Nitrogen Fixation (C2H2 Reduction) by Broad Bean (Vicia faba L.) Nodules and Bacteroids under Water-Restricted Conditions

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Abstract
Water potentials of leaves and nodules [Rhizobium leguminosarum] of broad bean (Vicia faba L.) cultivated on a sandy mixture were linearly and highly (r2 = 0.99) correlated throughout a water deprivation of plants. A decrease of 0.02 megapascal of the nodule water potential (.PSI.nod) induced an immediate 25% inhibition of the highest level of acetylene reduction of broad bean nodules attached to roots. This activity continued to be depressed when water stress increased, but the effect was less pronounced. Partial recovery of optimal C2H2 reduction capacity of mildly water stressed nodules (.PSI.nod = -1.2 megapascals) was possible by increasing the external O2 partial pressure up to 60 kilopascals. The dense packing of the cortical cells of nodules may be responsible for the limitation of O2 diffusion to the central tissue. Bacteroids isolated from broad bean nodules exhibited higher N2 fixation activity with glucose than with succinate as an energy-yielding substrate. Bacteroids from stressed nodules appeared more sensitive to O2, and their optimal activity declined with increasing nodule water deprivation. This effect could be partly due to decreased bacteroid respiration capacity with water stress. Water stress was also responsible for a decrease of the cytosolic protein content of the nodule and more specifically of leghemoglobin. The alteration of the bacteroid environment appears to contribute to the decline in N2 fixation under water restricted conditions.