Hardening enhances photoprotection in the moss Atrichum androgynum during rehydration by increasing fast- rather than slow-relaxing quenching

Chlorophyll fluorescence was used to study the effect of a hardening treatment on aspects of desiccation tolerance in the moss Atrichum androgynum. Mosses were hardened by reducing the relative water content of apical stem segments to 0.5 for 3 days, followed by storing the material fully hydrated for 1 day. Desiccation tolerance was assessed by storing plants over silica gel for 16 h, and then monitoring the recovery of photosystem II during rehydration. Hardening decreased the efficiency of photosystem II before desiccation, particularly at saturating light intensities. During rehydration, photosystem II activity recovered faster in hardened plants. Hardening also considerably increased non-photochemical quenching during the first few hours of rehydration. Previous data had shown that photophosphorylation, but not carbon fixation, is starting at this time, suggesting that the moss needs strong protection from excess light. Increased nonphotochemical quenching was the result of greater thylacoidal-based or fast-relaxing quenching (qE), rather than reaction centre-based or slow-relaxing quenching (ql). Slow-relaxing quenching was similar in hardened and non-hardened mosses, and was only important during the early stages of rehydration when the need for photoprotection was greatest. In A. androgynum hardening apparently shifts the photosynthetic apparatus from a 'high efficiency' state to a less efficient but 'photoprotected' state.