Crassulacean acid metabolism (CAM) in high elevation tropical cactus

Abstract. Several taxa of cacti are distributed in high elevation tropical alpine habitats between 4000–4700 m in central Peru. This region has a marked dry season with soil water potentials as low as – 25 MPa. The barrel type cactus Oroya peruviana and the low ceaspitose Tephrocactus floccosus (both the typical hairy form and a hairless form) all exhibited diurnal fluctuations of malic acid (10–100 μmol/g FW), indicative of CAM photosynthesis. δ3C carbon isotope ratios were – 13 to – 14 suggesting that for these CAM plants the bulk of the net carbon gain is through night‐time carbon uptake. This occurs in spite of overnight temperatures below 0°C. CAM activity was observed on nights when air temperature dropped to – 8°C and subepidermal temperatures reached as low as –3°C. In central Peru, the typical form of T. floccosus has a dense covering of long silvery white hairs. Comparisons with an adjacent ‘hairless’ form showed that the hairy morph maintained a subepidermal temperature several degrees higher during the night. At a site where the ‘hairless’ morph was rare, the hairy T. floccosus had substantially higher overnight acid accumulation. At another site where the ‘hairless’ morph was abundant, the hairy T. floccosus had substantially lower acid accumulation relative to the ‘hairless’ form.