PROTEOLYSIS IN FLAG LEAVES OF WHEAT AS RELATED TO DIFFERENT LEVELS AND TIMING OF NITROGEN SUPPLY

Senescence of wheat leaves is related to the increase of proteolytic activities and to lower photosynthetic performance. A good nitrogen fertilization management generally delays the senescence of the crop photosynthetic tissues; in environments without terminal water-stress this can improve crop yield and grain quality. The effects of two N levels, 70 and 210 kg/ha and three side-dressing timings (at double ridge, stem elongation and anthesis) on proteolytic activities and carbon exchange rate (CER) in the flag leaf were investigated from anthesis to maturity; the cultivar was Saliente, a high quality, bread-making winter wheat and the site was near Udine, Northern Italy. With high N level, endoprotease (pH 4.5) and carboxypeptidase (pH 5.5) activities increased up to the last sampling, while with low N these activities dropped earlier. Soluble protein decreased steadily, and its level was influenced by endoproteolytic activities. With high N, total chlorophyll content was degraded later if compared with low N treatment. The latest N side-dressing initially increased leaf chlorophyll and soluble protein contents, but they did not exceed the levels of second side-dressing timing. This was related to an increased auto-proteolytic activity. Leaf carbon exchange rate decreased after anthesis; comparing the two N levels, only at the last measurement the N210 level obtained higher values.