The Effects of Age, Darkness and Nitrate on Poly‐β‐hydroxybutyrate Levels and Nitrogen‐Fixing Ability of Rhizobium in Lupinus angustifolius

The effects of age, darkness and nitrate on nitrogenase activity, poly-β-hydroxybutyrate (PHB) and β-hydroxybutyrate dehydrogenase (BOHB DH) activity in Rhizobium lupini while in symbiosis with Lupinus angustifolius were assessed. It was concluded that PHB functions as a reserve material in root nodules and is involved in the maintenance of nitrogen fixation when the availability of photosynthates is restricted. PHB accumulation and the development of BOHB-DH activity did not take place until the peak of nitrogenase activity had passed. All three parameters reached a steady level when the host plant was approximately 24 days old. Deprivation of photosynthates due to continuous darkness resulted in major losses of PHB and BOHB-DH activity until N2-fixation ceased. Nitrate addition without restriction of photosynthates resulted in diminished nitrogenase activity. Apart from a slight delay in accumulation. PHB levels were not affected. It was found important to standardize sampling since N2-fixation was affected by plant culture and diurnal cycles.