Machinism of abscission of leaves and reproductive parts of plants - a model

Abstract

Abscission of leaves and reproductive parts of plants is presented in the form of a model which allows the reader to see the process as a whole. The process of abscission was divided into 2 parts: the first of which may be inhibited or reversed by auxin, and the second, which is mostly irreversible. Increased ethylene (ET) production links the earlier phases of senescence with abscission. ET has a double role in the initiation of the abscission process: (1) acting on the leaf blade, it withholds the export of auxin from a leaf, in consequence the abscission zone (AZ) is scarcely supplied with this hormone. When this state lasts sufficiently long, AZ tissues become sensitive to the second action of ET, (2) that is, to its dorect action on the AZ. When this occurs, the further stage of abscission is triggered. The possible roles of abscisic acid, tyloses and the influence of environmental factors are also discussed. During the further, mostly irreversible stage of abscission, marked stimulation of metabolic activity takes place in the AZ and its tissues attract nutrients. Sharp hormonal gradients appear in the AZ, leading probably to the precise localization of the "separation stratum" in the AZ where the detachment finally occurs. Further on, the detachment of the plant part may be accomplished in different ways, however, one "way" does not exclude the simultaneous participation of other(s). Thus, abscission may be accomplished by the enzymatic degradation of the middle lamella and primary cell wall, or by inducing cell divisions in the separation stratum with subsequent breaking of the newly formed cells. It can occur as well due to uneven augmentation of the cells belonging to the neighbouring cell layers which results in mechanical strain among them and in tearing apart. Separation may occur also due to a collapse of the cells of the AZ which leads to the formation of empty spaces. The problem of infallibility of physiological processes is briefly discussed.