Carbon Metabolism in Two Species of Pereskia (Cactaceae)

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Abstract
The Pereskia are morphologically primitive, leafed members of the Cactaceae. Gas exchange characteristics using a dual isotope porometer to monitor 14CO2 and tritiated water uptake, diurnal malic acid fluctuations, phosphoenolpyruvate carboxylase and malate dehydrogenase activities were examined in P. grandifolia and P. aculeata. Investigations were done on well watered (control) and water-stressed plants. Nonstressed plants showed a CO2 uptake pattern indicating C3 Ca metabolism. Diurnal fluctuations in titratable acidity were observed similar to Crassulacean acid metabolism Plants exposed to 10 days of water stress exhibited stomatal opening only during an early morning period. Titratable acidity, phosphoenolpyruvate carboxylase activity and malate dehydrogenase activity fluctuations were magnified in the stressed plants but showed the same diurnal pattern as controls. Water stress causes these cacti to shift to an internal CO2 recycling (idling) that has all attributes of Crassulacean acid metabolism except nocturnal stomata opening and CO2 uptake. The consequences of this shift, which has been observed in other succulents, are unknown and some possibilities are suggested.