Int. J. Plant Sci. 161(4):639-650. 2000. © 2000 by The University of Chicago. All rights reserved. 1058-5893/2000/16104-0010$03.00 PHOTOSYNTHETIC PATHWAY VARIATION IN LEAFY MEMBERS OF TWO SUBFAMILIES OF THE CACTACEAE Craig E. Martin 1 and Robert S. Wallace Department of Botany, University of Kansas, Lawrence, Kansas 66045-2106, U.S.A.; and Department of Botany, Iowa State University, Ames, Iowa 50011-1020, U.S.A. Patterns of 24-h C 0 2 exchange and diel fluctuations in tissue acid concentrations were measured in leafy and leafless shoots of 10 species in the Pereskioideae and eight species in the Opuntioideae (Cactaceae). The species were selected to represent a range of phylogenetic histories. Leafy shoots of all species in the Peres- kioideae exhibited C3 patterns of gas exchange, and net C 0 2 exchange of leafless stems in all but one species was negative during the day and night. Although nighttime C 0 2 uptake was not observed in shoots or stems of any of the pereskioid taxa, tissue acidity increased at night to a small degree in leaves of six species and stems of five species, indicative of low levels of CAM-cycling. In contrast, in leafy shoots of nearly all species in the Opuntioideae, C 0 2 uptake occurred during the day and the night. Gas-exchange rates were typically greater during the day. As is typical of CAM, nighttime maximal water use efficiency often greatly exceeded daytime values. Tissue malic acid concentrations increased overnight in leaves and stems of all eight opuntioid species. Examination of the data from a phylogenetic perspective illustrates evidence of low levels of CAM scattered among the primarily C 3 members of the more ancestral Pereskioideae. Furthermore, such consid- eration of the taxa in the more derived Opuntioideae (comparing the genera from most ancestral to most derived, that is, Austrocylindropuntia -* Quiabentia -> Pereskiopsis -* Cylindropuntia) revealed that CAM became increasingly less important in the leaves of the various taxa, whereas this water-conservative pathway of photosynthesis became increasingly more important in the stems. The results of this study indicate that members of the Pereskioideae should be restricted to moister habitats or must restrict the timing of growth to wet seasons, whereas the observed combinations of the C 3 and CAM pathways in the opuntioid taxa should prove beneficial in conserving water in the sporadically arid tropical and subtropical habitats of these plants. Keywords: C 3 photosynthesis, Cactaceae, CAM, evolution, gas exchange, leaves, stems. Introduction Until recently, plants could be easily categorized into three groups according to their photosynthetic pathway: C 3 , C 4 , and Crassulacean acid metabolism (CAM; Black 1973; Osmond et al. 1982; Edwards and Walker 1983). Now, however, plants with characteristics of more than one metabolic pathway are known (Koch and Kennedy 1980, 1982; Ting 1985; Griffiths 1988; Monson and Moore 1989; Raghavendra and Das 1993; Kraybill and Martin 1996; Martin 1996). Such plants vary considerably in the degree to which they engage in a particular photosynthetic pathway. Furthermore, different organs of the same plant may exhibit different photosynthetic pathways (Lange and Zuber 1977; Martin et al. 1990). The physiolog- ical, ecological, and evolutionary ramifications of such pho- tosynthetic pathway diversity are frequently subjects of spec- ulation (Ting 1985; Griffiths 1988; Monson 1989; Ehleringer and Monson 1993; Martin 1996); however, a complete un- derstanding of such diversity remains elusive. The Cactaceae is a large derived family, composed of three widely recognized subfamilies: the Pereskioideae, the Opun- 1 Author for correspondence; fax 785-864-5321; e-mail ecophys@ falcon.cc.ukans.edu. Manuscript received October 1999; revised manuscript received March 2000. tioideae, and the Cactoideae (Barthlott and Hunt 1993; Wal- lace 1995). Although many familiar cacti occur as stem suc- culents in arid regions, e.g., the deserts of the southwestern United States and northern Mexico, numerous taxa occur as epiphytes in the tropics, as thorny shrubs in subtropical areas, or as succulents at mesic sites at high elevations (Gibson and Nobel 1986). Ancestral taxa, especially those in the Peres- kioideae, have large long-lasting leaves that are less succulent than the small ephemeral leaves of the more derived taxa in the Cactaceae (Gibson and Nobel 1986; Barthlott and Hunt 1993; Wallace 1995; also see table 1). Although physiological investigations of cacti are numerous, the great majority of stud- ies include only a single species, typically a leafless stem suc- culent growing in or collected from the aforementioned arid regions (see references in Nobel 1988). The photosynthetic pathway of these cacti is exclusively CAM, often referred to as "obligate" or "constitutive" CAM. Investigations of the potential modes of photosynthesis of other cacti, especially the ancestral leafy forms found in mesic tropical and subtropical habitats, are much less common. In a survey similar to the current one, although covering fewer species, Nobel and Hartsock (1986) investigated the photo- synthetic pathways of both leaves and stems of taxa repre- senting the three subfamilies of the Cactaceae. Three species in the Pereskioideae were included: Pereskia aculeata, Pereskia 639