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Photosynthetic light responses of a widespread moss, Sanionia uncinata, from contrasting water regimes in the high Arctic tundra, Svalbard, Norway

Abstract Photosynthetic responses to light intensity were investigated under field conditions in Sanionia uncinata growing in contrasting water regimes in the high arctic tundra. Carbon dioxide exchange measurements were made using an infra-red gas analyser under ambient environmental conditions (CO2 concentration, 363–370 p.p.m.; air temperature, 12–15°C) at near optimum hydration on sunny days. The sunlight was reduced using layers of shade cloth. The photosynthetic photon flux density required to saturate net photosynthesis (PPFDsat) of Sanionia growing in a dry and wet site was 484 and 990 μmol m−2 s−1, respectively. The PPFD at which there was zero net exchange of CO2 (PPFDcomp) in dry and wet sites was 34 and 100 μmol m−2 s−1, respectively, and the PPFD at the point of 50% of the light saturated net photosyntheric rate (PPFDhalf) in dry and wet sites was 138 and 306 μmol m−2 s−1, respectively. The findings suggest that Sanionia growing in the wet site appears to require high radiation for photosynthesis. The light-saturated net photosynthetic rate (P max) in dry and wet sites was 0.84 and 1.42 mg CO2 g−1 s−1, respectively, and dark respiration rates (R dark) were 0.21 and 0.57 mg CO2 g−1 s−1, respectively. NPmax and R dark were >1.5-fold lower in the dry site than the wet site. These results suggest that Sanionia found in dry habitats is shade plants (sciophilous) while Sanionia in wet habitats is sun plants heliophilous.