Photosynthetic response to dynamic changes of light and air humidity in two moss species from the Tibetan Plateau

Bryophytes are widely distributed in alpine meadow and wetlands on the Tibetan Plateau, where light intensity and air moisture are highly variable in time. To address how bryophytes respond to their light and moisture environments, we examined dynamic photosynthesis in two moss species, Distichium inclinatum common in meadow, and Encalypta alpine frequently found in wetland. Photosynthetic induction response was faster in the two moss species than in most vascular species. In both species the 90% induction time after a sudden light increase from 50 to 600 μmol m⁻² s⁻¹ was within 3 min. The induction was faster in mosses experiencing a period of weak light than in those in total darkness. E. alpina, the wetland species, showed more rapid induction response and shorter post-illumination CO₂ fixation to sunflecks than D. inclinatum, the meadow species. Photosynthetic rate (A _max) under saturated light in the two species increased linearly with increasing air relative humidity (RH). The meadow species D. inclinatum showed higher A _max under low RHs, but exhibited lower A _max under high RHs in comparison with the wetland moss E. alpina. Moreover, the quantum efficiency increased linearly with increasing RH, indicating that air humidity plays a critical role in photochemical activities in the alpine mosses. The study suggests that there are acclimations in dynamic photosynthesis in response to light and humidity, and the acclimations would benefit a high leaf carbon gain in the two alpine moss species in their common habitats.