Abstract
Summary The degradation of protein, changes in the polysome population and amino acid levels were investigated during the growth and senescence of tobacco ( Nicotiana rustica L. cv. Gansu Yellow Flower) callus. During the growth period, the synthetic rate of protein increased concommitantly with the elevation of proteinase activity. After the onset of callus senescence, a dramatic decline in the polysome population was observed after day 25. The activity of proteinase was markedly enhanced between day 20 and day 25, and then decreased. The activity of aminopeptidase was low throughout the growth period and the early stage of senescence, and eventually decreased during the late stage of senescence. Therefore, it is suggested that the loss of proteins is not mainly due to the role of aminopeptidase but that of proteinase. Free amino acids accumulated rapidly during senescence. The levels of 14 free amino acids, except for alanine and cysteine, increased significantly after the onset of senescence.
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