The effect of reduction in the number of nodules on nodule activity of faba bean (Vicia faba cv. Fiord)
1990
Herdina. | Silsbury, J.H.
Faba bean (Vicia faba L. cv. Fiord) plants were raised in a growth room for 5 weeks and then transplanted to a hydroponic system. After 48 h for acclimation, nine plants were removed (day 0) for the measurement of nitrogenase activity by acetylene reduction (AR), for determination of nodule number and nodule weight, volume of the active N2 fixing region (VAR), and volume of the senescent N2, fixing region (VSR). Half the nodule population on a further 18 plants was excised, and nine of these plants were assayed for AR. The nine plants from which the nodules had been removed (treated plants) and nine control plants with a full complement of nodules, were left to grow for 5 d, after which they were all harvested and assayed. The average weight of nodules and VAR remained constant in the control plants between day 0 and day 5, whereas the nodules left on the treated plants increased in weight by 1.2 times and VAR by 2.2 times. By day 5, VAR per plant was the same in the control plants as in the treated, whereas VSR of control and treated plants increased by 4.6 and 2.2 times, respectively. Removal of half of the nodules at day 0 halved the AR activity per plant, but specific activity remained the same. After 5 d, however, the nodules of the treated plants showed the same total activity as those of the control. Thus the specific activity of the nodules left on the treated plants doubled after 5 d in response to excision. The indeterminate nodule of faba bean appears to be able to increase its specific activity substantially in response to increase in the demand for fixed N. N2 fixation per nodule was responsive to substrate supply, in that halving the number of nodules on a plant induced the remaining nodules to increase activity, presumably because they could use the assimilate previously distributed over a larger number of nodules.
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