Photoacclimation is a process by which photosynthetic capacity is regulated inresponse to environmental adjustments in terms of light regime. Photoacclimation isessential in determining the photosynthetic capacity to optimize light use and to avoidpotentially damaging effects.Previous work in our laboratory has identified a gene, gpt2 (At1g61800) that isessential for plants to acclimate to an increase in growth irradiance. Furthermore, weobserved that the accession Columbia-0 (Col-0) is unable to respond to increases inlight. Therefore, a Quantitative Trait Locus (QTL) mapping analysis was performed inLandsberg erecta (Ler)/Columbia (Col) recombinant inbred line population to identifynovel genes responsible for this variation to acclimation.In order to investigate the photoacclimation in Arabidopsis thaliana,photosynthetic capacity was measured in plants of the accession Wassileskija (WS) andin plants lacking expression of the gene At1g61800 (WS-gpt2) during acclimation fromhigh to low light. Plants were grown for 6 weeks under high light (400 micro metreol.m-2.s-1) andhalf of them were transferred to low light (100 micro metreol.m-2.s-1) after six weeks. Gasexchange measurements were performed in order to measure the maximum capacity forphotosynthesis. Acclimation to a decrease in light resulted in a decrease in thephotosynthetic capacity in WS and WS-gpt2 plants. This shows that under lower orlimiting light, photosynthesis was slowed down.Chlorophyll fluorescence analysis was carried out to measure changes in thequantum efficiency of PSII (ΦPSII) and nonphotochemical quenching (NPQ) duringacclimation. ΦPSII decreased in both WS and WS-gpt2 plants showing that under lowlight, PSII is more saturated However, it was found that there was no significantchanges in NPQ level for both WS and WS-gpt2.To estimate the total chlorophyll and chl a/b ratio, a chlorophyll compositionanalysis was performed. There was no significant changes in the total chlorophyll forboth WS and WS-gpt2. However, the chlorophyll a/b ratio was seen to be decreased inlow light plants representing an increase in light harvesting complexes relative toreaction centre core.Plants of WS and WS-gpt2 were also grown under natural variable light in anunheated greenhouse in Manchester, UK. This experiment was carried out to study thephotosynthetic acclimation of plants under fluctuating light condition.A preliminary work on gene expression of gpt2 was conducted by doing reversetranscriptase PCR (RT-PCR). It shows that the gene expression of gpt2 decreasedfollowing transfer to low light plants in WS. Microarray analysis was also performed toinvestigate the role of GPT2 (if any) and to identify any potential gene that is importantin high to low light acclimation.