Excised watermelon (Citrullus vulgaris Schrad.) cotyledons were grown in the dark in the presence of 0.1 mM benzyladenine (BA). Under these conditions reserve breakdown and organelle differentiation progress very slowly. Treatment with BA accelerates, breakdown of reserves and stimulates development of organelles. Electron micrographs of cells from treated cotyledons show a larger number of plastids with a more developed inner membrane system. The levels of plastid pigments and enzymes are increased while starch content is reduced. Glyoxysomal enzyme levels are increased by BA during the first three days of development and their decline is accelerated thereafter. Also the activity of hydroxypyruvate reductase (EC 1.1.1.81.), a peroxisomal enzyme, is increased, but this increase is not followed by a decay phase. In water controls, hydroxypyruvate reductase bands together with glyoxysomal enzymes after equilibrium centrifugation in a sucrose gradient. In treated cotyledons the equilibrium position of glyoxysomal enzymes is uchanged while that of hydroxypyruvate reductase is shifted to a lower density.

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