While the morphological changes in the leaves of low light adapted higher plants are well established, the architecture and lateral arrangement of the thylakoid membrane from plants grown under low light conditions are still not well explored. In the present work we compare the structural organization and thermal stability of thylakoid membranes isolated from pea plants adapted to moderate and low light conditions, and relate the observed structural changes to the functional capacity of the photosynthetic apparatus. In line with earlier reports we confirm that low light induces decrease in the chlorophyll a/b ratio and enlargement of grana membrane regions. Importantly, for the first time we demonstrate a significant thermal instability of low-light thylakoids that are reflected in lower heat needed to disassemble the lateral order of the photosynthetic complexes, as well as for the destabilization of the trimers and monomers of the major light-harvesting complex of photosystem II. Data suggest that this important regulatory complex might adopt different conformation at moderate and low light, which is caused by its specific lateral arrangement within the membrane and might be essential for its regulatory role. In functional terms low light decreases the photochemical activity of thylakoids due to partial photosystem II centers inactivation and impaired electron transport towards photosystem I without inhibiting the photosystems’ functionality, which suggests that the established structural changes play a part in the photosynthetic apparatus operation.
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