Investigation on certain aspects of fixation of nitrogen by legumes and non-legumes

Ferguson, Thomas P (1953) Investigation on certain aspects of fixation of nitrogen by legumes and non-legumes. PhD thesis, University of Glasgow.

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Section I: "The Effect of Oxygen Supply on the Functioning of the Root Nodules of Red Clover", In this part of the work the effect of oxygen supply on the growth and nitrogen accumulation of nodulated red clover plants in nitrogen-free rooting medium and of non-nodulated plants supplied with combined nitrogen has been investigated by two different techniques. In the first technique the root .systems only were exposed to adjusted oxygen supply. The plants were grown in water culture, and oxygen /nitrogen gas mixtures containing 21, 12, 5 and 1 per cent oxygen respectively were bubbled through the culture solution of different series of plants. Protection against ingress of atmospheric oxygen was provided. In the second technique the clover plants were grown in sand culture with the plants wholly enclosed within 5-litre aspirator bottles. The gas space in these bottles was kept filled with the gas mixtures listed in the preceding paragraph, except that 1 per cent carbon dioxide was now added to provide for photosynthesis. The reactions of the plants to variation in oxygen supply were in general similar in both types of experiments. With nodulated plants growth at 12 percent oxygen was significantly superior to that at 21 per cent. Reduction to 5 per cent led to a curtailment of growth, the same being true when oxygen was further lowered to 1 per cent. With the non-nodulated plants growth at 21 and 12 per cent oxygen was essentially similar, but was reduced at 5 per cent and again at 1 per cent. The extent of the response to different oxygen levels was, however, much less marked in the whole-plant enclosure experiment and this is considered to be due to the limitations necessarily imposed on growth by the experimental arrangement. By graphical means it is deduced from these experiments that the optimal oxygen level for the growth of nodulated clover plants is approximately 10 per cent, and that for non-nodulated plants approximately 15 per cent. It is assumed that the differences in response of the two types of plants to variation in oxygen supply is due to effects exerted in the nodules, so that clover nodules appear to function most effectively at a reduced oxygen level and that in fact nitrogen fixation is favoured by the prevalence of some degree of anaerobiosis in the nodule. In the root-system enclosure experiments the number of nodules formed per plant was essentially similar at 21, 12 and 5 per cent oxygen, but was much reduced at 1 per cent. This last I'esult could have been wholly due to the poor root growth at 1 per cent oxygen. It is concluded that the process of nodule initiation has a greater tolerance towards oxygen supply than the nodule has in its later stage as a nitrogen-fixing organ. Section II: Observations on the Formation and Significance of the Root Nodules of Alnus glutinosa (L,) Gaertn. Here the conclusion reached by previous investigators that nodulated alder plants can fix atmosi5heric nitrogen and display vigorous growth in water culture free of combined nitrogen is confirmed. Also evidence is advanced to show that the fixation of nitrogen actually takes place within the nodulated plant and probably within the nodule. There is no reason to doubt that fixation is associated with nodulated alder trees in the field and this fixation may be of considerable ecological and general importance. It has been shown that in water culture nodule formation occurred over the pH range 4,2 to 7.0 but most freely between 5,4 and 7,0, No nodules developed at pH 3,3, At favourable pH nodules visible to the naked eye developed as soon as 10 days after inoculation. The optimum pH for the growth of nodulated plants in solution free of combined nitrogen lies between pH 4.2 and 5.4. By growing non-nodulated alder plants supplied with combined, nitrogen in water culture it has been shown that the failure of the plants to develop nodules at pH 3.3 was due to the fact that although the host plant can tolerate much a low pH level the nodule organism cannot. It has also been shown that non-nodulated plants supplied with ammonium sulphate made more vigorous growth than those supplied with sodium nitrate. In experiments with both nodulated plants and plants supplied with combined nitrogen in which the culture solution in certain cases was aerated it was found that high oxygen supply had a beneficial effect on the growth of the former but had no effect on the latter. It is concluded from this finding that nodules have a higher oxygen requirement than roots.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: G Bond
Keywords: Plant sciences
Date of Award: 1953
Depositing User: Enlighten Team
Unique ID: glathesis:1953-73821
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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