Low nitrate uptake white clover genotype -: Nitrogen relations of a low nitrate uptake inbred line of white clover (Trifolium repens L.)

The nitrogen relations of an inbred line of white clover (Trifolium repens L.) thought to exhibit an abnormally low capacity for NO3- uptake (line LNU) were compared with a line regarded as normal with respect to NO3- uptake (line NNU). Growth, nodulation, N-2 fixation and NO3- uptake were measured over 7 weeks in flowing solution culture (Experiment 1) by plants dependent for N acquisition on either (i) NO3- uptake, (ii) NO3- uptake +N-2 fixation, or (iii) N-2 fixation only. Effects of plant N status on the short-term uptake and translocation of (NH4+)-N-15 and (NO3-)-N-15 were also investigated (Experiment 2). Nitrate uptake per plant by -fix/+NO3- line LNU was 50% of uptake by line NNU over 35 days, and there were significant differences in specific uptake rates of NO3- between the lines over the first 24 days. The 'low NO3- uptake' phenotype was indistinct under +fix/+NO3- treatment. Nitrate lowered specific rates of nitrogen fixation by line NNU but had no effect on line LNU. Only low N status line LNU plants had lower short-term rates of NH4+ and NO3- uptake than line NNU. It is concluded that the 'low NO3- uptake' phenotype of line LNU is inconsistently expressed. Circumstantial evidence points to increased NO3- efflux and decreased xylem translocation of NO3- as possible explanations for the lower NO3- uptake by line LNU.