Phosphoenolpynivate carboxylase, malate and alcohol dehydrogenase activities in alfalfa (Medicago sativa) nodules under water stress

The effect of drought upon phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), malate ddiydrogenase (MDH; EC 1.1.1.37), alcohol dehydrogenase (ADH; EC 1.1.1.1) and β -hydroxybulyrate dehydrogenase ( β -OH-BDH; EC 1.1.1.30) enzyme activities as well as the leghemoglobin (Lb), malate and ethanol contents of alfalfa nodules ( Medicago sativa L. cv. Aragon) were examined. Both the ieghemoglobin (Lb) content and the Lb/soluble protein ratio were significantly reduced at a nodule water potential (Ψ_nod) of—1.3 MPa. At lower Ψ_nod, Lb content decreased further, but the ratio remained unchanged. Slight stress (—1.3 MPa) drastically affected acetylene reduction activity (ARA; 60% reduction) whereas in vitro PEPC activity was main-tained at relatively constant values. As stress progressed (—2.0 MPa), a simultaneous reduction in both activities was observed. Severe stress (Ψ_nod lower than —2.0 MPa) stimulated in vitro PEPC. Bacteroid β -J-OH-BDH activity was stimulated by slight (—1.3 MPa) and moderate (—2.0 MPa) drought. MDH activity rose in slightly stressed nodules (Ψ_nod—1.3 MPa). Greater water deficits sharply decreased MDH activity to values significantly lower than those found in control nodules. Nodule malate content followed the same pattern as MDH. The plant fraction of the nodule showed constitutive ADH activity and contained ethanol. ADH was stimulated at slight (— 1.3 MPa) and moderate drought levels (—2.0 MPa). Ethanol content showed similar behavior to ADH activity. Inhibition of ARA, reduction of Lb content and stimulation of the fermentative metabolism induced by water stress suggest some reduction ira O₂ availability within the nodule.