Changes in Protease Activity in Leaves during Natural Development and Accelerated Ageing upon Detachment

Abstract

When detached oat leaves were aged in darkness, loss of protein and chlorophyll was similar whether leaf segments were placed horizontally in Petri dishes on wet filter paper, or held vertically in plastic holders with their bases in water. In horizontally placed leaves, the activities of the two proteases tended to increase but in leaves kept vertically protease activities decreased as in attached leaves. In light, both chlorophyll and protein loss occurred at about half the rate observed in darkness. However, in vertically held leaves the acidic protease started to increase by day 2, whereas the neutral protease first declined but increased later. These increases were not mimicked by incubation on sucrose in darkness and neither could they be attributed to a greater accumulation of free amino acids in the light. Kinetin and, to a lesser extent zeatin, maintained chlorophyll and protein levels and counteracted both the increases and decreases in protease activities observed during ageing in light or darkness, respectively. Since accelerated loss of protein in darkness was not accompanied by increases in protease activities, these observations are at variance with those of Martin and Thimann (Plant Physiol. 49: 64, 1972) and suggest that additional synthesis of the major proteases is not a prerequisite for protein breakdown during senescence. © 1987, Gustav Fischer Verlag, Stuttgart. All rights reserved.