Photosynthesis of lichen symbiotic alga Trebouxia erici as affected by irradiance and osmotic stress

The relation between oxygen evolution rate (OER) and quantum yield of photochemical reactions in photosystem 2 (ΦPS2) was examined in lichen symbiotic alga Trebouxia erici Ahmadjian (strain UTEX 911) exposed to different irradiances and osmotic stress (2 M sucrose for 60 h). Linear relationship was found between OER and ΦPS2 in control cell suspension within irradiance range of 0 - 500 μmol m-2 s-1. Under osmotic stress, OER and ΦPS2 were significantly reduced. Relation between OER and ΦPS2 was curvilinear due to strong osmotically-induced inhibition of OER at high irradiance. The highest used irradiance (500 μmol m-2 s-1) was photoinhibitory for osmotically-stressed T. erici because non-photochemical quenching (NPQ) increased substantially. Energy-dependent quenching represented major part of NPQ increase. Osmotic stress led also to the reduction of capacity of photochemical processes in PS 2 (FV/FM) and increase in F0/FM. These changes indicated negative effects of osmoticum on structure and function of photosynthetic apparatus.