Growth and carbon economy of nodulated white clover in the presence and absence of combined nitrogen

An investigation of the carbon economy of single S184 white clover plants nodulated with an effective strain of Rhizobium trifolii growing on N‐free nutrient solution and supplied with 150 parts/10 6 N as NH 4 NO 3 has shown that 10% more of the C fixed per day is available for growth in the plants supplied with combined N. The difference between the two groups of plants is a result of higher respiratory activity in the roots of plants growing exclusively on N 2 . In terms of shoot growth, however, the difference in growth rate is likely to be less than 10%, because the rate of root growth is greater in the plants supplied with a moderate level of combined N. There is no evidence that NO ‐ 3 and CO 2 compete for photosynthetically produced reductant in the leaves of plants grown on N 2 +NH 4 NO 3 , since no reduction in net photosynthesis was observed in plants assimilating combined N. An experiment carried out on detached leaflets of white clover grown on N 2 and on N 2 + NH 4 NO 3 has shown that NO ‐ 3 ‐reducing activity is present in the leaflets of plants grown on N 2 + NH 4 NO 3 but not in plants grown on N 2 and that the activity is light‐dependent. Measurements of 14 CO 2 respired in the light and dark by leaflets previously allowed to photosynthesize 14 CO 2 showed that the ratio of 14 CO 2 output in the light to 14 CO 2 output in the dark was no higher in plants grown on N 2 + NH 4 NO 3 than in plants grown on N 2 . This observation is consistent with the finding that N source does not affect net photosynthesis in whole plants.