Ecological and physiological investigations in continental Antarctic cryptogams II. Moisture relations and photosynthesis of lichens near Casey Station, Wilkes Land

Abstract

In the second of three field studies on the ecology and physiology of lichens on Clark Peninsula, Wilkes Land, Antarctica, photosynthetic activity due to natural and artificial soaking of lichen thalli was investigated. Gravimetric measurements were used to quantify water uptake by lichens in contact with snow or ice. Quantum flux density under a 15 cm thick layer of snow can reach light saturation for net photosynthesis of Usnea sphacelata at temperatures around 0°C. Measurements with a steady-state CO₂ diffusion porometer in the field reveal that, in Usnea antarctica, Umbilicaria decussata, and U. aprina, the optimum water content for net photosynthesis was 75–115 % d.wt. after the thalli were sprayed with water or submerged. The depression of net photosynthesis at super-optimal water content was strong in these species. In naturally soaked Usnea sphacelata this depression was less apparent. The water content resulting from contact with snow is frequently near the optimum for photosynthesis. In lichens of continental Antarctica it seems that super-optimal water contents are the exception rather than the rule.