Leaf Anatomy and CO₂ Recycling during Crassulacean Acid Metabolism in Twelve Epiphytic Species of Tillandsia (Bromeliaceae)

The relationship between leaf anatomy, specifically the percent of leaf volume occupied by waterstorage parenchyma (hydrenchyma), and the contribution of respiratory CO₂ during Crassulacean acid metabolism (CAM) was investigated in 12 epiphytic species of Tillandsia. It has been postulated that the hydrenchyma, which contributes to CO₂ exchange through respiration only, may be causally related to the recently observed phenomenon of CO₂ recycling during CAM. Among the 12 species of Tillandsia, leaves of T. usneoides and T. bergeri exhibited 0% hydrenchyma, while the hydrenchyma in the other species ranged from 2.9% to 53% of leaf cross-sectional area. Diurnal malate fluctuation and nighttime atmospheric CO₂ uptake were measured in at least four individuals of each species. A significant excess of diurnal malate fluctuation as compared with atmospheric CO₂ absorbed overnight was observed only in T. schiedeana. This species had an intermediate proportion (30%) of hydrenchyma in its leaves. Results of this study do not support the hypothesis that CO₂ recycling during CAM may reflect respiratory contributions of CO₂ from the tissue hydrenchyma.