Postanthesis nitrate assimilation in winter wheat : in situ flag leaf reduction

When adequate levels of soil NO(3) (-) are available, concurrent NO(3) (-) absorption and assimilation, and mobilization of vegetative N reserves accumulated prior to anthesis, may be used to supply N to developing wheat (Triticum aestivum L.) kernels. Vegetative wheat components (stems, leaves, spike) are known to possess NO(3) (-) reductase activity, but the in situ utilization of NO(3) (-) translocated to the shoot has not been studied. Assimilation and partitioning of (15)N was determined in winter wheat ;Doublecrop.' At 7 days after anthesis, the stem immediately above the peduncle node was heat girdled to block phloem export from the flag leaf. Control plants were not girdled. One day later, 50 micromoles of (15)NO(3) (-) (98 atom percent (15)N) was injected into the penultimate internodal lacuna, after which (15)NO(3) (-) utilization was determined sequentially over a 5 day period. Based on differences in spike accumulation of reduced (15)N excess between treatments and the amount of reduced (15)N excess remaining in the flag leaf, it was estimated that the flag leaf contributed 37% of the total reduced (15)N excess in the injected shoot. The lower shoot contribution was 18% and that of the peduncle plus spike was 45%.