Respiratory CO2 as Carbon Source for Nocturnal Acid Synthesis at High Temperatures in Three Species Exhibiting Crassulacean Acid Metabolism

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Abstract
Temperature effects on nocturnal carbon gain and nocturnal acid accumulation were studied in three species of plants exhibiting Crassulacean acid metabolism: Mamillaria woodsii, Opuntia vulgaris, and Kalanchoë daigremontiana. Under conditions of high soil moisture, nocturnal CO2 gain and acid accumulation had temperature optima at 15 to 20°C. Between 5 and 15°C, uptake of atmospheric CO2 largely accounted for acid accumulation. At higher tissue temperatures, acid accumulation exceeded net carbon gain indicating that acid synthesis was partly due to recycling of respiratory CO2. When plants were kept in CO2-free air, acid accumulation based on respiratory CO2 was highest at 25 to 35°C. Net acid synthesis occurred up to 45°C, although the nocturnal carbon balance became largely negative above 25 to 35°C. Under conditions of water stress, net CO2 exchange and nocturnal acid accumulation were reduced. Acid accumulation was proportionally more decreased at low than at high temperatures. Acid accumulation was either similar over the whole temperature range (5-45°C) or showed an optimum at high temperatures, although net carbon balance became very negative with increasing tissue temperatures. Conservation of carbon by recycling respiratory CO2 was temperature dependent. At 30°C, about 80% of the dark respiratory CO2 was conserved by dark CO2 fixation, in both well irrigated and water stressed plants.