Spinach plants maintained in a short day photoperiod of 8:16 (L:D) remain vegetative. Transition to flowering (induction) may be initiated by extending the light phase of the photoperiod (transfer). We have examined the free sugar content during this transition and observed a sudden and considerable increase (in the order of 3 to 10 times the baseline) in the concentration of glucose already 3-5 h after the start of light extension. Various sources of stress affect this response. Pricks and cuts, as well as a brief, strong increase of an atmospheric pollutant such as ozone turned out to be inhibitors. At the time of the experiments (4 weeks), spinach plants possess two symmetrical and fully developed primary leaves. The pricking of one leaf just before the transfer treatment inhibits this particular carbohydrate response, not only in the directly wounded leaf but also in the adjacent unwounded leaf. The transmission of the stress effect from one leaf to the other is undeniable for both glucose and saccharose, it implies the existence of a systemic mechanism concerning photoperiodic-controlled carbohydrate metabolism in plants.