Effect of soil type on nitrate uptake by wheat shoots characterized using 15N-labelled NH4 NO3-fertilizer and in vitro, leaf nitrate reductase activity

Wheat plants grown in a greenhouse in containers filled with chalky (CP) or loamy (LP) soil were fertilized with NH415NO3 or 15NH4 NO3 (5 atom% 15N), initial soil nitrate levels being lower in chalky soil. Both the total amount of nitrate and the proportion derived from fertilizer were higher in leaves of plants grown on chalky soil, however, increased inorganic N was not paralleled by a higher organic N content in the CP leaves. In vitro NR activity of the youngest fully expanded leaves confirmed that NO3 flux into the shoot was higher for CP than for LP. The ratio of the ‘proportion of fertilizer in the flag leaf NO3 pool divided by the proportion of fertilizer in the soil total N pool’ reached a maximum (0.8) at the onset of the flag leaves for CP but decreased to 0.5 at the time of flowering because stored NO3 from fertilizer was predominantly re-used to feed other parts of the plants. In LP, NO3 was not remobilized and the ratio remained at 0.8. Higher in vitro NR activity in the CP flag leaf confirmed that release and re-use of stored nitrate occurred and that plants grown on chalky soils appear to have an enhanced ability to utilise nitrate.