Seedlings of Brassica napus were cultivated on a slowly rotating clinostat (1 rpm) or in the vertical control for 5 d. The root growth, the cotyledonary reserves and the transport of 14C-labeled sucrose from cotyledons to root system were studied in both cultural conditions. The biomass (fresh weight) of the root system was 35% higher in the horizontally clinorotated seedlings than in the controls. This increase was correlated with a greater degradation of reserve lipids and faster accumulation of sucrose in the cotyledons. The activity of isocitrate lyase, one of the two enzymes necessary to conversion of lipids into glucids, was also greater in the cotyledons of clinorotated seedlings. The labeling distribution of 14C in the cotyledons, the hypocotyl and the root system after 30, 60 and 120 min of application of 14C-labeled sucrose on the cotyledons showed higher translocation of the cotyledonary sucrose to the root system of clinorotated seedlings. In addition, we studied the effects of clinorotation on the biomass of the excised root system (of 10 d old seedlings) cultivated in a medium containing 1% sucrose. The horizontally clinorotated root system grew more than that of the controls. These results showed that the horizontal clinorotation acted on the root system growth and provoked a higher sucrose translocation from source to sink, i.e. from cotyledons to root system.

Résumé.-Lorsque les racines de Lentille (Lens culinaris L., variété large blonde} sont gravistimulées en position horizontale durant 2h, les cellules corticales s'allongent plus rapidement dans la moitié supérieure que dans la moitié inférieure de la zone de courbure. La zone de graviréaction s'étend dans le méristème sensu stricto et dans la partie proximale de la zone d'élongation cellulaire.Dans le méristème, la courbure gravitropique se réalise par stimulation de l'élongation des cellules de la moitié supérieure. Dans la zone d'élongation cellulaire, la courbure est dûe à une inhibition de l'allongement des cellules de la moitié inférieure. L'analyse semi-quantitative de l'ultrastructure des cellules graviréactives révèle que la différenciation des organites est modifiée et que les modifications les plus remarquables concernent les cellules graviréactives méristématiques.La graviréaction entraîne une augmentation de l'activité mitotique dans l'ensemble de la ré-gion proximale et dans la moitié inférieure de la région distale de la zone méristématique. De plus, la gravistimulation provoque une augmentation des synthèses d'ADN nucléaire des cellules corticales dans la moitié supérieure de la région méristématique distale. Le cycle des cellules corticales est donc modifié lors de la cÔurbure gravitropique. Il est proposé que l'inhibiteur de croissance, responsable de la graviréaction racinaire, est un inhibiteur de la différenciation cellulaire et qu'un autre facteur hormonal, favorisant l'entrée en mitose, serait impliqué dans la réponse gravitropique.Dans les racines du Zea mays, dont la croissance s'est effectuée à bord du satellite COSMOS 1667, l'activité de prolifération des cellules corticales est plus intense et leur différenciation est plus rapide que dans les contrôles terrestres. Ces phénomènes sont comparés à ceux observés dans la réac-tion gravitropique de la racine de Lentille.Summary.-When lentil roots (Lens culinaris L., cv. large blonde} are gravistimulated in horizontal position for 2h, the length of the cortical cells is greater in the upper than in the lower hait of the zone of the gravitropic curvature. The bending begins in the meristem sensu stricto and is exten· ded to the proximal part of the cell elongation zone. ln the meristem, the curvature is due to astimulation of cell elongation in its upper half. ln the proximal part of the cell elongation zone, the curvature results from an inhibition of cell growth in its lower half. The semi-quantitative analyse of the ultrastructure of the gravireacting cells shows that the differentiation of organelles is changed during the gravireaction and the modifications are the greatest for the gravireacting meristematic ce lis.The gravireaction leads to an increase of the mitotic activity in the whole proximal part of the meristematic zone and also in the lower half of its distal part. Moreover, the gravistimulation brings about an increase of nuclear DNA synthesis of cortical cells in the upper half of the distal part of the meristematic zone. The ce...

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When roots of lentil (Lens culinaris L., cv. Large blonde) were placed in horizontal position for 2 h, their upper side elongated faster than their lower side, and also faster than vertical controls. The length of the cortical cells was greater in the upper half than in the lower half of roots which had been horizontally stimulated for 2 h. The zone of curvature extended from the distal part of the meristem to the proximal part of the cell elongation zone. The curvature in the meristem was due to early differentiation of the cells of its upper part. In the proximal part of the cell elongation zone, bending took place due to inhibition of cell growth in the lower half of the root. The results obtained are in agreement with the hypothesis of lateral transport of an inhibitor in gravistimulated roots. This inhibitor should be present in greater amounts in the lower side of the stimulated root and in lower amounts in its upper side than in the vertical controls.