No abstract

When applied to pea (Pisum sativum L. var. nain hâtif d'Annonay) cotyledonary bud at the time of decapitation, cis-abscisic acid (ABA) induced a decrease in the mitotic indices of the meristem and subapical regions of the bud, in comparison with decapitated but not ABA treated control plants. ABA treatment delayed the entry of the G0–1 nuclei of the inhibited bud into the S phase and then into mitosis. Whatever the ABA concentration (10−3 to 10−7 M), the overall growth of the bud and the elongation of the cortical cells in its subapical region were not affected during the first 48 h. ABA induced an inhibition of bud elongation, all the more important since it was applied later after decapitation. When applied on a 48-h released bud, ABA induced an inhibition of elongation of the bud, a decrease in its mitotic activity, as well as a concentration of the shoot apex nuclei in the G1 phase of the cell cycle. However, this exogenous ABA treatment could not entirely replace the main axis in maintaining the inhibited state of the bud.

For three components (apical meristem, subapical internodes, and foliar apparatus) of Pisum sativum (cv. nain hâtif d'Annonay) cotyledonary buds, DNA microdensitometry, mitotic indices, and release of growth have been used to detect changes produced by kinetin applications (50 μg/mL). One kinetin treatment is sufficient to release the cell cycle, but continuous kinetin supply is necessary to maintain bud elongation. At the apical, subapical, and foliar levels, inhibited cotyledonary buds contain a majority of nuclei with 2C DNA content, in the G1 phase of the cell cycle. Following hormonal treatment, the prereplicative phase of the noncycling cells is at least 6 h in the whole bud. The lag period for the release of bud growth is between 15 and 23 h in subapical regions (first and second internodes) and between 23 and 38 h at the foliar level. At subapical and foliar levels, a few mitoses precede and then follow the elongation process. The first mitoses are from G2 nuclei of the inhibited bud. The following ones, more numerous, are from noncycling cells of inhibited buds, which have entered the S phase in response to kinetin application. The degree of inhibition of an axillary bud, estimated by the distribution of the DNA content in its three components, conditions the events induced by hormonal treatment or decapitation. The discussion shows how the knowledge of the original nuclear states of inhibited buds is necessary to understand axillary bud reactivation.

The ability of the first internode of Pisum sativum epicotylary axis to produce adventitious roots was examined with respect to dark and light growing conditions and depth position of the soaked seeds in vermiculite. Root initiation began on the 7th day and moved acropetally until the 16th day, with no formation of new roots between older ones. DNA synthesis in the diploid interfascicular cells followed the first mitoses of root initiation. The cell layer that would normally have differentiated into an endodermis reacted more slowly and built a primordial root cap, protecting the root as it grew through the cortex. When the new root emerged a new root cap – protoderm complex was rebuilt from the subapical cells of the root meristem. The increase in cell number was highest during the first 24 h following root initiation, and the cell cycle was extremely short with a very reduced G1 phase. Only the elongation zone of the mature adventitious root showed polyploidy.

Plastochrone, cycle cellulaire et teneurs en ADN nucleaire du meristeme caulinaire de plants de Chrysanthemum segetum soumis a deux conditions lumineuses differentes, sous une photoperiode de 16 heures R e~u le 1" dtcembre 1989 NOUGAREDE, A,, DI MICHELE, M. N., RONDET, P., et SAINT-COME, R. 1990. Plastochrone, cycle cellulaire et teneurs en ADN nuclCaire du meristbme caulinaire de plants de Chrysanthemum segetum soumis a deux conditions lumineuses differentes, sous une photoperiode de 16 heures. Can. J. Eot. 68: 2389-2397. Des plants de Chrysanthemum segetum sont cultivCs en photopCriode de 16 h sous deux Cclairements diffkrents (70 ou 200 pmol m-2 s-I).Sous 200 pmol m -2 s -I, par comparaison B 70 pmol m-2 s-I, la phyllotaxie n'est pas modifiCe, mais le plastochrone dCcroit et le diambtre apical s'accroit par extension de la zone axiale. Le temps moyen de doublement cellulaire decroit de 36,2% en zone laterale, de 29% en zone axiale et de 13% seulement dans le mCristbme mCdullaire. La durCe de la mitose reste, par contre, constante. Sous les deux conditions lumineuses, les noyaux, dont les teneurs en ADN sont localisCes dans les limites de la repartition 2C, sont predominants, ce qui signifie que la phase G , du cycle cellulaire est la plus longue. Sous 200 pmol m-2 s -' , le raccourcissement du temps moyen de doublement cellulaire va de pair avec une rkduction du pourcentage des noyaux compris B I'intCrieur des limites de la repartition 2C; la reduction de ce pourcentage est la plus importante dans la zone latCrale et la moins importante dans le mCristbme medullaire, montrant qu'un contr6le de la prolifkration s'exerce par l'intermkdiaire de la phase G I du cycle cellulaire.Mots clPs : Chrysanthemum segetum, cycle cellulaire, duke de la mitose, plastochron:, teneurs en ADN nuclCaire, zonation. NOUGAREDE, A., DI MICHELE, M. N., RONDET, P., and SAINT-COME, R. 1990. Plastochrone, cycle cellulaire et teneurs en ADN nuclCaire du mtristbme caulinaire de plants de Chrysanthemum segetum soumis B deux conditions lumineuses diffkrentes, sous une photopCriode de 16 heures. Can. J. Bot. 68 : 2389-2397. Chrysanthemum segetum plants were grown from seeds under a 16-h photoperiod, at two different photon fluence rates (70 or 200 pmol m-2 s-I. At 200 pmol m-' s-', by comparison with 70 pmol m-2 s-I, phyllotaxy was not modified, but the plastochron decreased and the apical diameter increased by extension of the axial zone. The mean cell doubling time decreased 36.2% in the lateral zone, 29% in the axial zone, and only 13% in the rib meristem. In contrast, mitosis duration was constant. Under both light conditions, nuclei with a DNA content within the limits of the 2C range were always predominant, which means that the GI phase of the cell cycle was the longest. At 200 pmol m-2 sC1, the shortening of the mean cell doubling time is accompan~ed by a reduction of the percentage of nuclei with DNA levels within the limits of the 2C range. The decrease of the latter was the most important in the axial zone and the least important in ...