P-deficiency increases the O2 uptake per N2 reduced in alfalfa

Nodulated alfalfa (Medicago sativa L. cv. Saranac) plants were grown in hydroponics at P-sufficient and P-deficient supply levels. After 5 weeks of growth, dry matter accumulation, nodulation, total IN and P accumulation, as well as N-15(2) uptake, were measured. Moreover, the response of nodule O-2-uptake to raising external pO(2) was determined in an open-flow measurement system and nodule permeability was calculated. Plants in the P-deficient supply treatment had a lower P concentration in all organs. In both treatments the highest P concentration was found in nodules. In the P-deficient supply treatment plants formed less dry matter, had a lower shoot/root ratio, less nodulation, decreased total N accumulation, and lower N-15(2) uptake per dry matter nodule. Nodules in the P-deficient treatment were, on average, smaller and had a higher O-2 uptake per N-2 reduced, coinciding with increased nodule permeability and conductance. Thus increased oxygen uptake appears to be a mechanism to adjust nodule metabolism to P deficiency in indeterminate N-2-fixing nodules such as in alfalfa, as has previously been shown for determinate nodule forms.