Activation of rape (Brassica napus L.) embryo during seed germination. IV. Germinating embryo. The first zones of mitoses, starch and DNA synthesis and their expansion pattern

Abstract

The rape radicle is completely covered by the root cap. The successive lateral cell layers of the root cap are terminated by T-forming walls in the dermatogen layer in a statistically constant position. T-walls in dermatogen were utilized for delimitation of successive root sectors on longitudinal microtome sections at the succeeding germination stages. The length and the cell number of the corresponding sectors were studied and the starch and DNA synthesis sites as well as cell divisions localized. All these processes are initiated in a constant sequence and in specific embryo zones. The first symptom of activation of the embryo is starch synthesis. It begins in two centres: in the apical part of the radicle columella and in the hypoctyl dermalogen it moves deep into the cortex and in both directions along the columella the activation of starch synthesis shifts basipetally into the whole columella and the initial centre as well as the lateral parts of the root cap. From the hypocotyl dermatogen it moves into the cortex and in both directions along the embryo axis. In the root dermatogen and periblem the activation zone is first located in the basal sector and, then, gradually, in the lower ones. Just before germination the basal and apical zones of activation meet. Starch can then, be found throughout the root. During starch synthesis, in the basal part of the radicle. DNA synthesis and cell growth begin. In the root sectors which have already begun to grow and synthesize DNA, cell divisions start. The boundary of the dividing cells zone shifts acropetally at some distance above the lower boundary of DNA synthesis and the zone of cell premitotic growth. The acropetal shift of the mitotic activation zones can be described as wave expansion. Before the first mitotic wave reaches the promeristem it is followed by at least three acropetal waves, arising in the already activated basal sectors. and then the mitoses are asynchronised. Mitotic.: activation of the root cap is partly independent of the acropetal wave of cell activation and results from the expanding weaker basipetal wave.