Nitrogen fixation in sainfoin (Onobrychis viciifolia) 1. Responses to changes in nitrogen nutrition

Abstract

Development and nodulation of the forage legume sainfoin (Onobrychis viciifolia Scop.) were observed under glasshouse conditions for 50 days after sowing. Growth changes caused by different nitrogen (N) nutrition were investigated from Day 50. The quantity of nodule tissue produced compared favourably with values reported for a range oflegumes. In spite of this, the symbiotic N₂-fixing system of sainfoin appeared unable to provide sufficient N to meet the plant's requirements. The relative growth rate of the whole plant, and the relative N accumulation rate were substantially lower in plants dependent on N2 fixation compared to plants provided with 210 mg/litre nitrate N. Development of the below-ground portion of the plant was rapid relative to that of the top. Relative growth rates, top: root + nodule ratio, and leaf area ratio of sainfoin were lower than values reported for other legumes. The mode of N nutrition affected dry matter distribution, with proportionately much greater below-ground development in plants dependent on symbiotic N₂ fixation. This resulted in lower leaf area ratios and hence probably reduced the availability of energy for symbiotic N₂ fixation. The application of 35 mg/litre nitrate N to plants previously solely dependent on symbiotic N₂ fixation appeared to substitute for, rather than supplement, N₂ fixation. Applying 35 mg/litre nitrate N to plants previously given 210 mg/litre nitrate N resulted in nodulation, the initiation of symbiotic N₂ fixation, and a substantial change in dry matter distribution in favour of the root system.

Keywords:

• sainfoin
• Onobrychis viciifolia
• nitrogen fixation
• nodulation
• relative growth rate
• leaf area
• roots