Comparative Rates of Dark CO₂Uptake and Acidification in the Bromeliaceae, Orchidaceae, and Euphorbiaceae

Abstract

Dark CO2 uptake, titratable acid, pH changes, percentage dry weight, and leaf thickness were compared in 30 genera of angiosperms belonging to the Bromeliaceae, Orchidaceae, and Euphorbiaceae. In each family, the more primitive taxa did not exhibit dark CO2 uptake and dark acidification while the more specialized taxa generally exhibited this system. There appears to be a correlation between the physiological characters studied and the broad phylogeny of the families studied. It is suggested that there has been a concomitant development of dark CO2 fixation with a decreased transpiration ratio and that these have been factors in the successful adaptive radiation of these three families into more xeric environments. There was a significant correlation between dark CO2 exchange and dark acidification and between dark CO2 exchange and leaf thickness in all three families. Crassulacean acid metabolism appears to be associated with a complex of xeromorphic characters rather than strictly with succulence.