Effect of soil type on nitrate uptake by wheat shoots characterized using N-labelled NH NO -fertilizer and leaf nitrate reductase activity

Wheat plants grown in a greenhouse in containers filled with chalky (CP) or loamy (LP) soil were fertilized with NH 4 15 NO 3 or 15 NH 4 NO 3 (5 atom% 15 N), 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 NO 3 flux into the shoot was higher for CP than for LP. The ratio of the ‘proportion of fertilizer in the flag leaf NO 3 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 NO 3 from fertilizer was predominantly re-used to feed other parts of the plants. In LP, NO 3 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. Keywords: Triticum aestivum L, chalky soil, loamy soil, ammonium nitrate, 15 N, nitrate reductase activity. Australian Journal of Plant Physiology 25(4) 465 - 474 Full text doi:10.1071/PP97043 © CSIRO 1998