Effect of changed source/sink relations on proteolytic activities and on nitrogen mobilization in field-grown wheat (Triticum aestivum L.)

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Abstract
Nitrogen mobilization and the pattern of proteolytic enzymes were investigated in leaves and glumes of field-grown winter wheat (Triticum aestivum L.) during maturation. Source/sink relations were changed by removal of the ear, the flag leaf or the lower leaves shortly after anthesis. Removal of the ear was most effective, resulting in delayed senescence of the flag leaf with the chlorophyll, aminopeptidase and carboxypeptidase activities remaining high in contrast to the control, whereas neutral endopeptidase activity increased more slowly. No major changes were observed in the second leaf from the top in plants with either ears or flag leaves removed. Nitrogen mobilization and proteolytic activities in glumes and the remaining leaves were influenced only slightly by leaf removal. In earless plants, nitrogen was transported from the second leaf into the leaf sheath and stem, but in the flag leaf the total reduced nitrogen remained high and free amino groups increased. The increase in endopeptidase activity was influenced by the source/sink relations. However, the accumulation of amino groups and the increasing endopeptidase activity in the flag leaf of earless plants suggest that the nitrogen sink capacity did not greatly control protein degradation; it remains to be seen whether phytohormones, accumulated amino acids or other factors delayed the increase in endopeptidase activity.