Effect of nitrogen application on redistribution and transformation of photosynthesized 14C during grain formation in two maize cultivars with different senescence appearance

A field trial was conducted to investigate the effect of nitrogen (N) application on redistribution and transformation of photosynthesized C-14 during grain formation in two maize (Zea mays L.) cultivars with different senescence appearance, (CO2)-C-14 was introduced to ear leaf of maize at vegetative growth stage and grain-filling stage, respectively, the C-14-distribution to respired CO2, vegetative mass and grain as well as to crude chemical components including sugar, polysaccharide, organic acids, amino and protein in ear leaf of maize and grain at grain-filling stage was determined. Two maize hybrids included "Danyu 13", an early senescence hybrid, and "Zhongdan 306", a stay-green hybrid. There were four N treatments: (1) N1: 0 kg N ha(-1); (2) N2: 250 kg N ha(-1) (1/5 N as basal application and 4/5 N as topdressing at stalk elongation stage only); (3) N3: 250 kg N ha(-1) (1/5N as basal, 2/5N as top-dressing at stalk elongation stage and tasselling stage, respectively); (4) N4: 400 kg N ha(-1) (1/5 N as basal and 4/5 N as top-dressing at stalk elongation stage). For Zhongdan 306, a higher grain yield was ascribed to the less respiratory loss of C, larger translocation of current photo-synthesized C during ripening, and smaller translocation of C existed in vegetative mass, while grain formation for Danyu 13 depended upon the larger translocation of C existed in vegetative mass and smaller translocation of cur-rent photosynthesized C during ripening. Percentage of photosynthetically fixed C-14 distributed in both sugar and polysaccharide in ear leaf and grain for Zhongdan 306 was significantly larger than Danyu 13, while that in organic acids, amino and protein for Zhongdan 306 were markedly smaller than Danyu 13. The highest percentage of 14 C distributed in sugar, the main carbohydrate translocated from leaf to grain, could be achieved in N3 treatment for Zhongdan 306 and in N2 treatment for Danyu 13, respectively, which consequently benefited to grain formation. This distribution of photosynthesized C-14 between C-pool and N-pool may be related to the ratio of sucrose phosphate synthase (SPS) to phosphoenolpyruvate carboxylase (PEPC) in leaf of maize at grain-filling stage.