The efficiency of photosynthesis results from the composition and organization of the plant’s internal structural components as well as the capability of response to environmental fluctuations. This thesis aims at identifying the genetic loci that are regulating the (sub-) processes in photosynthetic acclimation to increased irradiance levels, in order to obtain the genetic information useful to breed for photosynthetic performance. It uses genome wide association studies (GWAS) to reveal which genetic loci are being exploited in nature for keeping good photosynthetic performances in natural conditions. Phenotypic variation among natural accessions in photosynthetic light use efficiency response to increased growth irradiance is related to its variation in genetics in order to identify the associated genetic loci. In Chapter 2 is described which light environment reveals most natural variation in photosynthetic performance and for which photosynthetic parameter this is. It shows different Arabidopsis accessions display different photosynthetic responses to various light environments, well relatable to genetic differences. A candidate gene list for the direct response to increased growth irradiance was revealed after performing genome wide association analysis. Chapter 3 elaborates on the genome wide association results by visualizing the dynamics of the associated genetic loci over the time course of the photosynthetic response to increased irradiance. It shows it is possible to simplify the complexity of photosynthetic physiology as well as the genetic analysis in such way to confirm the causal genes underlying the associated loci, by confirming this for the YELLOW SEEDLING 1 (YS1) gene, a gene encoding a Pentatrico-Peptide-Repeat (PPR) protein involved in RNA editing of plastid-encoded genes essential for photosystems I and II. Genetic variation for any trait can be on the transcriptional level or on the functional level. In Chapter 4, the gene regulation in three Arabidopsis accessions with contrasting photosynthesis efficiency responses to increased irradiance is studied. These differences in photosynthesis efficiency are associated to differences in activation extents of heat responsive genes as well as to differences in the presence of a gene activation pathway acting on membrane lipid remodelling, suggested to maintain balanced cellular phosphate concentrations. Chapter 5 confirms the significance of maintaining balanced cellular phosphate concentrations for photosynthesis efficiency responses to increased irradiance. It describes how genome wide association mapping and linkage mapping combine to reveal genetic epistatic interactions between PHOSPHATIDIC ACID PHOPSPHOHYDROLASE 2 (PAH2, phosphate metabolism gene) and ASPARAGINE SYNTHETASE 2 (ASN2, nitrogen metabolism gene), both acting in the delivery of orthophosphate in the chloroplast. In conclusion this thesis contributes new insights into the physiological and molecular pathways underlying photosynthesis responses to increased growth irradiances.