Leaf Proteolytic Activities and Senescence during Grain Development of Field-grown Corn (Zea mays L.)

Some proteolytic enzymes occurring in the leaves of field-grown corn (Zea mays) (B73) were identified and partially characterized. Changes in activities of several proteolytic enzymes and in concentrations of protein and chlorophyll as a function of intraleaf segments (tip to base), leaf position, and leaf senescence during grain development and maturation were followed in crude leaf extracts.The aminopeptidase (not affected by sulfhydryl or fluoride reagents) was most active at pH 7, while the carboxypeptidase(s) (sensitive to fluoride, but insensitive to sulfhydryl reagents) was most active in the acid range, pH 3 to 6. The presence of two or more endopeptidases is indicated. Endopeptidase (caseolytic) activity at pH 5.4 appeared to be stimulated by sulfhydryl groups or EDTA, while caseolytic activity at pH 7.5 was not.Visually, individual leaf senescence starts at the leaf tip and the necrotic (brown) V-shaped area enlarges progressively toward the leaf base. Canopy senescence occurs in two phases. Foliar symptoms are first observed on the bottom leaf and then in sequential order up the plant. Subsequently, senescence occurs on the top leaf and moves downward. These foliar senescence symptoms are paralleled by decreases in exopeptidase activities, protein, and chlorophyll concentrations and by increases in endopeptidase activities.During development and maturation of the grain, both aminopeptidase and carboxypeptidase activity of the middle half of the ear leaf increased (2- to 3-fold) during the onset of the visual reproductive phase (tassel and car emergence). However, during grain development and plant senescence, both activities decreased rapidly and concurrently with the loss of protein and chlorophyll from this leaf section. In contrast, caseolytic activity at both pH 5.4 and 7.5 increased gradually during the early reproductive phase and rapidly with leaf senescence. The fastest rate of increase in caseolytic activities was concurrent with the most rapid loss of protein from the leaves. The coincidence of these events suggests a major role for the caseolytic enzymes in initiating the rapid hydrolysis of leaf protein.