The acclimation to increased photoperiod length was studied in vegetative and floral spinach plants with respect to changes in photosynthetic activity, ontogenesis and dry weight yield. Heterogeneity in size and physiological activity between the primary and secondary leaves was found under short day as well as upon transfer to continuous light. The secondary leaves emerge in SD conditions at a time when the primary leaves are about half maximal size with about 50% of the final chloroplast components. Rapid enlargement of the secondary leaves is accompanied by a second rise in photosynthetic 0, production. Transfer of young plants primarily stimulates metabolic activity and growth of the primary leaves. However, plants whose first leaves were close to maturity upon transfer predominately increase development of secondary leaves. The physiological activities of leaves are higher in continuous light but the life span of the leaves is considerably shorter. Photosynthate production is limited in SD conditions, which in turn limites synthesis of chloroplast constituents (especially Rubisco capacity) and finally restricts leaf growth. Prolongation of the daily inci­dent radiation leads to increases in chloroplast components, growth and dry weight yield. The extent of acclimation is higher for Rubisco capacity, soluble protein, total soluble sugar content and dry weight com­pared to chlorophyll content, 0, evolution and leaf area expansion. Rapid modifications are observed once the critical photoperiod is overcome. These changes might however be concomitant with flowering without necessarily directly promoting floral induction