Five decades ago, a novel mode of CO2 assimilation that was later described as C4-photosynthesis was discovered on mature leaves of maize (Zea mays L.) plants. Here we show that 3- to 5-day-old developing maize leaves recapitulate the evolutionary advance from the ancient, inefficient C3 mode of photosynthesis to the C4 pathway, a mechanism for overcoming the wasteful process of photorespiration. Chlorophyll fluorescence measurements documented that photorespiration was high in 3-day-old juvenile primary leaves with non-specialized C3-like leaf anatomy and low in 5-day-old organs with the typical "Kranz-anatomy" of C4 leaves. Photosynthetic gas (CO2)-exchange measurements on 5-day-old leaves revealed the characteristic features of C4 photosynthesis, with a CO2 compensation point close to zero and little inhibition of photosynthesis by the normal oxygen concentration in the air. This indicates a very low photorespiratory activity in contrast to control experiments conducted with mature C3 sunflower (Helianthus annuus L.) leaves, which display a high rate of photorespiration. and U. Kutschera ... [et al.].