Aggregation of the Light‐Harvesting Complex in Intact Leaves of Tobacco Plants Stressed by CO2Deficit

Abstract

ABSTRACTPronounced aggregation of the photosystem II light‐harvesting complex (LHC II) was observed in low‐lightgrown tobacco plants stressed with a strong CO2deficit for 2–3 days. The LHC II aggregates showed a typical band at 697–700 nm (F699) in low‐temperature emission spectra. Its excitation spectrum corresponded to that of detergent‐solubilized LHC II. Formation of F699in stressed plants was not reversed in the dark and leaves did not contain any zeaxanthin showing that neither a light‐induced transthylakoid pH gradient nor zeaxanthin was required for LHC II aggregation. The CO2‐stressed plants showed clear signs of photodamage: depression of the potential yield of photosystem II photochemistry (F,/FM) by 50–70% and a decline in chlorophyll content by 10–15%. Therefore, we propose that the photodamage to the photosynthetic apparatus is the cause of the LHC II aggregation in plants. The F699exhibited a reversible decrease of its intensity upon irradiation of leaves with intensive light. There was no or only slight decrease around 700 nm in unstressed plants. The nonphotochemical quenching of chlorophyll fluorescence showed the opposite relation, being higher before than after the strong CO2deficit. This discrepancy was likely related to the different LHC II aggregation state in control and stressed plants.