Abstract: In order to investigate the photosynthetic responses of photoacclimation in Arabidopsis thaliana, photosynthetic capacity was measured in plants of the accession Wassilew Skija (WS) and in plants lacking expression of the gene At1g61800 (WS-gpt2) during acclimation from high to low light. Plants of WS and WS-gpt2 were grown for 6 weeks under high light (400 μmol.m^-2.sec^-1) and then half of them were transferred to low light (100 μmol.m^-2.sec^-1) to start the treatment. Gas exchange measurements were performed in order to measure the maximum capacity for photosynthesis. Meanwhile, chlorophy ll fluorescence analysis was carried out to measure changes in the quantum efficiency of PSII (ΦPSII) and Non-Photochemical Quenching (NPQ) during acclimation. Besides, a chlorophy ll composition analysis was performed to estimate the total chlorophyll and chl a/b ratio. Acclimation to a decrease in light resulted in a decrease in the photosynthetic capacity in WS and WS-gpt2 plants. Meanwhile, ΦPSII decreased in both WS and WS-gpt2 plants showing that under low light, PSII is more saturated. However, it was found that there were no significant changes in NPQ level for either WS or WS-gpt2. There were no significant changes in the total chlorophyll for both WS and WS-gpt2. However, the chlorophy ll a/b ratio was seen to be decreased in low light plants representing an increase in light harvesting complexes relative to reaction centre core. It is concluded that acclimation from high to low light is not a simple reversal of acclimation from low to high light but is mechanistically distinct process.