Photosynthesis on the edge: photoinhibition, desiccation and freezing tolerance of Antarctic bryophytes

In Antarctica, multiple stresses (low temperatures, drought and excessive irradiance) hamper photosynthesis even in summer. We hypothesize that controlled inactivation of PSII reaction centres, a mechanism widely studied by pioneer work of Fred Chow and co-workers, may effectively guarantee functional photosynthesis under these conditions. Thus, we analysed the energy partitioning through photosystems in response to temperature in 15 bryophyte species presenting different worldwide distributions but all growing in Livingston Island, under controlled and field conditions. We additionally tested their tolerance to desiccation and freezing and compared those with their capability for sexual reproduction in Antarctica (as a proxy to overall fitness). Under field conditions, when irradiance rules air temperature by the warming of shoots (up to 20 degrees C under sunny days), a predominance of sustained photoinhibition beyond dynamic heat dissipation was observed at low temperatures. Antarctic endemic and polar species showed the largest increases of photoinhibition at low temperatures. On the contrary, the variation of thermal dissipation with temperature was not linked to species distribution. Instead, maximum non-photochemical quenching at 20 degrees C was related (strongly and positively) with desiccation tolerance, which also correlated with fertility in Antarctica, but not with freezing tolerance. Although all the analysed species tolerated - 20 degrees C when dry, the tolerance to freezing in hydrated state ranged from the exceptional ability ofSchistidium rivulare(that survived for 14 months at - 80 degrees C) to the susceptibility ofBryum pseudotriquetrum(that died after 1 day at - 20 degrees C unless being desiccated before freezing).

Automated summary

The study found that in Antarctica, most plants experience sustained photoinhibition rather than dynamic heat dissipation at low temperatures. Antarctic endemic and polar species showed the largest increases of photoinhibition at low temperatures.