Effect of two contrasted N fertilisations on rapeseed growth and nitrate metabolism

Winter oilseed rape was grown under two nitrogen fertilisation conditions. The N1-plants and NS-plants were respectively supplied with 4.5 g N per pla nt (N-limiting condition) and 22.5 g N per plant (non-N-limiting condition) . Growth parameters and nitrate I educing capacity were monitored at five s ampling stages interspersed with ammonium nitrate applications. NS-plants s howed a higher growth rate producing more leaves and stems, early flower an d silique formation and delayed leaf senescence. They also contained more n itrate and a higher nitrate reductase activity (NRA) especially in leaves w hich represented the main site of nitrate reduction before flowering. Howev er, stems and siliques contributed to NRA especially in nitrogen-limited pl ants that lost their leaves early. This present study outlines the importan ce of siliques as individual sinks reducing nitrate essentially in the pod walls. The soluble protein content decreased in senescing leaves which was indicative of the reallocation of proteinic nitrogen towards stems and sili ques. In non-limiting conditions, other nitrogen compounds of leaves may ac count for such a reallocation. Hence, the timing of leaf fall could contrib ute to the low nitrogen recovery in rapeseed. (C) 2000 Editions scientifiqu es et medicales Elsevier SAS.