Characterization of novel inbred lines of white clover (Trifolium repens L.). II. Variation in N2 fixation, NO3- uptake and their interactions

Abstract

N2 fixation, nitrate uptake, and tissue N levels werecompared in four novel self-fertile inbred lines (referred to as A, B, C andD) of white clover (Trifolium repens L.) believed to differ markedly intheir nitrogen relations. Plants were inoculated with a mixture of Rhizobiumstrains and grown in flowing nutrient solutions without mineral N for 18 dprior to supplying half the plants with 20 µM\(NO_3^ - \) over a 36 d treatment period. Net uptakeof \(NO_3^ - \) was measured automatically on a dailybasis. The remaining plants constituted ‘controls’ solely dependent uponN2 fixation for acquisition of N. Total uptake of\(NO_3^ - \) over 36 days was 27.4 (A), 23.8 (B), 13.8(C) and 30.1 (D) mmol N/plant, compared with N2 fixation of7.2 (A), 16.9 (B), 26.2 (C) and 6.2 (D) mmol N/plant by the\(NO_3^ - \)-fed lines. Total N2 fixationby corresponding ‘control’ plants was 15.6 (A), 35.6 (B), 35.3 (C) and 2.2(D) mmol N/plant. Line D plants exhibited a 3:1 segregation with respect toeither ‘nil’ or ‘partially impaired’ N2 fixation; thecorresponding ratio for line A was 1:3. Line C plants had a ‘low\(NO_3^ - \) uptake’ phenotype characterised by specific\(NO_3^ - \) uptake rates approximately half those ofthe other lines, higher N2 fixation in the presence of\(NO_3^ - \), and low leaf + petiole concentrations of\(NO_3^ - \), although concentrations of\(NO_3^ - \) in stolon and root fractions were similarin all lines. These inbred lines provide a tool for (1) the analysis of thegenetic and physiological basis of variation in the efficacy with which\(NO_3^ - \) uptake down-regulates N2fixation, and (2) testing ecological hypotheses for temporal and spatialvariation in clover/grass coexistence in pastures.