Low Root Temperature Retardation of the Mineral Nitrogen Induced Decline in N2 Fixation by a Northern Ecotype of White Clover

Abstract

Abstract 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 51d after sowing, when it was lowered decrementally over 5d to 7°C in four of the eight plant culture units. After a further 24h, mineral N was supplied automatically at 20μ M NH 4 NO 3 in three culture units at each root temperature (7 and 17°C) over 17d. The remaining two units provided control plants solely dependent on N 2 fixation at 7 and 17°C. The supply of NH 4 NO 3 greatly reduced the nodule biomass per plant at 17°C over 17d 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 N 2 fixation, from 9mmolN d -1 g -1 nodule d.wt on day 0 to zero by day 10. Whilst initial rates of N 2 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. N 2 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 N 2 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 NH 4 NO 3 at 7°C had little effect on the gross amount of N 2 fixed subsequently. In contrast, total N 2 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 N 2 fixation are discussed.