Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation |
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Authors: | RYLE, G. J. A. POWELL, C. E. GORDON, A. J. |
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Affiliation: | The Grassland Research Institute Hurley, Maidenhead, Berkshire SL6 5LR, UK |
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Abstract: | Single plants of white clover, grown in a controlled environmentand dependent for nitrogen on fixation in their root nodules,were defoliated once by removing approximately half their shoottissue. Their regrowth was compared with the growth of comparableundefoliated plants. Two similar experiments were carried out:in the first, plants were defoliated at 2.5 g, and in the secondat 1.2 g total plant d. wt. Defoliation reduced rate of N2 fixation by > 70 per cent,rate of photosynthesis by 8396 per cent, and rate ofplant respiration by 3040 per cent. Nodule weights initiallydeclined following defoliation as a result of loss of carbohydratesand other unidentified components. No immediate shedding ofnodules was observed but nodules on the most severely defoliatedplants exhibited accelerated senescence. The original rates of N2 fixation were re-attained after 56or 9 d regrowth, with increase in plant size at defoliation.In general, the rate of recovery of N2 fixation was relatedto the re-establishment and increase of the plant's photosyntheticcapacity. Throughout the growth of both defoliated and undefoliatedplants nodule respiration (metabolism) accounted for at least23 ± 2 per cent of gross photosynthesis. The unit costof fixing N2 in root nodules, in terms of photosynthate, appearedto be unaffected by defoliation, except perhaps for plants veryrecently defoliated. Similarly, the percentage nitrogen contentsof shoot, root and nodules of defoliated plants became adaptedwithin a few days to those characteristic of undefoliated plants. Trifolium repens, white clover, N2 fixation, defoliation, photosynthesis, respiration |
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