Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation

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Abstract
Single plants of white clover, grown in a controlled environment and dependent for nitrogen on fixation in their root nodules, were defoliated once by removing approximately half their shoot tissue. Their regrowth was compared with the growth of comparable undefoliated plants. Two similar experiments were carried out: in the first, plants were defoliated at 2.5 g, and in the second at 1.2 g total plant d. wt. Defoliation reduced rate of N2 fixation by > 70 per cent, rate of photosynthesis by 83–96 per cent, and rate of plant respiration by 30–40 per cent. Nodule weights initially declined following defoliation as a result of loss of carbohydrates and other unidentified components. No immediate shedding of nodules was observed but nodules on the most severely defoliated plants exhibited accelerated senescence. The original rates of N2 fixation were re-attained after 5–6 or 9 d regrowth, with increase in plant size at defoliation. In general, the rate of recovery of N2 fixation was related to the re-establishment and increase of the plant's photosynthetic capacity. Throughout the growth of both defoliated and undefoliated plants nodule respiration (metabolism) accounted for at least 23 ± 2 per cent of gross photosynthesis. The unit ‘cost’ of fixing N2 in root nodules, in terms of photosynthate, appeared to be unaffected by defoliation, except perhaps for plants very recently defoliated. Similarly, the percentage nitrogen contents of shoot, root and nodules of defoliated plants became adapted within a few days to those characteristic of undefoliated plants.

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