresponsive to substrate supply, in that halving the number ofnodules on a plant induced the remaining nodules to increaseactivity, presumably because they could use the assimilate previouslydistributed over a larger number of nodules. Victa faba, faba bean, nodule number, nodule activity, acetylene reduction, volumes of active and senescent N₂ fixing regions of nodules, hydroponic system     

Nodulation and Nitrogen Fixation of Faba Bean (Vicia faba L.) as Affected by Removal of the Cotyledons and Nitrate Supply

Cotyledons of faba bean (^{Vicia faba} L. cv. Fiord) were removedto determine whether an apparent delay in nodulation of thiscultivar could be attributed to an inhibitor from these organs.Cotyledons were left intact or excised from seedling plants14 and 18 d after sowing and plants grown with or without 2·5mm NO₃^–. Seedling growth was depressed when cotyledons were removed onday 14 but not when removed on day 18. Removal of the cotyledonsat day 14 reduced nodule number and nodule weight in the absenceof NO₃^–, but in the presence of NO₂^–, nodule numberwas unaffected and only nodule weight was reduced. Cotyledonremoval at day 18 increased both nodule number and nodule weightwith +NO₃^– but not with –NO₂^–. Acetylene reduction(AR) was markedly depressed by NO₃^–. Cotyledon removalat day 14 decreased AR but removal at day 18 resulted in anincrease in AR. We suggest from these results that faba beancotyledons have an inhibitory effect on nodule activity andon nodulation and this interacts with NO₃^–. This can beexplained through a ‘feed-back’ regulation of N₂fixation by soluble N in the seedling. Vicia faba, faba bean, nodule number, nodulation, nodule activity, acetylene reduction, N₂ fixation, cotyledon removal, nitrate