Nitrogen fixation in sainfoin ( Onobrychis viciifolia ) 2. Effectiveness of the nitrogen‐fixing system

The growth and development of sainfoin ( Onobrychis viciifolia Scop.) plants dependent on symbiotic nitrogen (N) fixation were compared, under glasshouse conditions, with those of sainfoin supplied with 210 mg/litre nitrate N over the period 84–143 days from sowing. The particular sainfoin‐ Rhizobium combination used in this experiment produced a lower proportion of its potential N requirement from symbiotic fixation than many other commonly grown legumes. It appeared to have more nodule tissue per unit whole‐plant dry weight, but a lower specific N 2 [C 2 H 2 ]‐fixing activity. However, the overall N 2 [C 2 H 2 ]‐fixing activity per unit plant weight compared favourably with those of other forage legumes. The molar ratio of C 2 H 2 reduced: N 2 fixed was 3.9 : 1, which is higher than both the theoretical ratio of 3 : 1 and values reported for several other legumes. Diurnal variation in N 2 ‐fixing activity (N 2 [C 2 H 2 ]‐fixing activity was measured in the middle of the photoperiod) and/or wastage of energy by the N 2 ‐fixing enzyme may have occurred. In plants dependent on symbiotic N 2 fixation, leaf area appeared to be linked to total dry weight indirectly via total plant N. Leaf area was highly correlated with total plant Nand there was evidence that this link was via energy supply to the symbiotic N 2 ‐fixing system. Thus leaf area may have been limiting N 2 fixation and hence total plant N.