Evidence for decoupled water potential between myco- and photobionts during desiccation in the tripartite lichen <i>Peltigera britannica</i>

In lichens, accurate description of thallus water status is required to understand growth and photosynthesis dynamics. A recent model suggested that myco- and photobiont layers could have a different water energy status (i.e. a different water potential, Ψ) during thallus desiccation, but data supporting this hypothesis were presented only for one chlorolichen. We compared water relations and maximum photosystem II (PSII) efficiency during desiccation in the tripartite lichen Peltigera britannica and its axenically cultured photobionts, the green alga Coccomyxa sp. and the cyanobacterium Nostoc sp. To assess whether myco- and photobiont layers had different Ψ values during desiccation, we expected that (i) water relation parameters differ between cultured photobionts and entire thalli and (ii) Ψ values inducing a significant drop in PSII efficiency are lower for entire thalli than for cultured photobionts. We found that photobionts had very different water relation parameters than entire thalli, i.e. lower water potential at the turgor loss point and higher cell wall elasticity, irrespective of the photobiont type, potentially underlying a different drought tolerance. PSII efficiency in entire thalli and cultured photobionts started to decrease below Ψ values, inducing turgor loss. Importantly, PSII efficiency in entire thalli decreased at Ψ values significantly more negative than those inducing turgor loss in cultured photobionts. These data support the hypothesis of decoupled Ψ between myco- and photobionts in lichens during desiccation. A higher Ψ ensured to the photobiont layer might represent a key adaptation to prolong photosynthesis during desiccation.