STUDIES ON THE GERMINATION OF SEEDS OF THE ROOT PARASITES, ALECTRA VOGELII AND STRIGA GESNERIOIDES. II. DNA SYNTHESIS AND DEVELOPMENT OF THE QUIESCENT CENTER IN THE RADICLE

DNA synthetic activity in the radicle meristem of embryos of germinating seeds of the obligate root parasites, Alectra vogelii and Striga gesnerioides was followed by autoradiography of 3Hthymidine incorporation. Incorporation of 3H-thymidine occurred in the nuclei of cells destined to form the vascular tissues, ground meristem and epidermis. An analysis of the distribution of labeled nuclei demonstrated the presence of a quiescent center of 2-4 cells in the radicle at the beginning of seed germination, becoming more prominent at later stages of germination. During continued growth of the radicle which resulted in a reduction in size of the meristem, cells of the original auiescent center were activated to undergo DNA synthesis. THE ANATOMICAL CHANGES in the embryos of germinating seeds of the obligate root parasites, Alectra vogelii and Striga gesnerioides have been the subject of the first paper in this series (Okonkwo and Raghavan, 1982). This study showed that cell division activity during germination of seeds in the presence of the exudate of the host root (cowpea) was mostly confined to the radicle end of the embryo. However, due to the microscopic size of the radicle and the limited number of actively dividing cells present in this organ, the meristematic region could not be delimited very precisely during the different phases of germination. Therefore, an autoradiographic analysis of DNA synthesis in the embryos of germinating seeds of A. vogelii and S. gesnerioides is of interest to follow the organization of the root apical meristem and evolution of the quiescent center. Studies on the development of the quiescent center in the radicle meristem of early stage embryos are few, although several workers have examined the origin of the quiescent center in the embryonic radicle of germinating seeds. The consensus of current opinion is that in germinating seeds, the radicle emerges without a well-defined quiescent center, but it acquires one soon after (Clowes, 1958, 1978a; 1 Received for publication 22 August 1981; revision accepted 16 March 1982. 2 This work was done while S.N.C.O. was on sabbatical leave during the 1980-.81 academic year, and was supported by a grant from the Research Committee of the College of Biological Sciences, The Ohio State University. Permanent address: Department of Botany, University of Nioerrtia NJcil-lkk a.peric. Alfieri and Evert, 1968; Jones, 1977). Even in the radicle of mature embryos of Glycine max (Miksche and Greenwood, 1966) and Malva sylvestris (Byrne and Heimsch, 1970), where a quiescent center has been claimed to be present right from the start of germination, the evidence is considered to be equivocal (Clowes, 1978a). Based on mitotic activity and staining reactions, some evidence has been presented for the existence of a group of cells analogous to the quiescent center in the radicular tip of developing embryos of certain plants (Sterling, 1955; Rondet, 1962; Vallade, 1972). This observation was confirmed by Jones (1977) and Clowes (1978a, b) who followed the distribution of labeled nuclei and mitotic figures in the embryos of Pisum sativum and Zea mays, fed with 3H-thymidine. In both plants, a quiescent center was first noticed in embryonic radicles with well-developed root cap initials or a primitive root cap. As the embryos matured, there was a decay of meristematic activity throughout the radicle, but following germination the quiescent center reappeared. The presence of undifferentiated heart-shaped embryos in the mature seeds of A. vogelii and S. gesnerioides makes them particularly suitable to follow the development of the quiescent center in the radicle meristem of early-stage embryos and of germinating seeds. MATERIAL AND METHODs-Seeds of Alectra vogelii and Striga gesnerioides from the 1972-73 harvests used in the anatomical study were used in this investigation. Methods for inducing germination of seeds and the conditions for germination and seedling growth were