Annals of Botany 77: 615-621, 1996
© 1996 Annals of Botany Company
Low Root Temperature Retardation of the Mineral Nitrogen Induced Decline in N2 Fixation by a Northern Ecotype of White Clover
* Department of Plant Physiology and Microbiology, IBG, University of Troniso N-9037, Tromso, Norway
Institute of Grassland and Environmental Research Plas Gogerddan, Aberystwyth, SY23 3EB, UK
* For correspondence.
Received: 29 July 1995 Accepted: 2 December 1995
Nodulated white clover plants (Trifolium repens L.) of a Norwegian ecotype from Pasvik (70° N) were grown in flowing solution culture. Root temperature was 17 °C until 51 d after sowing, when it was lowered decrementally over 5 d to 7 °C in four of the eight plant culture units. After a further 24 h, mineral N was supplied automatically at 20 µM NH4NO3 in three culture units at each root temperature (7 and 17 °C) over 17 d. The remaining two units provided control plants solely dependent on N2 fixation at 7 and 17 °C.
The supply of NH4NO3 greatly reduced the nodule biomass per plant at 17 °C over 17 d compared with control plants, but had little effect at 7 °C. The nodule decline at 17 °C accompanied an acute and progressive decrease in specific rate of N2 fixation, from 9 mmol N d–1 g–1 nodule d.wt on day 0 to zero by day 10. Whilst initial rates of N2 fixation were lower at 7 °C, the mineral N-induced decrease in fixation rates was also less severe than at 17 °C and specific fixation rates recovered after reaching a minimum on day 11. N2 fixation accounted for 36% of the total uptake of N by +min.N plants during the treatment period at 7 °C as opposed to only 13% at 17 °C. The total N2 fixed at 7 °C was 86% of that fixed at 17 °C, although the specific growth rate (d.wt) at 7 °C was only 55 % of that at 17 °C. Addition of NH4NO3 at 7 °C had little effect on the gross amount of N2 fixed subsequently. In contrast, total N2 fixation by +min.N plants at 17 °C was only 24% of that fixed by the corresponding controls. The possible mechanisms by which mineral N affects N2 fixation are discussed.
White clover, Trifolium repens L., nitrogen uptake, nitrogen fixation, ammonium, nitrate, root temperature