Early morning photosynthesis of the moss Tortula ruralis following summer dew fall in a Hungarian temperate dry sandy grassland

Air temperature and humidity, moss surface temperature, moss water content, and photosynthetically active radi ation were measured through a clear dry night and early morning in July 1998: CO2 gas exchange of the moss was mea sured by infra-red gas analysis. The measurements showed progressive absorp tion of water by the moss through much of the night. The moss reached suffi cient water content for about 1.5 h of positive net CO2 uptake immediately after dawn. The cumulative net carbon balance on this occasion was negative , but mornings with heavier dew could give a positive daily carbon balance, and short, early morning periods of photosynthesis during prolonged dry we ather may mitigate long-term desiccation damage and allow for regular molec ular repair.