| Photosynthesis is the process by which light energy is converted into chemical energy. This process is performed in specialized organelles called chloroplasts. The chloroplast contains a unique membrane structure—thylakoid membranes—which contain the photosynthetic machinery. The individual components of the photosynthetic apparatus are maintained in a stoichiometric balance, adjusting to maintain optimal photosynthetic capacity. This acclimation of the photosynthetic apparatus occurs in response to light quality/quantity regimes that the plant perceives at different stages of development.; Inland plants, the mesophyll cell contains 80–100 chloroplasts. The reason for this high plastid number is unknown. Due to the non-motile nature of plants, it has been theorized that maintaining a large number of small chloroplasts allows them to better acclimate to their changing environment. Until recently, this area has been unexplored; however, availability of Arabidopsis mutants altered in the a&barbelow;ccumulation and r&barbelow;eplication of c&barbelow;hloroplasts (arc mutants), allows us to address such questions. In this work, the photosynthetic capacity of three arc mutants have been examined. Arc3, arc5, and arc6 accumulate only 2–15 abnormally large chloroplasts per mesophyll cell. The hypothesis that these mutants compensate for the lack of many small chloroplasts by increasing the chloroplast size, and therefore maintained the chloroplast area to mesophyll cell area ratio, is tested. It is shown that the arc mutants do not show normal patterns of acclimation to increasing growth light intensity, and in fact show a low light grown phenotype at all growth light intensities.; During the screening of Arabidopsis mutants, one line was identified, arc2, that had an extremely over-reduced plastoquinone pool (PQ-pool) in young leaves. Over-reduction of the PQ-pool is probably the result of increased excitation pressure due to the unusual thylakoid organization. It was found that young leaves of both wild type and arc2 are sensitive to n-propyl gallate, suggesting that electron flow through Photosystem II (PSII) is dependent on a plastid terminal oxidase (PTOX). In older leaves, the n-propyl gallate effect is not seen. This provides evidence that PTOX is an electron acceptor in developing chloroplasts that may serve to protect PSII prior to the establishment of full photosynthetic competence. |
