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 NO^sub 3^^sup -^ uptake (line LNU) were compared with a line regarded as normal with respect to NO^sub 3^^sup -^ uptake (line NNU). Growth, nodulation, N^sub 2^ fixation and NO^sub 3^^sup -^ uptake were measured over 7 weeks in flowing solution culture (Experiment 1) by plants dependent for N acquisition on either (i) NO^sub 3^^sup -^ uptake, (ii) NO^sub 3^^sup -^ uptake +N^sub 2^ fixation, or (iii) N^sub 2^ fixation only. Effects of plant N status on the short-term uptake and translocation of ^sup 15^NH^sub 4^^sup +^ and ^sup 15^NO^sub 3^^sup -^ were also investigated (Experiment 2). Nitrate uptake per plant by -fix/+NO^sub 3^^sup -^ line LNU was 50% of uptake by line NNU over 35 days, and there were significant differences in specific uptake rates of NO^sub 3^^sup -^ between the lines over the first 24 days. The `low NO^sub 3^^sup -^ uptake' phenotype was indistinct under +fix/+NO^sub 3^^sup -^ 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 NH^sub 4^^sup +^ and NO^sub 3^^sup -^ uptake than line NNU. It is concluded that the `low NO^sub 3^^sup -^ uptake' phenotype of line LNU is inconsistently expressed. Circumstantial evidence points to increased NO^sub 3^^sup -^ efflux and decreased xylem translocation of NO^sub 3^^sup -^ as possible explanations for the lower NO^sub 3^^sup -^ uptake by line LNU.[PUBLICATION ABSTRACT]