Photosynthesis and carbon metabolism were followed during the acclimatization of micropropagated Spathiphyllum cv. Petite shootlets cultured with two different sucrose concentrations (3 and 6. Increased sugar supply resulted in inhibition of photosynthesis, nonfunctional photosynthetic reaction centers, a more mixotrophic metabolism and higher starch and sucrose reserves at the end of the in vitro period. During the first days of acclimatization, net photosynthesis and adenosine diphosphoglucose (ADPG) pyrophosphorylase (EC 18.104.22.168) activity decreased for both treatments. In this period, sucrose was mainly used as nutrient reserve by plants cultured on 6% sucrose, as evidenced by a strong increase in sucrose synthase (EC 22.214.171.124) activity together with a severe decline in sucrose content. In the same period, sucrose breakdown and synthesis were in balance for plants cultured on 3% sucrose and the starch content increased slightly. After one week plants started to recover and full photosynthetic capacity developed. Two weeks after ex vitro transplantation, no differences between plants micropropagated with 3 or 6% sucrose could be found for photosynthesis, carbohydrate pools or enzyme activities. At the end of acclimatization the starch content increased again.