Figure 4.1. Net CO2 uptake over 24-hour periods for various platyo-puntias. Hatched bars indicate nighttime. Data for Opuntia ficus-indica are adapted from Cui et al. (1993) and Cui and Nobel (1994), those for O. humifusa from Koch and Kennedy (1980), and those for O. polyacantha from Gerwick and Williams (1978).

contrast to the case for the stems of species of Pereski-oideae, stems of all three leafy species of Opuntioideae had periods of positive net CO2 uptake, although maximal values averaged only 1 pmol m-2 s-1 (Nobel 1988).

For opuntias, the largest genus in the Opuntioideae, leaves tend to be ephemeral and hence do not contribute substantially to net CO2 uptake by the entire shoot. Net CO2 uptake thus occurs primarily by the stems at night (termed Phase I; Osmond 1978) in the Crassulacean acid metabolism (CAM) mode (Fig. 4.1). Some aspects of the gas exchange by an Opuntia sp. in Phase I were appreciated by Nicolas Théodore de Saussure in 1804, perhaps the earliest experimental insight into CAM (Richards 1915). The CO 2 taken up is incorporated into an organic acid such as malate using the enzyme phosphoenol pyruvate carboxylase (PEPCase), and the accumulating acids are sequestered into the large vacuoles of chlorenchyma cells (Kluge and Ting 1984). Thus the chlorenchyma becomes progressively more acidic during the night for CAM plants, and its osmotic pressure also increases (Luttge and Nobel 1984). Under wet conditions and moderate temperatures, some net CO2 uptake tends to occur in the early morning using 1,5-ribulosebisphosphate carboxylase/oxy-genase (Rubisco) via the C3 pathway before stem and air temperatures have risen appreciably from their minimum nighttime values (Phase II). In fact, carbon isotope evidence indicates that Opuntia polyacantha utilizes this phase of the CAM pathway more during wetter, cooler periods around 40,000 years before present (BP) than at 10,000 years BP or at the present time (Troughton et al. 1974).

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