Photosynthesis is the most important process for nearly all life on earth. Photosynthetic organisms capture and transfer light energy from the sun into chemical energy which in turn provides a resource base for heterotrophic organisms. Natural light regimes are irregular and vary over magnitudes. At a certain light intensity, metabolic processes cannot keep up with the electron flow produced by the primary photoreactions, and thus reactive oxygen species (ROS) are produced. ROS are highly reactive and can damage the photosynthesis apparatus and hence plants have evolved several photoprotection mechanisms to avoid the formation of ROS. The aim of this thesis was to examine the ecological effects of variations in photoprotection in plants. In particular I wanted to study the effect on fitness and the interaction with herbivorous insects of plants with different ability in photoprotection. To study this I used wild-type and transgenic Arabidopsis thaliana plants and grew them under natural conditions in field experiments in our botanical garden in Umeå, northern Sweden. For the investigation of the plant-insect interaction, a specialist on Brassicaceae (Plutella xylostella – diamondback moth) and a generalist herbivore (Spodoptera littoralis - Egyptian cotton worm) were used. Plants that are genetically deficient in one of the photoprotection mechanisms showed reduced fitness under natural conditions. I could thus show that feedback de-excitation (FDE) is the most important photoprotection mechanism, because a lack of FDE showed the highest reduction in fitness. The comparison of field grown wild-type with FDE mutant plants, using molecular biology methods, revealed large changes in gene transcription and metabolic composition. In particular, the jasmonate pathway was upregulated in light stressed plants, especially in plants lacking FDE. Jasmonate in turn is known to be a chemical compound which induces herbivore resistance genes and other stress responses. Specialist and generalist insect herbivores responded differently in feeding (dual-choice and no-choice) and oviposition experiments with field grown plants that differed in FDE. Female diamondback moths were attracted by induced defense compounds whereas the larvae avoided these plants in feeding experiments. Generalist larvae preferred, and showed a higher survival rate, on less light-stressed plants compared to more light-stressed plants. Combining molecular biology with ecological experiments is a challenging task. To summarize my experiences, I have produced a guide for experiments on transgenic plants in common gardens. In future investigations it is important to examine natural variations in photoprotection to elucidate selection pressures on specific genes.

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