Desiccation resistence of photosynthetic processes in photosystem II of Antarctic moss <i>Sanionia uncinata</i> assessed by two chlorophyll fluorescence methods

Dehydration-induced decrease of photosynthetic performance in mosses is a general response to thallus dessication. The decrease is caused by changes in photosystem II (PSII) functioning which is sensitively reflected in several chlorophyll fluorescence (ChlF) parameters. Apart from the decrease in potential yield of photosynthetic reactions in PSII (FV/FM), several protective mechanisms are activated during moss desiccation, non-photochemical quenching (NPQ) in particular. This is study focused on NPQ changes during the desiccation of Antarctic moss Sanionia uncinata. NPQ induction and relaxation curves were recorded during gradual dehydration from a fully wet state (relative water content [RWC] = 100%) to a fully dry state (RWC = 0%). Three key NPQ parameters were evaluated: NPQmax (maximum value), NPQtermin (at the end of induction) and NPQrelax (end of dark relaxation). The initial slope (a) and dark relaxation (b) of NPQ were analysed. Additionally, fast chlorophyll fluorescence transients (OJIPs) were measured and the parameters related to PSII functioning evaluated for decreasing RWC. The relationships between the ChlF parameters and RWC were investigated. Results indicated that NPQ induction and relaxation curves – along with the OJIP-derived parameters were sensitive to dehydration. Since critical RWC for all the investigated ChlF parameters were found below 20%, Sanionia uncinata might be ranked into drought-resistant moss species.