Document Type

Dissertation

Date of Award

1977

Keywords

Plants, Aging Leaves, Hydrolytic enzyme, Leaf senescense

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Alan H. Haber

Second Advisor

Fredrick J. Kull

Third Advisor

Herbert B. Posner

Abstract

Acid phosphatase activity has been interpreted as being associated with senescence in several plant systems. Consistent with this interpretation, this study demonstrates that in excised apical portions of wheat leaves kept in the light acid phosphatase activity slowly increases with time. Contrary to this interpretation, however, dark incubation of excised leaf tips diminishes the acid phosphatase response, although it accelerates chlorophyll loss. Acid phosphatase activity does not increase in intact illuminated leaf tissue over a period of three weeks, during which the leaf is nevertheless senescing. In fact, acid phosphatase activity declined in the intact, senescing leaf tip, and the activity declined first in the more apical, physiologically oldest portions. In eighteen-day old intact senescing leaves, in which the acid phosphatase activity is already declining, excision of the apical portion produces a significant increase in acid phosphatase activity. These results indicate that the increase in acid phosphatase activity is a result of excision rather than normal physiological senescence, and raise serious questions about the validity of the excised leaf as a system for the study of normal plant senescence.

Multiple cutting of the excised leaf tip significantly increases the post-excision acid phosphatase activity, suggesting that wound metabolism is important in the response. The acid phosphatase response differs from typical plant wound metabolism in that (a) the increased activity is not localized at the cut surface, and (b) the magnitude of the response is not proportional to the number of cuts. Cutting of leaf sections left attached to the plant does not bring about an increase in acid phosphatase. Thus, detachment, and not merely cutting, appears to be necessary for the excision-related increase in acid phosphatase activity.

Nuclease activity of wheat left apical segments was found to be increased in the following instances: (a) during intact leaf senescence; (b) after excision; (c) after exposure to sublethal doses of gamma radiation. Since all of these deleterious factors might be expected to lead to cell death, it was possible that the observed increase in nuclease activity might be (a) a general consequence following from cell death or (b) a response of living or senescing cells leading to death. The first hypothesis was tested by exposing leaf apical segments to a dose of gamma radiation which was lethal for the leaf tissue but did not alter the nuclease activity. Since there was no increase in nuclease activity following this acute lethal dose, it was concluded that the increased activity observed in deleterious circumstances is not a general consequence of cell death.

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