VARIATIONS OF WHEAT LEAF C-ISOTOPE AND N- ISOTOPE COMPOSITIONS AFTER CROP FERTILIZATION

Wheat fertilization given at stage 1 cm-ear was studied at 80 and 160 kg N ha(-1) levels in comparison with a control without fertilizer. As commercial fertilizers were depleted in N-15 compared to the natural N supply of this soil, we proposed to follow the fertilizer input unti l flowering by the variations in natural delta(15)N of leaves at the t op of the canopy. N from controls grown on unfertilized plots was enri ched in N-15 whereas that from fertilized plants decreased as the supp lied fertilized increased. The variations of leaf in vitro NR activity confirmed the arrival of the fertilizer. Other criteria varied with t he level of fertilization and exhibited varietal differences. These co ncerned %N and soluble proteins for N metabolism and %C, delta(13)C an d osmolarity of soluble extracts for water use efficiency and water co ntent. Experiments with later additions of fertilizers on 80 kg N hd(- 1) assays confirmed the preceding observations. Thus, soil N use by cr op plants can be followed by the variations of criteria determined on the canopy of a soil.