Genotypic variation in nodule conductance to the oxygen diffusion in common bean (Phaseolus vulgaris)

Nodule conductance to O-2 diffusion in Phaseolus vulgaris was calculated fr om the in situ measurement of the response of nodulated-root O-2 uptake to variations of rhizospheric partial pressure of O-2 at the flowering stage f or nodulated-plants grown hydroaeroponically in a glasshouse. Comparing com mon bean lines BAT477, CocoT, DOR364, Flamingo and NAG310, the nodule O-2 u ptake at ambient pO(2) was the highest (372 mu mol O-2.h(-1).pl(-1)) for BA T477 in symbiosis with the reference strain Rhizobium tropici CIAT899. The maximal value (2800 mu mol O-2.h(-1).pl(-1)) was observed at 50 kPa O-2. Th is was associated with the lowest nodule conductance (4.5 mum.s(-1)), thoug h the latter was higher with the native strain Sinorhizobium fredii 1a6. Th e highest nodule conductance (7.5 mum.s(-1)) was observed for the less effi cient symbiosis CocoT-CIAT899. It is argued that nodule conductance is an a daptation mechanism to respond to plant N demand in compensation for low no dulation or the high respiratory cost for nitrogen fixation in some symbios es.