The long-day plant spinach (Spinacia oleracea cv. Nobel) remains vegetative in short-days (SD) of 8 hours of light (8:16 L:D). With 4-weeks old vegetative plants, the photosynthetic activity directly controls the total sugar content in the primary leaves and petioles. When the plants are transferred to continuous light (photo periodic transfer in LL), the total sugar content is sharply increased 2 to 3 hours after the beginning of the photoperiodic transfer. Glucose, fructose and sucrose are the main soluble sugars present in spinach leaves and petioles as indicated by TLC and HPLC techniques. During the photoperiodic transfer the glucose and fructose content increases 5 to 8 fold in the leaves and petioles by comparison with the content at the end of the light period during SDs. The sucrose content is also increased during photoperiodic transfer, but to a lesser extent. This phenomenon is called the "glucose effect" and is also observed in some other photoperiodic plants such as the long-day plants Coleus (Coleus blumei Benth.) and mustard (Sinapis alba L.), and the short-day plant Chenopodium rubrum (ecotype 184). In SD plants, however, the "glucose effect" is expressed as a decrease in glucose contents. The "glucose effect" in spinach is sensitive to different kinds of stresses. If leaves are pricked or fumigated with ozone, e.g., at the end of the SD light span (16 hours) and the plants transferred to LL, the "glucose effect" is more or less abolished. Pricking only one leaf also modifies the response in the opposite leaf and petiole. This kind of behaviour clearly indicates that the stress effect is transmissible within the whole plant. In spinach the photosynthetic activity in the leaves produces the substrates necessary for the expression of the "glucose effect" in the petioles. As fructose increases concomitantly with glucose in the petioles and that it is known that assimilates are transported in the phloem in the form of sucrose, it is proposed that the "glucose effect" results from the local enzymatic hydrolysis of sucrose by invertase. This enzyme is in f act present in the petioles and its potential activity, though it does not vary after the photoperiodic transfer, is already sufficient in SDs to account for the "glucose effect". Therefore a model is proposed based on a change in compartmentation of sucrose after the photoperiodic transfer. This hypothesis permits one to explain why the glucose content of petioles seems to be distributed unevenly between two compartments or tissues during the photoperiodic transfer. Accordingly, one of these compartments is mainly responsible for the SD pattern, the other one being responsible for the pattern obtained during photoperiodic transfer. Using the properties of circadian rhythm of leaf movement in spinach plants and by displacing the SD light period, it can be demonstrated that the "glucose effect" is primarily dependent on the moment when light is given and not on the absolute duration of the light period. This result indicates the existence of an internal rhythm of sensitivity towards light with respect to the expression of the "glucose effect” ... Thus, this latter could result from the coincidence between this internal sensitivity to light and the external photoperiod, so pro vi ding a way for the plant to measure the lengthening of the photoperiod. Assuming that the modification of sucrose compartmentation might be a more general phenomenon, it is hypothesized that the lengthening of the photoperiod in long-day plants results in a change of compartmentation of other specific molecules, such as growth factors so allowing their contact with corresponding specific receptors. This contact, which is only possible e after a photo periodic transfer, can depend on genotypic variations, thus controlling the establishment of new physiological or developmental specific properties related to the photoperiod.