An investigation into the efficiency of nitrogen fixation in Sainfoin (Onobrychis viciifolia scop.) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Science at Massey University.

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Massey University
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Earlier reports have indicated that growth of the forage legume sainfoin (Onobrychis viciifolia Scop.) is limited by its capacity to fix adequate quantities of N2 . Symbiotic N2 fixation and development of sainfoin up to the flowering stage was studied under glasshouse conditions. Growth and development of plants that were dependent solely on fixed N2 for their N requirements, were compared with plants supplied with abundant combined (nitrate) N. The effect of a low rate of combined N on symbiotic N2 fixing activity and plant growth was also investigated. From an early stage, plants dependent on symbiotic N2 fixation had lower relative growth rates than plants supplied with combined N, indicating that the N2 fixing system of sainfoin was not capable of providing enough N to meet the requirements of the plant, or that N2 fixation required an energy input greater than that for the assimilation of mineral N. The mode of N nutrition was found to influence the dry matter distribution in sainfoin to a greater extent than reported for most other legumes. Plants dependent on symbiotic N2 fixation allocated a substantially greater proportion of dry matter to root and nodule growth and consequently had lower top:root + nodule ratios than plants provided with combined N. Sainfoin was found to produce abundant nodules, and had a relatively high nodule weight in relation to total plant weight, compared to other legumes. Specific nodule activity, however, was found to be relatively low, and possible reasons for this are discussed. For plants dependent on symbiotic N2 fixation, total plant N, and hence N2 fixation appeared to be the major factor limiting plant growth. Evidence was obtained which indicated that the N2 fixing system of sainfoin may be relatively inefficient. The observed ratio of C2 H2 reduced:N2 fixed, was higher than the theoretical ratio, and appeared to be high relative to other legumes, which suggested possiblewastage of energy by the N2 fixing enzyme. The addition of a low rate of combined Nhad the effect of immediately reducing N2 [C2 H2 ] fixing activity, and the combined N appeared to substitute for, rather than supplement, symbiotic N2 fixation, further indicating an inefficient symbiotic N2 fixing system. Leaf area ratio was found to be lower in sainfoin dependent on N2 fixation than reported values for other N2 fixing legumes; this suggests that sainfoin is less efficient at intercepting photosynthetically active radiation. Leaf area was highly correlated withtotal plant N, and there was evidence that this link was via energy supply to the symbiotic N2 fixing system. Thus leaf area may have been limiting N2 fixation and hence total plant. N. Overall, a mutual dependence between the ability of the root nodules to fix N2 and the ability of the leaves to supply energy was indicated. There was evidence that both of these factors may play a role in limiting the growth of sainfoin, relative to other more productive legumes, such as lucerne.
Nitrogen Fixation, Sainfoin