2 DISTINCT MODES OF ACTION OF LIGHT ON THE INVITRO DEVELOPMENT OF ROOT EXPLANTS OF CICHORIUM-INTYBUS L

Modalities of action of light on the in vitro development of endive (Cichorium intybus L.) root tissues were examined by comparing the effects of two photoperiodic conditions (i.e. SD of 9 h light, 5.6 Wm-2, and LD of 16 h light with equal wattage). The influence of LD and SD was first studied on the structural events that take place from the beginning target cells until the appearance of regenerated buds. In both LD and SD conditions, the typical elements of a cellular dedifferentiation can be detected but LD supply resulted in the formation of more buds that also emerged more rapidly than those in SD. The increase of light intensity to 10 Wm-2 or the exogenous supply of sucrose (2-5%) in SD conditions enhanced the amount of the newly formed buds in comparison with controls, whereas the addition of DCMU to the culture medium in LD partially inhibited bud regeneration, 5.10(-6)M DCMU being the most effective concentration. After day 10 of the culture, the mitotic activity was given major attention to characterize the functioning of the newly formed buds. LD induced a progressively prefloral stage, the culmination of which was generally achieved on day 16. In SD conditions, some (20%) intermediate meristems could be observed with some delay (on day 24), but flowering was never completed, even after transfer to LD. Increasing the quantity of light in SD to an equivalent LD level resulted in a significant rise in the mitotic activity in the peripheral zone only; consequently, the zonation was retained. It is concluded that light acts first as an energizing source to control the bud's <<setting-up>> from the start to day 10 of the culture; secondly, light intervenes as an informative source to determine the differentiation, vegetative or reproductive, of the newly formed buds.